Merge remote-tracking branch 'upstream/master' into derivation-header-include-order

58 files changed, 957 insertions, 441 deletions

diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 2e2a17b14..83854df49 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -130,7 +130,7 @@ Pos findDerivationFilename(EvalState & state, Value & v, std::string what)
 
     Symbol file = state.symbols.create(filename);
 
-    return { file, lineno, 0 };
+    return { foFile, file, lineno, 0 };
 }
 
 
diff --git a/src/libexpr/attr-set.hh b/src/libexpr/attr-set.hh
index c601d09c2..7eaa16c59 100644
--- a/src/libexpr/attr-set.hh
+++ b/src/libexpr/attr-set.hh
@@ -78,7 +78,7 @@ public:
         if (!a)
             throw Error({
                 .hint = hintfmt("attribute '%s' missing", name),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
         return *a;
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 3d544c903..30f6ec7db 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -11,7 +11,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s))
 {
     throw EvalError({
         .hint = hintfmt(s),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -25,7 +25,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 {
     throw TypeError({
         .hint = hintfmt(s, showType(v)),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index b90a64357..c1a9af9b2 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -529,7 +529,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const
 {
     throw EvalError({
         .hint = hintfmt(s, s2),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -542,7 +542,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const
 {
     throw EvalError({
         .hint = hintfmt(s, s2, s3),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -551,7 +551,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & p1, const char * s, const
     // p1 is where the error occurred; p2 is a position mentioned in the message.
     throw EvalError({
         .hint = hintfmt(s, sym, p2),
-        .nixCode = NixCode { .errPos = p1 }
+        .errPos = p1
     });
 }
 
@@ -559,7 +559,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 {
     throw TypeError({
         .hint = hintfmt(s),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -572,7 +572,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 {
     throw TypeError({
         .hint = hintfmt(s, fun.showNamePos(), s2),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -580,7 +580,7 @@ LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s,
 {
     throw AssertionError({
         .hint = hintfmt(s, s1),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -588,23 +588,18 @@ LocalNoInlineNoReturn(void throwUndefinedVarError(const Pos & pos, const char *
 {
     throw UndefinedVarError({
         .hint = hintfmt(s, s1),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
-LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2))
+LocalNoInline(void addErrorTrace(Error & e, const char * s, const string & s2))
 {
-    e.addPrefix(format(s) % s2);
+    e.addTrace(std::nullopt, s, s2);
 }
 
-LocalNoInline(void addErrorPrefix(Error & e, const char * s, const ExprLambda & fun, const Pos & pos))
+LocalNoInline(void addErrorTrace(Error & e, const Pos & pos, const char * s, const string & s2))
 {
-    e.addPrefix(format(s) % fun.showNamePos() % pos);
-}
-
-LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2, const Pos & pos))
-{
-    e.addPrefix(format(s) % s2 % pos);
+    e.addTrace(pos, s, s2);
 }
 
 
@@ -818,7 +813,7 @@ void EvalState::evalFile(const Path & path_, Value & v)
     try {
         eval(e, v);
     } catch (Error & e) {
-        addErrorPrefix(e, "while evaluating the file '%1%':\n", path2);
+        addErrorTrace(e, "while evaluating the file '%1%':", path2);
         throw;
     }
 
@@ -1068,8 +1063,8 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 
     } catch (Error & e) {
         if (pos2 && pos2->file != state.sDerivationNix)
-            addErrorPrefix(e, "while evaluating the attribute '%1%' at %2%:\n",
-                showAttrPath(state, env, attrPath), *pos2);
+            addErrorTrace(e, *pos2, "while evaluating the attribute '%1%'",
+                showAttrPath(state, env, attrPath));
         throw;
     }
 
@@ -1237,11 +1232,15 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
 
     /* Evaluate the body.  This is conditional on showTrace, because
        catching exceptions makes this function not tail-recursive. */
-    if (settings.showTrace)
+    if (loggerSettings.showTrace.get())
         try {
             lambda.body->eval(*this, env2, v);
         } catch (Error & e) {
-            addErrorPrefix(e, "while evaluating %1%, called from %2%:\n", lambda, pos);
+            addErrorTrace(e, lambda.pos, "while evaluating %s", 
+              (lambda.name.set() 
+                  ? "'" + (string) lambda.name + "'" 
+                  : "anonymous lambdaction"));
+            addErrorTrace(e, pos, "from call site%s", "");
             throw;
         }
     else
@@ -1516,7 +1515,7 @@ void EvalState::forceValueDeep(Value & v)
                 try {
                     recurse(*i.value);
                 } catch (Error & e) {
-                    addErrorPrefix(e, "while evaluating the attribute '%1%' at %2%:\n", i.name, *i.pos);
+                    addErrorTrace(e, *i.pos, "while evaluating the attribute '%1%'", i.name);
                     throw;
                 }
         }
@@ -1936,7 +1935,7 @@ string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, boo
 {
     throw TypeError({
         .hint = hintfmt("cannot coerce %1% to a string", showType()),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 863365259..0d52a7f63 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -250,7 +250,7 @@ private:
     friend struct ExprAttrs;
     friend struct ExprLet;
 
-    Expr * parse(const char * text, const Path & path,
+    Expr * parse(const char * text, FileOrigin origin, const Path & path,
         const Path & basePath, StaticEnv & staticEnv);
 
 public:
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index b4b65883d..d5698011f 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -197,7 +197,22 @@ std::ostream & operator << (std::ostream & str, const Pos & pos)
     if (!pos)
         str << "undefined position";
     else
-        str << (format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%") % (string) pos.file % pos.line % pos.column).str();
+    {
+        auto f = format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%");
+        switch (pos.origin) {
+            case foFile:
+                f % (string) pos.file;
+                break;
+            case foStdin:
+            case foString:
+                f % "(string)";
+                break;
+            default:
+                throw Error("unhandled Pos origin!");
+        }
+        str << (f % pos.line % pos.column).str();
+    }
+
     return str;
 }
 
@@ -270,7 +285,7 @@ void ExprVar::bindVars(const StaticEnv & env)
     if (withLevel == -1)
         throw UndefinedVarError({
             .hint = hintfmt("undefined variable '%1%'", name),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     fromWith = true;
     this->level = withLevel;
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index ec6fd3190..e4cbc660f 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -24,11 +24,12 @@ MakeError(RestrictedPathError, Error);
 
 struct Pos
 {
+    FileOrigin origin;
     Symbol file;
     unsigned int line, column;
-    Pos() : line(0), column(0) { };
-    Pos(const Symbol & file, unsigned int line, unsigned int column)
-        : file(file), line(line), column(column) { };
+    Pos() : origin(foString), line(0), column(0) { };
+    Pos(FileOrigin origin, const Symbol & file, unsigned int line, unsigned int column)
+        : origin(origin), file(file), line(line), column(column) { };
     operator bool() const
     {
         return line != 0;
@@ -238,7 +239,7 @@ struct ExprLambda : Expr
         if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
             throw ParseError({
                 .hint = hintfmt("duplicate formal function argument '%1%'", arg),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
     };
     void setName(Symbol & name);
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index a639be64e..878f06c96 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -30,7 +30,8 @@ namespace nix {
         SymbolTable & symbols;
         Expr * result;
         Path basePath;
-        Symbol path;
+        Symbol file;
+        FileOrigin origin;
         ErrorInfo error;
         Symbol sLetBody;
         ParseData(EvalState & state)
@@ -65,18 +66,17 @@ namespace nix {
 static void dupAttr(const AttrPath & attrPath, const Pos & pos, const Pos & prevPos)
 {
     throw ParseError({
-        .hint = hintfmt("attribute '%1%' already defined at %2%",
+         .hint = hintfmt("attribute '%1%' already defined at %2%",
             showAttrPath(attrPath), prevPos),
-        .nixCode = NixCode { .errPos = pos },
+         .errPos = pos
     });
 }
 
-
 static void dupAttr(Symbol attr, const Pos & pos, const Pos & prevPos)
 {
     throw ParseError({
         .hint = hintfmt("attribute '%1%' already defined at %2%", attr, prevPos),
-        .nixCode = NixCode { .errPos = pos },
+        .errPos = pos
     });
 }
 
@@ -148,7 +148,7 @@ static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
         throw ParseError({
             .hint = hintfmt("duplicate formal function argument '%1%'",
                 formal.name),
-            .nixCode = NixCode { .errPos = pos },
+            .errPos = pos
         });
     formals->formals.push_front(formal);
 }
@@ -246,7 +246,7 @@ static Expr * stripIndentation(const Pos & pos, SymbolTable & symbols, vector<Ex
 
 static inline Pos makeCurPos(const YYLTYPE & loc, ParseData * data)
 {
-    return Pos(data->path, loc.first_line, loc.first_column);
+    return Pos(data->origin, data->file, loc.first_line, loc.first_column);
 }
 
 #define CUR_POS makeCurPos(*yylocp, data)
@@ -259,7 +259,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
 {
     data->error = {
         .hint = hintfmt(error),
-        .nixCode = NixCode { .errPos = makeCurPos(*loc, data) }
+        .errPos = makeCurPos(*loc, data)
     };
 }
 
@@ -339,7 +339,7 @@ expr_function
     { if (!$2->dynamicAttrs.empty())
         throw ParseError({
             .hint = hintfmt("dynamic attributes not allowed in let"),
-            .nixCode = NixCode { .errPos = CUR_POS },
+            .errPos = CUR_POS
         });
       $$ = new ExprLet($2, $4);
     }
@@ -419,7 +419,7 @@ expr_simple
       if (noURLLiterals)
           throw ParseError({
               .hint = hintfmt("URL literals are disabled"),
-              .nixCode = NixCode { .errPos = CUR_POS }
+              .errPos = CUR_POS
           });
       $$ = new ExprString(data->symbols.create($1));
   }
@@ -492,7 +492,7 @@ attrs
       } else
           throw ParseError({
               .hint = hintfmt("dynamic attributes not allowed in inherit"),
-              .nixCode = NixCode { .errPos = makeCurPos(@2, data) },
+              .errPos = makeCurPos(@2, data)
           });
     }
   | { $$ = new AttrPath; }
@@ -569,13 +569,24 @@ formal
 namespace nix {
 
 
-Expr * EvalState::parse(const char * text,
+Expr * EvalState::parse(const char * text, FileOrigin origin,
     const Path & path, const Path & basePath, StaticEnv & staticEnv)
 {
     yyscan_t scanner;
     ParseData data(*this);
+    data.origin = origin;
+    switch (origin) {
+        case foFile: 
+            data.file = data.symbols.create(path);
+            break;
+        case foStdin:
+        case foString:
+            data.file = data.symbols.create(text);
+            break;
+        default:
+            assert(false);
+    }
     data.basePath = basePath;
-    data.path = data.symbols.create(path);
 
     yylex_init(&scanner);
     yy_scan_string(text, scanner);
@@ -625,13 +636,13 @@ Expr * EvalState::parseExprFromFile(const Path & path)
 
 Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
 {
-    return parse(readFile(path).c_str(), path, dirOf(path), staticEnv);
+    return parse(readFile(path).c_str(), foFile, path, dirOf(path), staticEnv);
 }
 
 
 Expr * EvalState::parseExprFromString(std::string_view s, const Path & basePath, StaticEnv & staticEnv)
 {
-    return parse(s.data(), "(string)", basePath, staticEnv);
+    return parse(s.data(), foString, "", basePath, staticEnv);
 }
 
 
@@ -644,7 +655,7 @@ Expr * EvalState::parseExprFromString(std::string_view s, const Path & basePath)
 Expr * EvalState::parseStdin()
 {
     //Activity act(*logger, lvlTalkative, format("parsing standard input"));
-    return parseExprFromString(drainFD(0), absPath("."));
+    return parse(drainFD(0).data(), foStdin, "", absPath("."), staticBaseEnv);
 }
 
 
@@ -693,7 +704,7 @@ Path EvalState::findFile(SearchPath & searchPath, const string & path, const Pos
             ? "cannot look up '<%s>' in pure evaluation mode (use '--impure' to override)"
             : "file '%s' was not found in the Nix search path (add it using $NIX_PATH or -I)",
             path),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 16b181f3a..f9e03a09f 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -96,7 +96,7 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     } catch (InvalidPathError & e) {
         throw EvalError({
             .hint = hintfmt("cannot import '%1%', since path '%2%' is not valid", path, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 
@@ -177,7 +177,7 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
             .hint = hintfmt(
                 "cannot import '%1%', since path '%2%' is not valid",
                 path, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 
@@ -215,7 +215,7 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     if (count == 0) {
         throw EvalError({
             .hint = hintfmt("at least one argument to 'exec' required"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
     PathSet context;
@@ -230,7 +230,7 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
         throw EvalError({
             .hint = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
                 program, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 
@@ -239,13 +239,13 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         parsed = state.parseExprFromString(output, pos.file);
     } catch (Error & e) {
-        e.addPrefix(fmt("While parsing the output from '%1%', at %2%\n", program, pos));
+        e.addTrace(pos, "While parsing the output from '%1%'", program);
         throw;
     }
     try {
         state.eval(parsed, v);
     } catch (Error & e) {
-        e.addPrefix(fmt("While evaluating the output from '%1%', at %2%\n", program, pos));
+        e.addTrace(pos, "While evaluating the output from '%1%'", program);
         throw;
     }
 }
@@ -385,7 +385,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     if (startSet == args[0]->attrs->end())
         throw EvalError({
             .hint = hintfmt("attribute 'startSet' required"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     state.forceList(*startSet->value, pos);
 
@@ -399,7 +399,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     if (op == args[0]->attrs->end())
         throw EvalError({
             .hint = hintfmt("attribute 'operator' required"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     state.forceValue(*op->value, pos);
 
@@ -421,7 +421,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         if (key == e->attrs->end())
             throw EvalError({
                 .hint = hintfmt("attribute 'key' required"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         state.forceValue(*key->value, pos);
 
@@ -471,7 +471,7 @@ static void prim_addErrorContext(EvalState & state, const Pos & pos, Value * * a
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addPrefix(format("%1%\n") % state.coerceToString(pos, *args[0], context));
+        e.addTrace(std::nullopt, state.coerceToString(pos, *args[0], context));
         throw;
     }
 }
@@ -556,14 +556,14 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     if (attr == args[0]->attrs->end())
         throw EvalError({
             .hint = hintfmt("required attribute 'name' missing"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     string drvName;
     Pos & posDrvName(*attr->pos);
     try {
         drvName = state.forceStringNoCtx(*attr->value, pos);
     } catch (Error & e) {
-        e.addPrefix(fmt("while evaluating the derivation attribute 'name' at %1%:\n", posDrvName));
+        e.addTrace(posDrvName, "while evaluating the derivation attribute 'name'");
         throw;
     }
 
@@ -603,7 +603,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             else
                 throw EvalError({
                     .hint = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
-                    .nixCode = NixCode { .errPos = posDrvName }
+                    .errPos = posDrvName
                 });
         };
 
@@ -613,7 +613,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                 if (outputs.find(j) != outputs.end())
                     throw EvalError({
                         .hint = hintfmt("duplicate derivation output '%1%'", j),
-                        .nixCode = NixCode { .errPos = posDrvName }
+                        .errPos = posDrvName
                     });
                 /* !!! Check whether j is a valid attribute
                    name. */
@@ -623,14 +623,14 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                 if (j == "drv")
                     throw EvalError({
                         .hint = hintfmt("invalid derivation output name 'drv'" ),
-                        .nixCode = NixCode { .errPos = posDrvName }
+                        .errPos = posDrvName
                     });
                 outputs.insert(j);
             }
             if (outputs.empty())
                 throw EvalError({
                     .hint = hintfmt("derivation cannot have an empty set of outputs"),
-                    .nixCode = NixCode { .errPos = posDrvName }
+                    .errPos = posDrvName
                 });
         };
 
@@ -696,8 +696,9 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             }
 
         } catch (Error & e) {
-            e.addPrefix(format("while evaluating the attribute '%1%' of the derivation '%2%' at %3%:\n")
-                % key % drvName % posDrvName);
+            e.addTrace(posDrvName, 
+                "while evaluating the attribute '%1%' of the derivation '%2%'",
+                key, drvName);
             throw;
         }
     }
@@ -745,20 +746,20 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     if (drv.builder == "")
         throw EvalError({
             .hint = hintfmt("required attribute 'builder' missing"),
-            .nixCode = NixCode { .errPos = posDrvName }
+            .errPos = posDrvName
         });
 
     if (drv.platform == "")
         throw EvalError({
             .hint = hintfmt("required attribute 'system' missing"),
-            .nixCode = NixCode { .errPos = posDrvName }
+            .errPos = posDrvName
         });
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
         throw EvalError({
             .hint = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
-            .nixCode = NixCode { .errPos = posDrvName }
+            .errPos = posDrvName
         });
 
     if (outputHash) {
@@ -766,7 +767,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
             throw Error({
                 .hint = hintfmt("multiple outputs are not supported in fixed-output derivations"),
-                .nixCode = NixCode { .errPos = posDrvName }
+                .errPos = posDrvName
             });
 
         std::optional<HashType> ht = parseHashTypeOpt(outputHashAlgo);
@@ -776,7 +777,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out", DerivationOutput {
             .path = std::move(outPath),
-            .hash = DerivationOutputHash {
+            .hash = FixedOutputHash {
                 .method = ingestionMethod,
                 .hash = std::move(h),
             },
@@ -795,7 +796,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             drv.outputs.insert_or_assign(i,
                 DerivationOutput {
                     .path = StorePath::dummy,
-                    .hash = std::optional<DerivationOutputHash> {},
+                    .hash = std::optional<FixedOutputHash> {},
                 });
         }
 
@@ -807,7 +808,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             drv.outputs.insert_or_assign(i,
                 DerivationOutput {
                     .path = std::move(outPath),
-                    .hash = std::optional<DerivationOutputHash>(),
+                    .hash = std::optional<FixedOutputHash>(),
                 });
         }
     }
@@ -880,7 +881,7 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
     if (!state.store->isInStore(path))
         throw EvalError({
             .hint = hintfmt("path '%1%' is not in the Nix store", path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     Path path2 = state.store->toStorePath(path);
     if (!settings.readOnlyMode)
@@ -901,7 +902,7 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
             .hint = hintfmt(
                 "cannot check the existence of '%1%', since path '%2%' is not valid",
                 path, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 
@@ -947,7 +948,7 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     } catch (InvalidPathError & e) {
         throw EvalError({
             .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid", path, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
     string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
@@ -978,7 +979,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         if (i == v2.attrs->end())
             throw EvalError({
                 .hint = hintfmt("attribute 'path' missing"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
         PathSet context;
@@ -989,7 +990,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         } catch (InvalidPathError & e) {
             throw EvalError({
                 .hint = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         }
 
@@ -1009,7 +1010,7 @@ static void prim_hashFile(EvalState & state, const Pos & pos, Value * * args, Va
     if (!ht)
       throw Error({
           .hint = hintfmt("unknown hash type '%1%'", type),
-          .nixCode = NixCode { .errPos = pos }
+          .errPos = pos
       });
 
     PathSet context; // discarded
@@ -1028,7 +1029,7 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
     } catch (InvalidPathError & e) {
         throw EvalError({
             .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid", path, e.path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 
@@ -1104,7 +1105,7 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
                     "in 'toFile': the file named '%1%' must not contain a reference "
                     "to a derivation but contains (%2%)",
                     name, path),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         refs.insert(state.store->parseStorePath(path));
     }
@@ -1175,7 +1176,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
     if (!context.empty())
         throw EvalError({
             .hint = hintfmt("string '%1%' cannot refer to other paths", path),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     state.forceValue(*args[0], pos);
@@ -1184,7 +1185,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
             .hint = hintfmt(
                 "first argument in call to 'filterSource' is not a function but %1%",
                 showType(*args[0])),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, Hash(), v);
@@ -1207,7 +1208,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
             if (!context.empty())
                 throw EvalError({
                     .hint = hintfmt("string '%1%' cannot refer to other paths", path),
-                    .nixCode = NixCode { .errPos = *attr.pos }
+                    .errPos = *attr.pos
                 });
         } else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, *attr.pos);
@@ -1221,13 +1222,13 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         else
             throw EvalError({
                 .hint = hintfmt("unsupported argument '%1%' to 'addPath'", attr.name),
-                .nixCode = NixCode { .errPos = *attr.pos }
+                .errPos = *attr.pos
             });
     }
     if (path.empty())
         throw EvalError({
             .hint = hintfmt("'path' required"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     if (name.empty())
         name = baseNameOf(path);
@@ -1288,7 +1289,7 @@ void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
     if (i == args[1]->attrs->end())
         throw EvalError({
             .hint = hintfmt("attribute '%1%' missing", attr),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     // !!! add to stack trace?
     if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
@@ -1371,7 +1372,7 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
         if (j == v2.attrs->end())
             throw TypeError({
                 .hint = hintfmt("'name' attribute missing in a call to 'listToAttrs'"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         string name = state.forceStringNoCtx(*j->value, pos);
 
@@ -1381,7 +1382,7 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
             if (j2 == v2.attrs->end())
                 throw TypeError({
                     .hint = hintfmt("'value' attribute missing in a call to 'listToAttrs'"),
-                    .nixCode = NixCode { .errPos = pos }
+                    .errPos = pos
                 });
             v.attrs->push_back(Attr(sym, j2->value, j2->pos));
         }
@@ -1457,7 +1458,7 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
     if (args[0]->type != tLambda)
         throw TypeError({
             .hint = hintfmt("'functionArgs' requires a function"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     if (!args[0]->lambda.fun->matchAttrs) {
@@ -1513,7 +1514,7 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
     if (n < 0 || (unsigned int) n >= list.listSize())
         throw Error({
             .hint = hintfmt("list index %1% is out of bounds", n),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     state.forceValue(*list.listElems()[n], pos);
     v = *list.listElems()[n];
@@ -1543,7 +1544,7 @@ static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value
     if (args[0]->listSize() == 0)
         throw Error({
             .hint = hintfmt("'tail' called on an empty list"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     state.mkList(v, args[0]->listSize() - 1);
@@ -1688,7 +1689,7 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
     if (len < 0)
         throw EvalError({
             .hint = hintfmt("cannot create list of size %1%", len),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     state.mkList(v, len);
@@ -1850,7 +1851,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
     if (f2 == 0)
         throw EvalError({
             .hint = hintfmt("division by zero"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     if (args[0]->type == tFloat || args[1]->type == tFloat) {
@@ -1862,7 +1863,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
             throw EvalError({
                 .hint = hintfmt("overflow in integer division"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
         mkInt(v, i1 / i2);
@@ -1923,7 +1924,7 @@ static void prim_substring(EvalState & state, const Pos & pos, Value * * args, V
     if (start < 0)
         throw EvalError({
             .hint = hintfmt("negative start position in 'substring'"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     mkString(v, (unsigned int) start >= s.size() ? "" : string(s, start, len), context);
@@ -1946,7 +1947,7 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
     if (!ht)
         throw Error({
             .hint = hintfmt("unknown hash type '%1%'", type),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     PathSet context; // discarded
@@ -1992,12 +1993,12 @@ void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
             throw EvalError({
                 .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         } else {
             throw EvalError({
                 .hint = hintfmt("invalid regular expression '%s'", re),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         }
     }
@@ -2065,12 +2066,12 @@ static void prim_split(EvalState & state, const Pos & pos, Value * * args, Value
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
             throw EvalError({
                 .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         } else {
             throw EvalError({
                 .hint = hintfmt("invalid regular expression '%s'", re),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
         }
     }
@@ -2104,7 +2105,7 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     if (args[0]->listSize() != args[1]->listSize())
         throw EvalError({
             .hint = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
 
     vector<string> from;
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 301e8c5dd..dbb93bae6 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -148,7 +148,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (!state.store->isStorePath(i.name))
             throw EvalError({
                 .hint = hintfmt("Context key '%s' is not a store path", i.name),
-                .nixCode = NixCode { .errPos = *i.pos }
+                .errPos = *i.pos
             });
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(i.name));
@@ -165,7 +165,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
                 if (!isDerivation(i.name)) {
                     throw EvalError({
                         .hint = hintfmt("Tried to add all-outputs context of %s, which is not a derivation, to a string", i.name),
-                        .nixCode = NixCode { .errPos = *i.pos }
+                        .errPos = *i.pos
                     });
                 }
                 context.insert("=" + string(i.name));
@@ -178,7 +178,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
             if (iter->value->listSize() && !isDerivation(i.name)) {
                 throw EvalError({
                     .hint = hintfmt("Tried to add derivation output context of %s, which is not a derivation, to a string", i.name),
-                    .nixCode = NixCode { .errPos = *i.pos }
+                    .errPos = *i.pos
                 });
             }
             for (unsigned int n = 0; n < iter->value->listSize(); ++n) {
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index dd7229a3d..36b0db2bd 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -37,14 +37,14 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
             else
                 throw EvalError({
                     .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
-                    .nixCode = NixCode { .errPos = *attr.pos }
+                    .errPos = *attr.pos
                 });
         }
 
         if (url.empty())
             throw EvalError({
                 .hint = hintfmt("'url' argument required"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
     } else
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index 9bace8f89..59166b777 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -40,14 +40,14 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
             else
                 throw EvalError({
                     .hint = hintfmt("unsupported argument '%s' to 'fetchMercurial'", attr.name),
-                    .nixCode = NixCode { .errPos = *attr.pos }
+                    .errPos = *attr.pos
                 });
         }
 
         if (url.empty())
             throw EvalError({
                 .hint = hintfmt("'url' argument required"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
     } else
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 9be93710a..01d6ad8b0 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -68,7 +68,7 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
         if (!attrs.count("type"))
             throw Error({
                 .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
 
         input = fetchers::inputFromAttrs(attrs);
@@ -112,14 +112,14 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             else
                 throw EvalError({
                     .hint = hintfmt("unsupported argument '%s' to '%s'", attr.name, who),
-                    .nixCode = NixCode { .errPos = *attr.pos }
+                    .errPos = *attr.pos
                 });
             }
 
         if (!url)
             throw EvalError({
                 .hint = hintfmt("'url' argument required"),
-                .nixCode = NixCode { .errPos = pos }
+                .errPos = pos
             });
     } else
         url = state.forceStringNoCtx(*args[0], pos);
diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index 7615d1379..b00827a4b 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -83,7 +83,7 @@ static void prim_fromTOML(EvalState & state, const Pos & pos, Value * * args, Va
     } catch (std::runtime_error & e) {
         throw EvalError({
             .hint = hintfmt("while parsing a TOML string: %s", e.what()),
-            .nixCode = NixCode { .errPos = pos }
+            .errPos = pos
         });
     }
 }
diff --git a/src/libfetchers/cache.hh b/src/libfetchers/cache.hh
index d76ab1233..3db4f081c 100644
--- a/src/libfetchers/cache.hh
+++ b/src/libfetchers/cache.hh
@@ -6,6 +6,8 @@ namespace nix::fetchers {
 
 struct Cache
 {
+    virtual ~Cache() { }
+
     virtual void add(
         ref<Store> store,
         const Attrs & inAttrs,
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index ac83d52b9..f5356f0af 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -70,7 +70,10 @@ DownloadFileResult downloadFile(
         ValidPathInfo info(store->makeFixedOutputPath(FileIngestionMethod::Flat, hash, name));
         info.narHash = hashString(htSHA256, *sink.s);
         info.narSize = sink.s->size();
-        info.ca = makeFixedOutputCA(FileIngestionMethod::Flat, hash);
+        info.ca = FixedOutputHash {
+            .method = FileIngestionMethod::Flat,
+            .hash = hash,
+        };
         auto source = StringSource { *sink.s };
         store->addToStore(info, source, NoRepair, NoCheckSigs);
         storePath = std::move(info.path);
diff --git a/src/libmain/loggers.cc b/src/libmain/loggers.cc
index c44bb6408..fa18f991d 100644
--- a/src/libmain/loggers.cc
+++ b/src/libmain/loggers.cc
@@ -26,7 +26,7 @@ Logger * makeDefaultLogger() {
     case LogFormat::rawWithLogs:
         return makeSimpleLogger(true);
     case LogFormat::internalJson:
-        return makeJSONLogger(*makeSimpleLogger());
+        return makeJSONLogger(*makeSimpleLogger(true));
     case LogFormat::bar:
         return makeProgressBar();
     case LogFormat::barWithLogs:
diff --git a/src/libmain/progress-bar.cc b/src/libmain/progress-bar.cc
index 95a9187de..3f7d99a1d 100644
--- a/src/libmain/progress-bar.cc
+++ b/src/libmain/progress-bar.cc
@@ -131,7 +131,7 @@ public:
         auto state(state_.lock());
 
         std::stringstream oss;
-        oss << ei;
+        showErrorInfo(oss, ei, loggerSettings.showTrace.get());
 
         log(*state, ei.level, oss.str());
     }
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 1cb422967..52718c231 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -323,10 +323,8 @@ int handleExceptions(const string & programName, std::function<void()> fun)
         printError("Try '%1% --help' for more information.", programName);
         return 1;
     } catch (BaseError & e) {
-        if (settings.showTrace && e.prefix() != "")
-            printError(e.prefix());
         logError(e.info());
-        if (e.prefix() != "" && !settings.showTrace)
+        if (e.hasTrace() && !loggerSettings.showTrace.get())
             printError("(use '--show-trace' to show detailed location information)");
         return e.status;
     } catch (std::bad_alloc & e) {
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 82a2ab831..347fe1b99 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1950,8 +1950,11 @@ void linkOrCopy(const Path & from, const Path & to)
         /* Hard-linking fails if we exceed the maximum link count on a
            file (e.g. 32000 of ext3), which is quite possible after a
            'nix-store --optimise'. FIXME: actually, why don't we just
-           bind-mount in this case? */
-        if (errno != EMLINK)
+           bind-mount in this case?
+           
+           It can also fail with EPERM in BeegFS v7 and earlier versions
+           which don't allow hard-links to other directories */
+        if (errno != EMLINK && errno != EPERM)
             throw SysError("linking '%s' to '%s'", to, from);
         copyPath(from, to);
     }
@@ -2750,8 +2753,8 @@ struct RestrictedStore : public LocalFSStore
     void queryReferrers(const StorePath & path, StorePathSet & referrers) override
     { }
 
-    StorePathSet queryDerivationOutputs(const StorePath & path) override
-    { throw Error("queryDerivationOutputs"); }
+    OutputPathMap queryDerivationOutputMap(const StorePath & path) override
+    { throw Error("queryDerivationOutputMap"); }
 
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override
     { throw Error("queryPathFromHashPart"); }
@@ -3714,7 +3717,7 @@ void DerivationGoal::registerOutputs()
         /* Check that fixed-output derivations produced the right
            outputs (i.e., the content hash should match the specified
            hash). */
-        std::string ca;
+        std::optional<ContentAddress> ca;
 
         if (fixedOutput) {
 
@@ -3764,7 +3767,10 @@ void DerivationGoal::registerOutputs()
             else
                 assert(worker.store.parseStorePath(path) == dest);
 
-            ca = makeFixedOutputCA(i.second.hash->method, h2);
+            ca = FixedOutputHash {
+                .method = i.second.hash->method,
+                .hash = h2,
+            };
         }
 
         /* Get rid of all weird permissions.  This also checks that
@@ -3837,7 +3843,10 @@ void DerivationGoal::registerOutputs()
         info.ca = ca;
         worker.store.signPathInfo(info);
 
-        if (!info.references.empty()) info.ca.clear();
+        if (!info.references.empty()) {
+            // FIXME don't we have an experimental feature for fixed output with references?
+            info.ca = {};
+        }
 
         infos.emplace(i.first, std::move(info));
     }
diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index 1cfe4a46a..e630cf6f1 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -58,13 +58,16 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         }
     };
 
+    /* We always have one output, and if it's a fixed-output derivation (as
+       checked below) it must be the only output */
+    auto & output = drv.outputs.begin()->second;
+
     /* Try the hashed mirrors first. */
-    if (getAttr("outputHashMode") == "flat")
+    if (output.hash && output.hash->method == FileIngestionMethod::Flat)
         for (auto hashedMirror : settings.hashedMirrors.get())
             try {
                 if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                auto ht = parseHashTypeOpt(getAttr("outputHashAlgo"));
-                auto h = Hash(getAttr("outputHash"), ht);
+                auto & h = output.hash->hash;
                 fetch(hashedMirror + printHashType(*h.type) + "/" + h.to_string(Base16, false));
                 return;
             } catch (Error & e) {
diff --git a/src/libstore/content-address.cc b/src/libstore/content-address.cc
new file mode 100644
index 000000000..3d753836f
--- /dev/null
+++ b/src/libstore/content-address.cc
@@ -0,0 +1,85 @@
+#include "content-address.hh"
+
+namespace nix {
+
+std::string FixedOutputHash::printMethodAlgo() const {
+    return makeFileIngestionPrefix(method) + printHashType(*hash.type);
+}
+
+std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
+    switch (m) {
+    case FileIngestionMethod::Flat:
+        return "";
+    case FileIngestionMethod::Recursive:
+        return "r:";
+    default:
+        throw Error("impossible, caught both cases");
+    }
+}
+
+std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
+{
+    return "fixed:"
+        + makeFileIngestionPrefix(method)
+        + hash.to_string(Base32, true);
+}
+
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+std::string renderContentAddress(ContentAddress ca) {
+    return std::visit(overloaded {
+        [](TextHash th) {
+            return "text:" + th.hash.to_string(Base32, true);
+        },
+        [](FixedOutputHash fsh) {
+            return makeFixedOutputCA(fsh.method, fsh.hash);
+        }
+    }, ca);
+}
+
+ContentAddress parseContentAddress(std::string_view rawCa) {
+    auto prefixSeparator = rawCa.find(':');
+    if (prefixSeparator != string::npos) {
+        auto prefix = string(rawCa, 0, prefixSeparator);
+        if (prefix == "text") {
+            auto hashTypeAndHash = rawCa.substr(prefixSeparator+1, string::npos);
+            Hash hash = Hash(string(hashTypeAndHash));
+            if (*hash.type != htSHA256) {
+                throw Error("parseContentAddress: the text hash should have type SHA256");
+            }
+            return TextHash { hash };
+        } else if (prefix == "fixed") {
+            // This has to be an inverse of makeFixedOutputCA
+            auto methodAndHash = rawCa.substr(prefixSeparator+1, string::npos);
+            if (methodAndHash.substr(0,2) == "r:") {
+                std::string_view hashRaw = methodAndHash.substr(2,string::npos);
+                return FixedOutputHash {
+                    .method = FileIngestionMethod::Recursive,
+                    .hash = Hash(string(hashRaw)),
+                };
+            } else {
+                std::string_view hashRaw = methodAndHash;
+                return FixedOutputHash {
+                    .method = FileIngestionMethod::Flat,
+                    .hash = Hash(string(hashRaw)),
+                };
+            }
+        } else {
+            throw Error("parseContentAddress: format not recognized; has to be text or fixed");
+        }
+    } else {
+        throw Error("Not a content address because it lacks an appropriate prefix");
+    }
+};
+
+std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt) {
+    return rawCaOpt == "" ? std::optional<ContentAddress> {} : parseContentAddress(rawCaOpt);
+};
+
+std::string renderContentAddress(std::optional<ContentAddress> ca) {
+    return ca ? renderContentAddress(*ca) : "";
+}
+
+}
diff --git a/src/libstore/content-address.hh b/src/libstore/content-address.hh
new file mode 100644
index 000000000..ba4797f5b
--- /dev/null
+++ b/src/libstore/content-address.hh
@@ -0,0 +1,56 @@
+#pragma once
+
+#include <variant>
+#include "hash.hh"
+
+namespace nix {
+
+enum struct FileIngestionMethod : uint8_t {
+    Flat = false,
+    Recursive = true
+};
+
+struct TextHash {
+    Hash hash;
+};
+
+/// Pair of a hash, and how the file system was ingested
+struct FixedOutputHash {
+    FileIngestionMethod method;
+    Hash hash;
+    std::string printMethodAlgo() const;
+};
+
+/*
+  We've accumulated several types of content-addressed paths over the years;
+  fixed-output derivations support multiple hash algorithms and serialisation
+  methods (flat file vs NAR). Thus, ‘ca’ has one of the following forms:
+
+  * ‘text:sha256:<sha256 hash of file contents>’: For paths
+    computed by makeTextPath() / addTextToStore().
+
+  * ‘fixed:<r?>:<ht>:<h>’: For paths computed by
+    makeFixedOutputPath() / addToStore().
+*/
+typedef std::variant<
+    TextHash, // for paths computed by makeTextPath() / addTextToStore
+    FixedOutputHash // for path computed by makeFixedOutputPath
+> ContentAddress;
+
+/* Compute the prefix to the hash algorithm which indicates how the files were
+   ingested. */
+std::string makeFileIngestionPrefix(const FileIngestionMethod m);
+
+/* Compute the content-addressability assertion (ValidPathInfo::ca)
+   for paths created by makeFixedOutputPath() / addToStore(). */
+std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash);
+
+std::string renderContentAddress(ContentAddress ca);
+
+std::string renderContentAddress(std::optional<ContentAddress> ca);
+
+ContentAddress parseContentAddress(std::string_view rawCa);
+
+std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt);
+
+}
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index e370e278c..a8fb62e0a 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -78,10 +78,10 @@ struct TunnelLogger : public Logger
         if (ei.level > verbosity) return;
 
         std::stringstream oss;
-        oss << ei;
+        showErrorInfo(oss, ei, false);
 
         StringSink buf;
-        buf << STDERR_NEXT << oss.str() << "\n"; // (fs.s + "\n");
+        buf << STDERR_NEXT << oss.str() << "\n";
         enqueueMsg(*buf.s);
     }
 
@@ -347,6 +347,15 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         break;
     }
 
+    case wopQueryDerivationOutputMap: {
+        auto path = store->parseStorePath(readString(from));
+        logger->startWork();
+        OutputPathMap outputs = store->queryDerivationOutputMap(path);
+        logger->stopWork();
+        writeOutputPathMap(*store, to, outputs);
+        break;
+    }
+
     case wopQueryDeriver: {
         auto path = store->parseStorePath(readString(from));
         logger->startWork();
@@ -652,7 +661,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
                 to << info->ultimate
                    << info->sigs
-                   << info->ca;
+                   << renderContentAddress(info->ca);
             }
         } else {
             assert(GET_PROTOCOL_MINOR(clientVersion) >= 17);
@@ -710,7 +719,8 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         info.references = readStorePaths<StorePathSet>(*store, from);
         from >> info.registrationTime >> info.narSize >> info.ultimate;
         info.sigs = readStrings<StringSet>(from);
-        from >> info.ca >> repair >> dontCheckSigs;
+        info.ca = parseContentAddressOpt(readString(from));
+        from >> repair >> dontCheckSigs;
         if (!trusted && dontCheckSigs)
             dontCheckSigs = false;
         if (!trusted)
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 9f2c9b2cb..f0f5c95bf 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -7,11 +7,6 @@
 
 namespace nix {
 
-std::string DerivationOutputHash::printMethodAlgo() const {
-    return makeFileIngestionPrefix(method) + printHashType(*hash.type);
-}
-
-
 const StorePath & BasicDerivation::findOutput(const string & id) const
 {
     auto i = outputs.find(id);
@@ -112,7 +107,7 @@ static DerivationOutput parseDerivationOutput(const Store & store, istringstream
     expect(str, ","); const auto hash = parseString(str);
     expect(str, ")");
 
-    std::optional<DerivationOutputHash> fsh;
+    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
@@ -120,7 +115,7 @@ static DerivationOutput parseDerivationOutput(const Store & store, istringstream
             hashAlgo = string(hashAlgo, 2);
         }
         const HashType hashType = parseHashType(hashAlgo);
-        fsh = DerivationOutputHash {
+        fsh = FixedOutputHash {
             .method = std::move(method),
             .hash = Hash(hash, hashType),
         };
@@ -380,17 +375,17 @@ static DerivationOutput readDerivationOutput(Source & in, const Store & store)
 {
     auto path = store.parseStorePath(readString(in));
     auto hashAlgo = readString(in);
-    const auto hash = readString(in);
+    auto hash = readString(in);
 
-    std::optional<DerivationOutputHash> fsh;
+    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
             method = FileIngestionMethod::Recursive;
             hashAlgo = string(hashAlgo, 2);
         }
-        const HashType hashType = parseHashType(hashAlgo);
-        fsh = DerivationOutputHash {
+        auto hashType = parseHashType(hashAlgo);
+        fsh = FixedOutputHash {
             .method = std::move(method),
             .hash = Hash(hash, hashType),
         };
@@ -439,11 +434,16 @@ Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv)
 void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv)
 {
     out << drv.outputs.size();
-    for (auto & i : drv.outputs)
+    for (auto & i : drv.outputs) {
         out << i.first
-            << store.printStorePath(i.second.path)
-            << i.second.hash->printMethodAlgo()
-            << i.second.hash->hash.to_string(Base16, false);
+            << store.printStorePath(i.second.path);
+        if (i.second.hash) {
+            out << i.second.hash->printMethodAlgo()
+                << i.second.hash->hash.to_string(Base16, false);
+        } else {
+            out << "" << "";
+        }
+    }
     writeStorePaths(store, out, drv.inputSrcs);
     out << drv.platform << drv.builder << drv.args;
     out << drv.env.size();
diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index d09db53f2..6a195d314 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -3,6 +3,7 @@
 #include "path.hh"
 #include "types.hh"
 #include "hash.hh"
+#include "content-address.hh"
 
 #include <map>
 
@@ -12,18 +13,10 @@ namespace nix {
 
 /* Abstract syntax of derivations. */
 
-/// Pair of a hash, and how the file system was ingested
-struct DerivationOutputHash {
-    FileIngestionMethod method;
-    Hash hash;
-    std::string printMethodAlgo() const;
-};
-
 struct DerivationOutput
 {
     StorePath path;
-    std::optional<DerivationOutputHash> hash; /* hash used for expected hash computation */
-    void parseHashInfo(FileIngestionMethod & recursive, Hash & hash) const;
+    std::optional<FixedOutputHash> hash; /* hash used for expected hash computation */
 };
 
 typedef std::map<string, DerivationOutput> DerivationOutputs;
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index bee94cbd8..683fa5196 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -35,7 +35,7 @@ Settings::Settings()
     , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR").value_or(NIX_LIBEXEC_DIR)))
     , nixBinDir(canonPath(getEnv("NIX_BIN_DIR").value_or(NIX_BIN_DIR)))
     , nixManDir(canonPath(NIX_MAN_DIR))
-    , nixDaemonSocketFile(canonPath(nixStateDir + DEFAULT_SOCKET_PATH))
+    , nixDaemonSocketFile(canonPath(getEnv("NIX_DAEMON_SOCKET_PATH").value_or(nixStateDir + DEFAULT_SOCKET_PATH)))
 {
     buildUsersGroup = getuid() == 0 ? "nixbld" : "";
     lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 2fbcafff8..58cf08763 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -196,10 +196,6 @@ public:
     /* Whether to lock the Nix client and worker to the same CPU. */
     bool lockCPU;
 
-    /* Whether to show a stack trace if Nix evaluation fails. */
-    Setting<bool> showTrace{this, false, "show-trace",
-        "Whether to show a stack trace on evaluation errors."};
-
     Setting<SandboxMode> sandboxMode{this,
         #if __linux__
           smEnabled
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 45c70fad6..5657aa593 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -114,7 +114,7 @@ struct LegacySSHStore : public Store
             if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
                 auto s = readString(conn->from);
                 info->narHash = s.empty() ? Hash() : Hash(s);
-                conn->from >> info->ca;
+                info->ca = parseContentAddressOpt(readString(conn->from));
                 info->sigs = readStrings<StringSet>(conn->from);
             }
 
@@ -146,7 +146,7 @@ struct LegacySSHStore : public Store
                 << info.narSize
                 << info.ultimate
                 << info.sigs
-                << info.ca;
+                << renderContentAddress(info.ca);
             try {
                 copyNAR(source, conn->to);
             } catch (...) {
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index c6b55ff7c..eed225349 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -580,7 +580,7 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
 uint64_t LocalStore::addValidPath(State & state,
     const ValidPathInfo & info, bool checkOutputs)
 {
-    if (info.ca != "" && !info.isContentAddressed(*this))
+    if (info.ca.has_value() && !info.isContentAddressed(*this))
         throw Error("cannot add path '%s' to the Nix store because it claims to be content-addressed but isn't",
             printStorePath(info.path));
 
@@ -592,7 +592,7 @@ uint64_t LocalStore::addValidPath(State & state,
         (info.narSize, info.narSize != 0)
         (info.ultimate ? 1 : 0, info.ultimate)
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
-        (info.ca, !info.ca.empty())
+        (renderContentAddress(info.ca), (bool) info.ca)
         .exec();
     uint64_t id = sqlite3_last_insert_rowid(state.db);
 
@@ -666,7 +666,7 @@ void LocalStore::queryPathInfoUncached(const StorePath & path,
             if (s) info->sigs = tokenizeString<StringSet>(s, " ");
 
             s = (const char *) sqlite3_column_text(state->stmtQueryPathInfo, 7);
-            if (s) info->ca = s;
+            if (s) info->ca = parseContentAddressOpt(s);
 
             /* Get the references. */
             auto useQueryReferences(state->stmtQueryReferences.use()(info->id));
@@ -689,7 +689,7 @@ void LocalStore::updatePathInfo(State & state, const ValidPathInfo & info)
         (info.narHash.to_string(Base16, true))
         (info.ultimate ? 1 : 0, info.ultimate)
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
-        (info.ca, !info.ca.empty())
+        (renderContentAddress(info.ca), (bool) info.ca)
         (printStorePath(info.path))
         .exec();
 }
@@ -774,17 +774,20 @@ StorePathSet LocalStore::queryValidDerivers(const StorePath & path)
 }
 
 
-StorePathSet LocalStore::queryDerivationOutputs(const StorePath & path)
+OutputPathMap LocalStore::queryDerivationOutputMap(const StorePath & path)
 {
-    return retrySQLite<StorePathSet>([&]() {
+    return retrySQLite<OutputPathMap>([&]() {
         auto state(_state.lock());
 
         auto useQueryDerivationOutputs(state->stmtQueryDerivationOutputs.use()
             (queryValidPathId(*state, path)));
 
-        StorePathSet outputs;
+        OutputPathMap outputs;
         while (useQueryDerivationOutputs.next())
-            outputs.insert(parseStorePath(useQueryDerivationOutputs.getStr(1)));
+            outputs.emplace(
+                useQueryDerivationOutputs.getStr(0),
+                parseStorePath(useQueryDerivationOutputs.getStr(1))
+            );
 
         return outputs;
     });
@@ -985,15 +988,15 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
             deletePath(realPath);
 
-            if (info.ca != "" &&
-                !((hasPrefix(info.ca, "text:") && !info.references.count(info.path))
-                    || info.references.empty()))
+            // text hashing has long been allowed to have non-self-references because it is used for drv files.
+            bool refersToSelf = info.references.count(info.path) > 0;
+            if (info.ca.has_value() && !info.references.empty() && !(std::holds_alternative<TextHash>(*info.ca) && !refersToSelf))
                 settings.requireExperimentalFeature("ca-references");
 
             /* While restoring the path from the NAR, compute the hash
                of the NAR. */
             std::unique_ptr<AbstractHashSink> hashSink;
-            if (info.ca == "" || !info.references.count(info.path))
+            if (!info.ca.has_value() || !info.references.count(info.path))
                 hashSink = std::make_unique<HashSink>(htSHA256);
             else
                 hashSink = std::make_unique<HashModuloSink>(htSHA256, std::string(info.path.hashPart()));
@@ -1079,7 +1082,7 @@ StorePath LocalStore::addToStoreFromDump(const string & dump, const string & nam
             ValidPathInfo info(dstPath);
             info.narHash = hash.first;
             info.narSize = hash.second;
-            info.ca = makeFixedOutputCA(method, h);
+            info.ca = FixedOutputHash { .method = method, .hash = h };
             registerValidPath(info);
         }
 
@@ -1143,7 +1146,7 @@ StorePath LocalStore::addTextToStore(const string & name, const string & s,
             info.narHash = narHash;
             info.narSize = sink.s->size();
             info.references = references;
-            info.ca = "text:" + hash.to_string(Base32, true);
+            info.ca = TextHash { .hash = hash };
             registerValidPath(info);
         }
 
@@ -1254,7 +1257,7 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                 printMsg(lvlTalkative, "checking contents of '%s'", printStorePath(i));
 
                 std::unique_ptr<AbstractHashSink> hashSink;
-                if (info->ca == "" || !info->references.count(info->path))
+                if (!info->ca || !info->references.count(info->path))
                     hashSink = std::make_unique<HashSink>(*info->narHash.type);
                 else
                     hashSink = std::make_unique<HashModuloSink>(*info->narHash.type, std::string(info->path.hashPart()));
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 872500957..03837bc5f 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -130,7 +130,7 @@ public:
 
     StorePathSet queryValidDerivers(const StorePath & path) override;
 
-    StorePathSet queryDerivationOutputs(const StorePath & path) override;
+    OutputPathMap queryDerivationOutputMap(const StorePath & path) override;
 
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
 
diff --git a/src/libstore/nar-info-disk-cache.cc b/src/libstore/nar-info-disk-cache.cc
index 552970248..012dea6ea 100644
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@@ -203,7 +203,7 @@ public:
                 narInfo->deriver = StorePath(queryNAR.getStr(9));
             for (auto & sig : tokenizeString<Strings>(queryNAR.getStr(10), " "))
                 narInfo->sigs.insert(sig);
-            narInfo->ca = queryNAR.getStr(11);
+            narInfo->ca = parseContentAddressOpt(queryNAR.getStr(11));
 
             return {oValid, narInfo};
         });
@@ -237,7 +237,7 @@ public:
                     (concatStringsSep(" ", info->shortRefs()))
                     (info->deriver ? std::string(info->deriver->to_string()) : "", (bool) info->deriver)
                     (concatStringsSep(" ", info->sigs))
-                    (info->ca)
+                    (renderContentAddress(info->ca))
                     (time(0)).exec();
 
             } else {
diff --git a/src/libstore/nar-info.cc b/src/libstore/nar-info.cc
index bb4448c90..04550ed97 100644
--- a/src/libstore/nar-info.cc
+++ b/src/libstore/nar-info.cc
@@ -67,8 +67,9 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "Sig")
             sigs.insert(value);
         else if (name == "CA") {
-            if (!ca.empty()) corrupt();
-            ca = value;
+            if (ca) corrupt();
+            // FIXME: allow blank ca or require skipping field?
+            ca = parseContentAddressOpt(value);
         }
 
         pos = eol + 1;
@@ -104,8 +105,8 @@ std::string NarInfo::to_string(const Store & store) const
     for (auto sig : sigs)
         res += "Sig: " + sig + "\n";
 
-    if (!ca.empty())
-        res += "CA: " + ca + "\n";
+    if (ca)
+        res += "CA: " + renderContentAddress(*ca) + "\n";
 
     return res;
 }
diff --git a/src/libstore/path.hh b/src/libstore/path.hh
index aaebd3ec3..e43a8b50c 100644
--- a/src/libstore/path.hh
+++ b/src/libstore/path.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include "content-address.hh"
 #include "types.hh"
 
 namespace nix {
@@ -61,15 +62,11 @@ public:
 
 typedef std::set<StorePath> StorePathSet;
 typedef std::vector<StorePath> StorePaths;
+typedef std::map<string, StorePath> OutputPathMap;
 
 /* Extension of derivations in the Nix store. */
 const std::string drvExtension = ".drv";
 
-enum struct FileIngestionMethod : uint8_t {
-    Flat = false,
-    Recursive = true
-};
-
 struct StorePathWithOutputs
 {
     StorePath path;
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index f5f2ab7fd..118aadf7e 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -8,6 +8,7 @@
 #include "derivations.hh"
 #include "pool.hh"
 #include "finally.hh"
+#include "logging.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -38,6 +39,29 @@ void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths
         out << store.printStorePath(i);
 }
 
+std::map<string, StorePath> readOutputPathMap(const Store & store, Source & from)
+{
+    std::map<string, StorePath> pathMap;
+    auto rawInput = readStrings<Strings>(from);
+    if (rawInput.size() % 2)
+        throw Error("got an odd number of elements from the daemon when trying to read a output path map");
+    auto curInput = rawInput.begin();
+    while (curInput != rawInput.end()) {
+        auto thisKey = *curInput++;
+        auto thisValue = *curInput++;
+        pathMap.emplace(thisKey, store.parseStorePath(thisValue));
+    }
+    return pathMap;
+}
+
+void writeOutputPathMap(const Store & store, Sink & out, const std::map<string, StorePath> & pathMap)
+{
+    out << 2*pathMap.size();
+    for (auto & i : pathMap) {
+        out << i.first;
+        out << store.printStorePath(i.second);
+    }
+}
 
 /* TODO: Separate these store impls into different files, give them better names */
 RemoteStore::RemoteStore(const Params & params)
@@ -197,7 +221,7 @@ void RemoteStore::setOptions(Connection & conn)
         overrides.erase(settings.maxSilentTime.name);
         overrides.erase(settings.buildCores.name);
         overrides.erase(settings.useSubstitutes.name);
-        overrides.erase(settings.showTrace.name);
+        overrides.erase(loggerSettings.showTrace.name);
         conn.to << overrides.size();
         for (auto & i : overrides)
             conn.to << i.first << i.second.value;
@@ -381,7 +405,7 @@ void RemoteStore::queryPathInfoUncached(const StorePath & path,
             if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
                 conn->from >> info->ultimate;
                 info->sigs = readStrings<StringSet>(conn->from);
-                conn->from >> info->ca;
+                info->ca = parseContentAddressOpt(readString(conn->from));
             }
         }
         callback(std::move(info));
@@ -412,12 +436,24 @@ StorePathSet RemoteStore::queryValidDerivers(const StorePath & path)
 StorePathSet RemoteStore::queryDerivationOutputs(const StorePath & path)
 {
     auto conn(getConnection());
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 0x16) {
+        return Store::queryDerivationOutputs(path);
+    }
     conn->to << wopQueryDerivationOutputs << printStorePath(path);
     conn.processStderr();
     return readStorePaths<StorePathSet>(*this, conn->from);
 }
 
 
+OutputPathMap RemoteStore::queryDerivationOutputMap(const StorePath & path)
+{
+    auto conn(getConnection());
+    conn->to << wopQueryDerivationOutputMap << printStorePath(path);
+    conn.processStderr();
+    return readOutputPathMap(*this, conn->from);
+
+}
+
 std::optional<StorePath> RemoteStore::queryPathFromHashPart(const std::string & hashPart)
 {
     auto conn(getConnection());
@@ -465,7 +501,7 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
                  << info.narHash.to_string(Base16, false);
         writeStorePaths(*this, conn->to, info.references);
         conn->to << info.registrationTime << info.narSize
-                 << info.ultimate << info.sigs << info.ca
+                 << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
         bool tunnel = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21;
         if (!tunnel) copyNAR(source, conn->to);
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 80c8e9f11..fb2052752 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -51,6 +51,7 @@ public:
 
     StorePathSet queryDerivationOutputs(const StorePath & path) override;
 
+    OutputPathMap queryDerivationOutputMap(const StorePath & path) override;
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index cffa40f8c..3cb3bae4f 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -242,6 +242,16 @@ bool Store::PathInfoCacheValue::isKnownNow()
     return std::chrono::steady_clock::now() < time_point + ttl;
 }
 
+StorePathSet Store::queryDerivationOutputs(const StorePath & path)
+{
+    auto outputMap = this->queryDerivationOutputMap(path);
+    StorePathSet outputPaths;
+    for (auto & i: outputMap) {
+        outputPaths.emplace(std::move(i.second));
+    }
+    return outputPaths;
+}
+
 bool Store::isValidPath(const StorePath & storePath)
 {
     std::string hashPart(storePath.hashPart());
@@ -471,8 +481,8 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
                     jsonRefs.elem(printStorePath(ref));
             }
 
-            if (info->ca != "")
-                jsonPath.attr("ca", info->ca);
+            if (info->ca)
+                jsonPath.attr("ca", renderContentAddress(info->ca));
 
             std::pair<uint64_t, uint64_t> closureSizes;
 
@@ -757,41 +767,35 @@ void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
     sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 
 bool ValidPathInfo::isContentAddressed(const Store & store) const
 {
-    auto warn = [&]() {
-        logWarning(
-            ErrorInfo{
-                .name = "Path not content-addressed",
-                .hint = hintfmt("path '%s' claims to be content-addressed but isn't", store.printStorePath(path))
-            });
-    };
+    if (! ca) return false;
 
-    if (hasPrefix(ca, "text:")) {
-        Hash hash(ca.substr(5));
-        if (store.makeTextPath(path.name(), hash, references) == path)
-            return true;
-        else
-            warn();
-    }
-
-    else if (hasPrefix(ca, "fixed:")) {
-        FileIngestionMethod recursive { ca.compare(6, 2, "r:") == 0 };
-        Hash hash(ca.substr(recursive == FileIngestionMethod::Recursive ? 8 : 6));
-        auto refs = references;
-        bool hasSelfReference = false;
-        if (refs.count(path)) {
-            hasSelfReference = true;
-            refs.erase(path);
+    auto caPath = std::visit(overloaded {
+        [&](TextHash th) {
+            return store.makeTextPath(path.name(), th.hash, references);
+        },
+        [&](FixedOutputHash fsh) {
+            auto refs = references;
+            bool hasSelfReference = false;
+            if (refs.count(path)) {
+                hasSelfReference = true;
+                refs.erase(path);
+            }
+            return store.makeFixedOutputPath(fsh.method, fsh.hash, path.name(), refs, hasSelfReference);
         }
-        if (store.makeFixedOutputPath(recursive, hash, path.name(), refs, hasSelfReference) == path)
-            return true;
-        else
-            warn();
-    }
+    }, *ca);
+
+    bool res = caPath == path;
+
+    if (!res)
+        printError("warning: path '%s' claims to be content-addressed but isn't", store.printStorePath(path));
 
-    return false;
+    return res;
 }
 
 
@@ -822,25 +826,6 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
-std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
-    switch (m) {
-    case FileIngestionMethod::Flat:
-        return "";
-    case FileIngestionMethod::Recursive:
-        return "r:";
-    default:
-        throw Error("impossible, caught both cases");
-    }
-}
-
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
-{
-    return "fixed:"
-        + makeFileIngestionPrefix(method)
-        + hash.to_string(Base32, true);
-}
-
-
 Derivation Store::derivationFromPath(const StorePath & drvPath)
 {
     ensurePath(drvPath);
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index dc47f4d6e..b122e05d6 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -2,6 +2,7 @@
 
 #include "path.hh"
 #include "hash.hh"
+#include "content-address.hh"
 #include "serialise.hh"
 #include "crypto.hh"
 #include "lru-cache.hh"
@@ -18,6 +19,7 @@
 #include <memory>
 #include <string>
 #include <chrono>
+#include <variant>
 
 
 namespace nix {
@@ -109,7 +111,6 @@ struct SubstitutablePathInfo
 
 typedef std::map<StorePath, SubstitutablePathInfo> SubstitutablePathInfos;
 
-
 struct ValidPathInfo
 {
     StorePath path;
@@ -138,21 +139,11 @@ struct ValidPathInfo
        that a particular output path was produced by a derivation; the
        path then implies the contents.)
 
-       Ideally, the content-addressability assertion would just be a
-       Boolean, and the store path would be computed from
-       the name component, ‘narHash’ and ‘references’. However,
-       1) we've accumulated several types of content-addressed paths
-       over the years; and 2) fixed-output derivations support
-       multiple hash algorithms and serialisation methods (flat file
-       vs NAR). Thus, ‘ca’ has one of the following forms:
-
-       * ‘text:sha256:<sha256 hash of file contents>’: For paths
-         computed by makeTextPath() / addTextToStore().
-
-       * ‘fixed:<r?>:<ht>:<h>’: For paths computed by
-         makeFixedOutputPath() / addToStore().
+       Ideally, the content-addressability assertion would just be a Boolean,
+       and the store path would be computed from the name component, ‘narHash’
+       and ‘references’. However, we support many types of content addresses.
     */
-    std::string ca;
+    std::optional<ContentAddress> ca;
 
     bool operator == (const ValidPathInfo & i) const
     {
@@ -427,8 +418,11 @@ public:
     virtual StorePathSet queryValidDerivers(const StorePath & path) { return {}; };
 
     /* Query the outputs of the derivation denoted by `path'. */
-    virtual StorePathSet queryDerivationOutputs(const StorePath & path)
-    { unsupported("queryDerivationOutputs"); }
+    virtual StorePathSet queryDerivationOutputs(const StorePath & path);
+
+    /* Query the mapping outputName=>outputPath for the given derivation */
+    virtual OutputPathMap queryDerivationOutputMap(const StorePath & path)
+    { unsupported("queryDerivationOutputMap"); }
 
     /* Query the full store path given the hash part of a valid store
        path, or empty if the path doesn't exist. */
@@ -837,15 +831,6 @@ std::optional<ValidPathInfo> decodeValidPathInfo(
     std::istream & str,
     bool hashGiven = false);
 
-/* Compute the prefix to the hash algorithm which indicates how the files were
-   ingested. */
-std::string makeFileIngestionPrefix(const FileIngestionMethod m);
-
-/* Compute the content-addressability assertion (ValidPathInfo::ca)
-   for paths created by makeFixedOutputPath() / addToStore(). */
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash);
-
-
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri);
 
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index ac42457fc..8b538f6da 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x115
+#define PROTOCOL_VERSION 0x116
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -30,7 +30,7 @@ typedef enum {
     wopSetOptions = 19,
     wopCollectGarbage = 20,
     wopQuerySubstitutablePathInfo = 21,
-    wopQueryDerivationOutputs = 22,
+    wopQueryDerivationOutputs = 22, // obsolete
     wopQueryAllValidPaths = 23,
     wopQueryFailedPaths = 24,
     wopClearFailedPaths = 25,
@@ -49,6 +49,7 @@ typedef enum {
     wopNarFromPath = 38,
     wopAddToStoreNar = 39,
     wopQueryMissing = 40,
+    wopQueryDerivationOutputMap = 41,
 } WorkerOp;
 
 
@@ -69,5 +70,6 @@ template<class T> T readStorePaths(const Store & store, Source & from);
 
 void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths);
 
+void writeOutputPathMap(const Store & store, Sink & out, const OutputPathMap & paths);
 
 }
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 6a8484705..51c88537e 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -262,7 +262,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                             names[name] = 0;
                     }
                 } else if (s == "node") {
-                    if (s.empty()) throw badArchive("entry name missing");
+                    if (name.empty()) throw badArchive("entry name missing");
                     parse(sink, source, path + "/" + name);
                 } else
                     throw badArchive("unknown field " + s);
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 0fad9ae42..a4ee7afc2 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -7,14 +7,11 @@
 
 namespace nix {
 
-
 const std::string nativeSystem = SYSTEM;
 
-// addPrefix is used for show-trace.  Strings added with addPrefix
-// will print ahead of the error itself.
-BaseError & BaseError::addPrefix(const FormatOrString & fs)
+BaseError & BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
 {
-    prefix_ = fs.s + prefix_;
+    err.traces.push_front(Trace { .pos = e, .hint = hint});
     return *this;
 }
 
@@ -28,7 +25,7 @@ const string& BaseError::calcWhat() const
         err.name = sname();
 
         std::ostringstream oss;
-        oss << err;
+        showErrorInfo(oss, err, false);
         what_ = oss.str();
 
         return *what_;
@@ -56,28 +53,114 @@ string showErrPos(const ErrPos &errPos)
     }
 }
 
-// if nixCode contains lines of code, print them to the ostream, indicating the error column.
-void printCodeLines(std::ostream &out, const string &prefix, const NixCode &nixCode)
+std::optional<LinesOfCode> getCodeLines(const ErrPos &errPos)
+{
+    if (errPos.line <= 0)
+        return std::nullopt;
+
+    if (errPos.origin == foFile) {
+        LinesOfCode loc;
+        try {
+            AutoCloseFD fd = open(errPos.file.c_str(), O_RDONLY | O_CLOEXEC);
+            if (!fd) {
+                logError(SysError("opening file '%1%'", errPos.file).info());
+                return std::nullopt;
+            }
+            else
+            {
+                // count the newlines.
+                int count = 0;
+                string line;
+                int pl = errPos.line - 1;
+                do
+                {
+                    line = readLine(fd.get());
+                    ++count;
+                    if (count < pl)
+                    {
+                        ;
+                    }
+                    else if (count == pl) {
+                        loc.prevLineOfCode = line;
+                    } else if (count == pl + 1) {
+                        loc.errLineOfCode = line;
+                    } else if (count == pl + 2) {
+                        loc.nextLineOfCode = line;
+                        break;
+                    }
+                } while (true);
+                return loc;
+            }
+        }
+        catch (EndOfFile &eof) {
+            if (loc.errLineOfCode.has_value())
+                return loc;
+            else
+                return std::nullopt;
+        }
+        catch (std::exception &e) {
+            printError("error reading nix file: %s\n%s", errPos.file, e.what());
+            return std::nullopt;
+        }
+    } else {
+        std::istringstream iss(errPos.file);
+        // count the newlines.
+        int count = 0;
+        string line;
+        int pl = errPos.line - 1;
+
+        LinesOfCode loc;
+
+        do
+        {
+            std::getline(iss, line);
+            ++count;
+            if (count < pl)
+            {
+                ;
+            }
+            else if (count == pl) {
+                loc.prevLineOfCode = line;
+            } else if (count == pl + 1) {
+                loc.errLineOfCode = line;
+            } else if (count == pl + 2) {
+                loc.nextLineOfCode = line;
+                break;
+            }
+
+            if (!iss.good())
+                break;
+        } while (true);
+
+        return loc;
+    }
+}
+
+// print lines of code to the ostream, indicating the error column.
+void printCodeLines(std::ostream &out,
+    const string &prefix,
+    const ErrPos &errPos,
+    const LinesOfCode &loc)
 {
     // previous line of code.
-    if (nixCode.prevLineOfCode.has_value()) {
+    if (loc.prevLineOfCode.has_value()) {
         out << std::endl
             << fmt("%1% %|2$5d|| %3%",
-                prefix,
-                (nixCode.errPos.line - 1),
-                *nixCode.prevLineOfCode);
+            prefix,
+            (errPos.line - 1),
+            *loc.prevLineOfCode);
     }
 
-    if (nixCode.errLineOfCode.has_value()) {
+    if (loc.errLineOfCode.has_value()) {
         // line of code containing the error.
         out << std::endl
             << fmt("%1% %|2$5d|| %3%",
-                prefix,
-                (nixCode.errPos.line),
-                *nixCode.errLineOfCode);
+            prefix,
+            (errPos.line),
+            *loc.errLineOfCode);
         // error arrows for the column range.
-        if (nixCode.errPos.column > 0) {
-            int start = nixCode.errPos.column;
+        if (errPos.column > 0) {
+            int start = errPos.column;
             std::string spaces;
             for (int i = 0; i < start; ++i) {
                 spaces.append(" ");
@@ -87,23 +170,49 @@ void printCodeLines(std::ostream &out, const string &prefix, const NixCode &nixC
 
             out << std::endl
                 << fmt("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL,
-                    prefix,
-                    spaces,
-                    arrows);
+                prefix,
+                spaces,
+                arrows);
         }
     }
 
     // next line of code.
-    if (nixCode.nextLineOfCode.has_value()) {
+    if (loc.nextLineOfCode.has_value()) {
         out << std::endl
             << fmt("%1% %|2$5d|| %3%",
-                prefix,
-                (nixCode.errPos.line + 1),
-                *nixCode.nextLineOfCode);
+            prefix,
+            (errPos.line + 1),
+            *loc.nextLineOfCode);
+    }
+}
+
+void printAtPos(const string &prefix, const ErrPos &pos, std::ostream &out)
+{
+    if (pos)
+    {
+        switch (pos.origin) {
+            case foFile: {
+                out << prefix << ANSI_BLUE << "at: " << ANSI_YELLOW << showErrPos(pos) <<
+                    ANSI_BLUE << " in file: " << ANSI_NORMAL << pos.file;
+                break;
+            }
+            case foString: {
+                out << prefix << ANSI_BLUE << "at: " << ANSI_YELLOW << showErrPos(pos) <<
+                    ANSI_BLUE << " from string" << ANSI_NORMAL;
+                break;
+            }
+            case foStdin: {
+                out << prefix << ANSI_BLUE << "at: " << ANSI_YELLOW << showErrPos(pos) <<
+                    ANSI_BLUE << " from stdin" << ANSI_NORMAL;
+                break;
+            }
+            default:
+                throw Error("invalid FileOrigin in errPos");
+        }
     }
 }
 
-std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
+std::ostream& showErrorInfo(std::ostream &out, const ErrorInfo &einfo, bool showTrace)
 {
     auto errwidth = std::max<size_t>(getWindowSize().second, 20);
     string prefix = "";
@@ -158,8 +267,12 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
         }
     }
 
-    auto ndl = prefix.length() + levelString.length() + 3 + einfo.name.length() + einfo.programName.value_or("").length();
-    auto dashwidth = ndl > (errwidth - 3) ? 3 : errwidth - ndl;
+    auto ndl = prefix.length()
+        + filterANSIEscapes(levelString, true).length()
+        + 7
+        + einfo.name.length()
+        + einfo.programName.value_or("").length();
+    auto dashwidth = std::max<int>(errwidth - ndl, 3);
 
     std::string dashes(dashwidth, '-');
 
@@ -179,16 +292,9 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
             einfo.programName.value_or(""));
 
     bool nl = false;  // intersperse newline between sections.
-    if (einfo.nixCode.has_value()) {
-        if (einfo.nixCode->errPos.file != "") {
-            // filename, line, column.
-            out << std::endl << fmt("%1%in file: " ANSI_BLUE "%2% %3%" ANSI_NORMAL,
-                prefix,
-                einfo.nixCode->errPos.file,
-                showErrPos(einfo.nixCode->errPos));
-        } else {
-            out << std::endl << fmt("%1%from command line argument", prefix);
-        }
+    if (einfo.errPos.has_value() && (*einfo.errPos)) {
+        out << prefix << std::endl;
+        printAtPos(prefix, *einfo.errPos, out);
         nl = true;
     }
 
@@ -200,12 +306,16 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
         nl = true;
     }
 
-    // lines of code.
-    if (einfo.nixCode.has_value() && einfo.nixCode->errLineOfCode.has_value()) {
-        if (nl)
-            out << std::endl << prefix;
-        printCodeLines(out, prefix, *einfo.nixCode);
-        nl = true;
+    if (einfo.errPos.has_value() && (*einfo.errPos)) {
+        auto loc = getCodeLines(*einfo.errPos);
+
+        // lines of code.
+        if (loc.has_value()) {
+            if (nl)
+                out << std::endl << prefix;
+            printCodeLines(out, prefix, *einfo.errPos, *loc);
+            nl = true;
+        }
     }
 
     // hint
@@ -216,6 +326,59 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
         nl = true;
     }
 
+    // traces
+    if (showTrace && !einfo.traces.empty())
+    {
+        const string tracetitle(" show-trace ");
+
+        int fill = errwidth - tracetitle.length();
+        int lw = 0;
+        int rw = 0;
+        const int min_dashes = 3;
+        if (fill > min_dashes * 2) {
+            if (fill % 2 != 0) {
+                lw = fill / 2;
+                rw = lw + 1;
+            }
+            else
+            {
+                lw = rw = fill / 2;
+            }
+        }
+        else
+            lw = rw = min_dashes;
+
+        if (nl)
+            out << std::endl << prefix;
+
+        out << ANSI_BLUE << std::string(lw, '-') << tracetitle << std::string(rw, '-') << ANSI_NORMAL;
+
+        for (auto iter = einfo.traces.rbegin(); iter != einfo.traces.rend(); ++iter)
+        {
+            try {
+                out << std::endl << prefix;
+                out << ANSI_BLUE << "trace: " << ANSI_NORMAL << iter->hint.str();
+
+                nl = true;
+                if (*iter->pos) {
+                    auto pos = iter->pos.value();
+                    out << std::endl << prefix;
+
+                    printAtPos(prefix, pos, out);
+                    auto loc = getCodeLines(pos);
+                    if (loc.has_value())
+                    {
+                        out << std::endl << prefix;
+                        printCodeLines(out, prefix, pos, *loc);
+                        out << std::endl << prefix;
+                    }
+                }
+            } catch(const std::bad_optional_access& e) {
+                out << iter->hint.str() << std::endl;
+            }
+        }
+    }
+
     return out;
 }
 }
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 1e6102ce1..1b0fb43b8 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -25,20 +25,20 @@ namespace nix {
 
 /*
 
-This file defines two main structs/classes used in nix error handling.
+   This file defines two main structs/classes used in nix error handling.
 
-ErrorInfo provides a standard payload of error information, with conversion to string
-happening in the logger rather than at the call site.
+   ErrorInfo provides a standard payload of error information, with conversion to string
+   happening in the logger rather than at the call site.
 
-BaseError is the ancestor of nix specific exceptions (and Interrupted), and contains
-an ErrorInfo.
+   BaseError is the ancestor of nix specific exceptions (and Interrupted), and contains
+   an ErrorInfo.
 
-ErrorInfo structs are sent to the logger as part of an exception, or directly with the
-logError or logWarning macros.
+   ErrorInfo structs are sent to the logger as part of an exception, or directly with the
+   logError or logWarning macros.
 
-See the error-demo.cc program for usage examples.
+   See the error-demo.cc program for usage examples.
 
-*/
+ */
 
 typedef enum {
     lvlError = 0,
@@ -50,11 +50,25 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
+typedef enum {
+    foFile,
+    foStdin,
+    foString
+} FileOrigin;
+
+// the lines of code surrounding an error.
+struct LinesOfCode {
+    std::optional<string> prevLineOfCode;
+    std::optional<string> errLineOfCode;
+    std::optional<string> nextLineOfCode;
+};
+
 // ErrPos indicates the location of an error in a nix file.
 struct ErrPos {
     int line = 0;
     int column = 0;
     string file;
+    FileOrigin origin;
 
     operator bool() const
     {
@@ -65,9 +79,14 @@ struct ErrPos {
     template <class P>
     ErrPos& operator=(const P &pos)
     {
+        origin = pos.origin;
         line = pos.line;
         column = pos.column;
-        file = pos.file;
+        // is file symbol null?
+        if (pos.file.set())
+            file = pos.file;
+        else
+            file = "";
         return *this;
     }
 
@@ -78,11 +97,9 @@ struct ErrPos {
     }
 };
 
-struct NixCode {
-    ErrPos errPos;
-    std::optional<string> prevLineOfCode;
-    std::optional<string> errLineOfCode;
-    std::optional<string> nextLineOfCode;
+struct Trace {
+    std::optional<ErrPos> pos;
+    hintformat hint;
 };
 
 struct ErrorInfo {
@@ -90,19 +107,19 @@ struct ErrorInfo {
     string name;
     string description;
     std::optional<hintformat> hint;
-    std::optional<NixCode> nixCode;
+    std::optional<ErrPos> errPos;
+    std::list<Trace> traces;
 
     static std::optional<string> programName;
 };
 
-std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo);
+std::ostream& showErrorInfo(std::ostream &out, const ErrorInfo &einfo, bool showTrace);
 
 /* BaseError should generally not be caught, as it has Interrupted as
    a subclass. Catch Error instead. */
 class BaseError : public std::exception
 {
 protected:
-    string prefix_; // used for location traces etc.
     mutable ErrorInfo err;
 
     mutable std::optional<string> what_;
@@ -113,23 +130,23 @@ public:
 
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
-        : err { .level = lvlError,
-                .hint = hintfmt(args...)
-              }
+        : err {.level = lvlError,
+            .hint = hintfmt(args...)
+            }
         , status(status)
     { }
 
     template<typename... Args>
     BaseError(const std::string & fs, const Args & ... args)
-        : err { .level = lvlError,
-                .hint = hintfmt(fs, args...)
-              }
+        : err {.level = lvlError,
+            .hint = hintfmt(fs, args...)
+            }
     { }
 
     BaseError(hintformat hint)
-        : err { .level = lvlError,
-                .hint = hint
-              }
+        : err {.level = lvlError,
+            .hint = hint
+            }
     { }
 
     BaseError(ErrorInfo && e)
@@ -150,10 +167,17 @@ public:
 #endif
 
     const string & msg() const { return calcWhat(); }
-    const string & prefix() const { return prefix_; }
-    BaseError & addPrefix(const FormatOrString & fs);
-
     const ErrorInfo & info() { calcWhat(); return err; }
+
+    template<typename... Args>
+    BaseError & addTrace(std::optional<ErrPos> e, const string &fs, const Args & ... args)
+    {
+        return addTrace(e, hintfmt(fs, args...));
+    }
+
+    BaseError & addTrace(std::optional<ErrPos> e, hintformat hint);
+
+    bool hasTrace() const { return !err.traces.empty(); }
 };
 
 #define MakeError(newClass, superClass) \
@@ -173,7 +197,7 @@ public:
 
     template<typename... Args>
     SysError(const Args & ... args)
-      :Error("")
+        : Error("")
     {
         errNo = errno;
         auto hf = hintfmt(args...);
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index 12ab9c407..a39de041f 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <boost/format.hpp>
+#include <boost/algorithm/string/replace.hpp>
 #include <string>
 #include "ansicolor.hh"
 
@@ -103,7 +104,9 @@ class hintformat
 public:
     hintformat(const string &format) :fmt(format)
     {
-        fmt.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
+        fmt.exceptions(boost::io::all_error_bits ^ 
+                       boost::io::too_many_args_bit ^
+                       boost::io::too_few_args_bit);
     }
 
     hintformat(const hintformat &hf)
@@ -117,6 +120,13 @@ public:
         return *this;
     }
 
+    template<class T>
+    hintformat& operator%(const normaltxt<T> &value)
+    {
+        fmt % value.value;
+        return *this;
+    }
+
     std::string str() const
     {
         return fmt.str();
@@ -136,4 +146,9 @@ inline hintformat hintfmt(const std::string & fs, const Args & ... args)
     return f;
 }
 
+inline hintformat hintfmt(std::string plain_string)
+{
+    // we won't be receiving any args in this case, so just print the original string
+    return hintfmt("%s", normaltxt(plain_string));
+}
 }
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index e49eb4569..1a3e7c5d8 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -19,7 +19,7 @@ namespace nix {
 
 void Hash::init()
 {
-    if (!type) abort();
+    assert(type);
     switch (*type) {
     case htMD5: hashSize = md5HashSize; break;
     case htSHA1: hashSize = sha1HashSize; break;
@@ -101,15 +101,15 @@ static string printHash32(const Hash & hash)
 
 string printHash16or32(const Hash & hash)
 {
+    assert(hash.type);
     return hash.to_string(hash.type == htMD5 ? Base16 : Base32, false);
 }
 
 
-HashType assertInitHashType(const Hash & h) {
-    if (h.type)
-        return *h.type;
-    else
-        abort();
+HashType assertInitHashType(const Hash & h)
+{
+    assert(h.type);
+    return *h.type;
 }
 
 std::string Hash::to_string(Base base, bool includeType) const
@@ -223,7 +223,7 @@ Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht)
         if (!ht)
             throw BadHash("empty hash requires explicit hash type");
         Hash h(*ht);
-        warn("found empty hash, assuming '%s'", h.to_string(Base::SRI, true));
+        warn("found empty hash, assuming '%s'", h.to_string(SRI, true));
         return h;
     } else
         return Hash(hashStr, ht);
@@ -363,14 +363,15 @@ HashType parseHashType(const string & s)
 string printHashType(HashType ht)
 {
     switch (ht) {
-    case htMD5: return "md5"; break;
-    case htSHA1: return "sha1"; break;
-    case htSHA256: return "sha256"; break;
-    case htSHA512: return "sha512"; break;
+    case htMD5: return "md5";
+    case htSHA1: return "sha1";
+    case htSHA256: return "sha256";
+    case htSHA512: return "sha512";
+    default:
+        // illegal hash type enum value internally, as opposed to external input
+        // which should be validated with nice error message.
+        assert(false);
     }
-    // illegal hash type enum value internally, as opposed to external input
-    // which should be validated with nice error message.
-    abort();
 }
 
 }
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 0d9916508..23259dced 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -10,7 +10,7 @@ namespace nix {
 MakeError(BadHash, Error);
 
 
-enum HashType : char { htMD5, htSHA1, htSHA256, htSHA512 };
+enum HashType : char { htMD5 = 42, htSHA1, htSHA256, htSHA512 };
 
 
 const int md5HashSize = 16;
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 105fadb15..90c6afe81 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -1,5 +1,6 @@
 #include "logging.hh"
 #include "util.hh"
+#include "config.hh"
 
 #include <atomic>
 #include <nlohmann/json.hpp>
@@ -7,6 +8,10 @@
 
 namespace nix {
 
+LoggerSettings loggerSettings;
+
+static GlobalConfig::Register r1(&loggerSettings);
+
 static thread_local ActivityId curActivity = 0;
 
 ActivityId getCurActivity()
@@ -72,10 +77,11 @@ public:
     void logEI(const ErrorInfo & ei) override
     {
         std::stringstream oss;
-        oss << ei;
+        showErrorInfo(oss, ei, loggerSettings.showTrace.get());
 
         log(ei.level, oss.str());
     }
+    
 
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
         const std::string & s, const Fields & fields, ActivityId parent)
@@ -173,7 +179,7 @@ struct JSONLogger : Logger {
     void logEI(const ErrorInfo & ei) override
     {
         std::ostringstream oss;
-        oss << ei;
+        showErrorInfo(oss, ei, loggerSettings.showTrace.get());
 
         nlohmann::json json;
         json["action"] = "msg";
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index b1583eced..09619aac6 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -2,6 +2,7 @@
 
 #include "types.hh"
 #include "error.hh"
+#include "config.hh"
 
 namespace nix {
 
@@ -34,6 +35,16 @@ typedef enum {
 
 typedef uint64_t ActivityId;
 
+struct LoggerSettings : Config
+{
+    Setting<bool> showTrace{this,
+        false,
+        "show-trace",
+        "Whether to show a stack trace on evaluation errors."};
+};
+
+extern LoggerSettings loggerSettings;
+
 class Logger
 {
     friend struct Activity;
diff --git a/src/libutil/tests/logging.cc b/src/libutil/tests/logging.cc
index 4cb54995b..ef22e9966 100644
--- a/src/libutil/tests/logging.cc
+++ b/src/libutil/tests/logging.cc
@@ -1,6 +1,7 @@
 #include "logging.hh"
 #include "nixexpr.hh"
 #include "util.hh"
+#include <fstream>
 
 #include <gtest/gtest.h>
 
@@ -10,6 +11,13 @@ namespace nix {
      * logEI
      * --------------------------------------------------------------------------*/
 
+    const char *test_file =
+        "previous line of code\n"
+        "this is the problem line of code\n"
+        "next line of code\n";
+    const char *one_liner =
+        "this is the other problem line of code";
+
     TEST(logEI, catpuresBasicProperties) {
 
         MakeError(TestError, Error);
@@ -42,7 +50,7 @@ namespace nix {
             logger->logEI(ei);
             auto str = testing::internal::GetCapturedStderr();
 
-            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- TestError --- error-unit-test\x1B[0m\n\x1B[33;1m\x1B[0minitial error\x1B[0m; subsequent error message.\n");
+            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- TestError --- error-unit-test\x1B[0m\ninitial error; subsequent error message.\n");
         }
 
     }
@@ -60,8 +68,7 @@ namespace nix {
             logError(e.info());
             auto str = testing::internal::GetCapturedStderr();
 
-            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SysError --- error-unit-test\x1B[0m\n\x1B[33;1m\x1B[0mstatting file\x1B[0m: \x1B[33;1mBad file descriptor\x1B[0m\n");
-
+            ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SysError --- error-unit-test\x1B[0m\nstatting file: \x1B[33;1mBad file descriptor\x1B[0m\n");
         }
     }
 
@@ -69,9 +76,9 @@ namespace nix {
         testing::internal::CaptureStderr();
 
         logger->logEI({ .level = lvlInfo,
-                .name = "Info name",
-                .description = "Info description",
-                });
+                        .name = "Info name",
+                        .description = "Info description",
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[32;1minfo:\x1B[0m\x1B[34;1m --- Info name --- error-unit-test\x1B[0m\nInfo description\n");
@@ -85,7 +92,7 @@ namespace nix {
         logger->logEI({ .level = lvlTalkative,
                         .name = "Talkative name",
                         .description = "Talkative description",
-                        });
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[32;1mtalk:\x1B[0m\x1B[34;1m --- Talkative name --- error-unit-test\x1B[0m\nTalkative description\n");
@@ -99,7 +106,7 @@ namespace nix {
         logger->logEI({ .level = lvlChatty,
                         .name = "Chatty name",
                         .description = "Talkative description",
-                        });
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[32;1mchat:\x1B[0m\x1B[34;1m --- Chatty name --- error-unit-test\x1B[0m\nTalkative description\n");
@@ -113,7 +120,7 @@ namespace nix {
         logger->logEI({ .level = lvlDebug,
                         .name = "Debug name",
                         .description = "Debug description",
-                        });
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[33;1mdebug:\x1B[0m\x1B[34;1m --- Debug name --- error-unit-test\x1B[0m\nDebug description\n");
@@ -127,7 +134,7 @@ namespace nix {
         logger->logEI({ .level = lvlVomit,
                         .name = "Vomit name",
                         .description = "Vomit description",
-                        });
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[32;1mvomit:\x1B[0m\x1B[34;1m --- Vomit name --- error-unit-test\x1B[0m\nVomit description\n");
@@ -137,14 +144,13 @@ namespace nix {
      * logError
      * --------------------------------------------------------------------------*/
 
-
     TEST(logError, logErrorWithoutHintOrCode) {
         testing::internal::CaptureStderr();
 
         logError({
                 .name = "name",
                 .description = "error description",
-                });
+            });
 
         auto str = testing::internal::GetCapturedStderr();
         ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- name --- error-unit-test\x1B[0m\nerror description\n");
@@ -152,7 +158,7 @@ namespace nix {
 
     TEST(logError, logErrorWithPreviousAndNextLinesOfCode) {
         SymbolTable testTable;
-        auto problem_file = testTable.create("myfile.nix");
+        auto problem_file = testTable.create(test_file);
 
         testing::internal::CaptureStderr();
 
@@ -160,53 +166,43 @@ namespace nix {
                 .name = "error name",
                 .description = "error with code lines",
                 .hint = hintfmt("this hint has %1% templated %2%!!",
-                        "yellow",
-                        "values"),
-                .nixCode = NixCode {
-                .errPos = Pos(problem_file, 40, 13),
-                .prevLineOfCode = "previous line of code",
-                .errLineOfCode = "this is the problem line of code",
-                .nextLineOfCode = "next line of code",
-                }});
-
+                    "yellow",
+                    "values"),
+                .errPos = Pos(foString, problem_file, 02, 13),
+            });
 
         auto str = testing::internal::GetCapturedStderr();
-        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nerror with code lines\n\n    39| previous line of code\n    40| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n    41| next line of code\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(2:13)\x1B[34;1m from string\x1B[0m\n\nerror with code lines\n\n     1| previous line of code\n     2| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n     3| next line of code\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
     }
 
-    TEST(logError, logErrorWithoutLinesOfCode) {
+    TEST(logError, logErrorWithInvalidFile) {
         SymbolTable testTable;
-        auto problem_file = testTable.create("myfile.nix");
+        auto problem_file = testTable.create("invalid filename");
         testing::internal::CaptureStderr();
 
         logError({
                 .name = "error name",
                 .description = "error without any code lines.",
                 .hint = hintfmt("this hint has %1% templated %2%!!",
-                        "yellow",
-                        "values"),
-                .nixCode = NixCode {
-                .errPos = Pos(problem_file, 40, 13)
-                }});
+                    "yellow",
+                    "values"),
+                .errPos = Pos(foFile, problem_file, 02, 13)
+            });
 
         auto str = testing::internal::GetCapturedStderr();
-        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nerror without any code lines.\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- SysError --- error-unit-test\x1B[0m\nopening file '\x1B[33;1minvalid filename\x1B[0m': \x1B[33;1mNo such file or directory\x1B[0m\n\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(2:13)\x1B[34;1m in file: \x1B[0minvalid filename\n\nerror without any code lines.\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
     }
 
     TEST(logError, logErrorWithOnlyHintAndName) {
-        SymbolTable testTable;
-        auto problem_file = testTable.create("myfile.nix");
         testing::internal::CaptureStderr();
 
         logError({
                 .name = "error name",
                 .hint = hintfmt("hint %1%", "only"),
-                .nixCode = NixCode {
-                .errPos = Pos(problem_file, 40, 13)
-                }});
+            });
 
         auto str = testing::internal::GetCapturedStderr();
-        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nhint \x1B[33;1monly\x1B[0m\n");
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- error name --- error-unit-test\x1B[0m\nhint \x1B[33;1monly\x1B[0m\n");
 
     }
 
@@ -218,19 +214,19 @@ namespace nix {
         testing::internal::CaptureStderr();
 
         logWarning({
-            .name = "name",
-            .description = "error description",
-            .hint = hintfmt("there was a %1%", "warning"),
-        });
+                .name = "name",
+                .description = "warning description",
+                .hint = hintfmt("there was a %1%", "warning"),
+            });
 
         auto str = testing::internal::GetCapturedStderr();
-        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- name --- error-unit-test\x1B[0m\nerror description\n\nthere was a \x1B[33;1mwarning\x1B[0m\n");
+        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- name --- error-unit-test\x1B[0m\nwarning description\n\nthere was a \x1B[33;1mwarning\x1B[0m\n");
     }
 
     TEST(logWarning, logWarningWithFileLineNumAndCode) {
 
         SymbolTable testTable;
-        auto problem_file = testTable.create("myfile.nix");
+        auto problem_file = testTable.create(test_file);
 
         testing::internal::CaptureStderr();
 
@@ -238,18 +234,125 @@ namespace nix {
                 .name = "warning name",
                 .description = "warning description",
                 .hint = hintfmt("this hint has %1% templated %2%!!",
-                        "yellow",
-                        "values"),
-                .nixCode = NixCode {
-                .errPos = Pos(problem_file, 40, 13),
-                .prevLineOfCode = std::nullopt,
-                .errLineOfCode = "this is the problem line of code",
-                .nextLineOfCode = std::nullopt
-                }});
+                    "yellow",
+                    "values"),
+                .errPos = Pos(foStdin, problem_file, 2, 13),
+            });
+
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- warning name --- error-unit-test\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(2:13)\x1B[34;1m from stdin\x1B[0m\n\nwarning description\n\n     1| previous line of code\n     2| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n     3| next line of code\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+    }
+
+    /* ----------------------------------------------------------------------------
+     * traces
+     * --------------------------------------------------------------------------*/
+
+    TEST(addTrace, showTracesWithShowTrace) {
+        SymbolTable testTable;
+        auto problem_file = testTable.create(test_file);
+
+        auto oneliner_file = testTable.create(one_liner);
+
+        auto e = AssertionError(ErrorInfo {
+                .name = "wat",
+                .description = "a well-known problem occurred",
+                .hint = hintfmt("it has been %1% days since our last error", "zero"),
+                .errPos = Pos(foString, problem_file, 2, 13),
+            });
+
+        e.addTrace(Pos(foStdin, oneliner_file, 1, 19), "while trying to compute %1%", 42);
+        e.addTrace(std::nullopt, "while doing something without a %1%", "pos");
+
+        testing::internal::CaptureStderr();
+
+        loggerSettings.showTrace.assign(true);
+
+        logError(e.info());
+
+        auto str = testing::internal::GetCapturedStderr();
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- AssertionError --- error-unit-test\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(2:13)\x1B[34;1m from string\x1B[0m\n\na well-known problem occurred\n\n     1| previous line of code\n     2| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n     3| next line of code\n\nit has been \x1B[33;1mzero\x1B[0m days since our last error\n\x1B[34;1m---- show-trace ----\x1B[0m\n\x1B[34;1mtrace: \x1B[0mwhile trying to compute \x1B[33;1m42\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(1:19)\x1B[34;1m from stdin\x1B[0m\n\n     1| this is the other problem line of code\n      |                   \x1B[31;1m^\x1B[0m\n\n\x1B[34;1mtrace: \x1B[0mwhile doing something without a \x1B[33;1mpos\x1B[0m\n");
+    }
+
+    TEST(addTrace, hideTracesWithoutShowTrace) {
+        SymbolTable testTable;
+        auto problem_file = testTable.create(test_file);
+
+        auto oneliner_file = testTable.create(one_liner);
+
+        auto e = AssertionError(ErrorInfo {
+                .name = "wat",
+                .description = "a well-known problem occurred",
+                .hint = hintfmt("it has been %1% days since our last error", "zero"),
+                .errPos = Pos(foString, problem_file, 2, 13),
+            });
+
+        e.addTrace(Pos(foStdin, oneliner_file, 1, 19), "while trying to compute %1%", 42);
+        e.addTrace(std::nullopt, "while doing something without a %1%", "pos");
 
+        testing::internal::CaptureStderr();
+
+        loggerSettings.showTrace.assign(false);
+
+        logError(e.info());
 
         auto str = testing::internal::GetCapturedStderr();
-        ASSERT_STREQ(str.c_str(), "\x1B[33;1mwarning:\x1B[0m\x1B[34;1m --- warning name --- error-unit-test\x1B[0m\nin file: \x1B[34;1mmyfile.nix (40:13)\x1B[0m\n\nwarning description\n\n    40| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n\nthis hint has \x1B[33;1myellow\x1B[0m templated \x1B[33;1mvalues\x1B[0m!!\n");
+        ASSERT_STREQ(str.c_str(), "\x1B[31;1merror:\x1B[0m\x1B[34;1m --- AssertionError --- error-unit-test\x1B[0m\n\x1B[34;1mat: \x1B[33;1m(2:13)\x1B[34;1m from string\x1B[0m\n\na well-known problem occurred\n\n     1| previous line of code\n     2| this is the problem line of code\n      |             \x1B[31;1m^\x1B[0m\n     3| next line of code\n\nit has been \x1B[33;1mzero\x1B[0m days since our last error\n");
+    }
+
+    /* ----------------------------------------------------------------------------
+     * hintfmt
+     * --------------------------------------------------------------------------*/
+
+    TEST(hintfmt, percentStringWithoutArgs) {
+
+        const char *teststr = "this is 100%s correct!";
+
+        ASSERT_STREQ(
+            hintfmt(teststr).str().c_str(),
+            teststr);
+
+    }
+
+    TEST(hintfmt, fmtToHintfmt) {
+
+        ASSERT_STREQ(
+            hintfmt(fmt("the color of this this text is %1%", "not yellow")).str().c_str(),
+            "the color of this this text is not yellow");
+
+    }
+
+    TEST(hintfmt, tooFewArguments) {
+
+        ASSERT_STREQ(
+            hintfmt("only one arg %1% %2%", "fulfilled").str().c_str(),
+            "only one arg " ANSI_YELLOW "fulfilled" ANSI_NORMAL " ");
+
+    }
+
+    TEST(hintfmt, tooManyArguments) {
+
+        ASSERT_STREQ(
+            hintfmt("what about this %1% %2%", "%3%", "one", "two").str().c_str(),
+            "what about this " ANSI_YELLOW "%3%" ANSI_NORMAL " " ANSI_YELLOW "one" ANSI_NORMAL);
+
+    }
+
+    /* ----------------------------------------------------------------------------
+     * ErrPos
+     * --------------------------------------------------------------------------*/
+
+    TEST(errpos, invalidPos) {
+
+      // contains an invalid symbol, which we should not dereference!
+      Pos invalid;
+
+      // constructing without access violation.
+      ErrPos ep(invalid);
+    
+      // assignment without access violation.
+      ep = invalid;
+
     }
 
 }
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 8b0692035..c992b7d74 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -593,7 +593,7 @@ static void upgradeDerivations(Globals & globals,
                 } else newElems.push_back(i);
 
             } catch (Error & e) {
-                e.addPrefix(fmt("while trying to find an upgrade for '%s':\n", i.queryName()));
+                e.addTrace(std::nullopt, "while trying to find an upgrade for '%s'", i.queryName());
                 throw;
             }
         }
@@ -1185,7 +1185,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         } catch (AssertionError & e) {
             printMsg(lvlTalkative, "skipping derivation named '%1%' which gives an assertion failure", i.queryName());
         } catch (Error & e) {
-            e.addPrefix(fmt("while querying the derivation named '%1%':\n", i.queryName()));
+            e.addTrace(std::nullopt, "while querying the derivation named '%1%'", i.queryName());
             throw;
         }
     }
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 4e02aa2bf..7d81bf54f 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -864,7 +864,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                         out << info->narSize // downloadSize
                             << info->narSize;
                         if (GET_PROTOCOL_MINOR(clientVersion) >= 4)
-                            out << (info->narHash ? info->narHash.to_string(Base32, true) : "") << info->ca << info->sigs;
+                            out << (info->narHash ? info->narHash.to_string(Base32, true) : "") << renderContentAddress(info->ca) << info->sigs;
                     } catch (InvalidPath &) {
                     }
                 }
@@ -952,7 +952,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                 info.references = readStorePaths<StorePathSet>(*store, in);
                 in >> info.registrationTime >> info.narSize >> info.ultimate;
                 info.sigs = readStrings<StringSet>(in);
-                in >> info.ca;
+                info.ca = parseContentAddressOpt(readString(in));
 
                 if (info.narSize == 0)
                     throw Error("narInfo is too old and missing the narSize field");
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index f43f774c1..f9d6de16e 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -48,7 +48,10 @@ struct CmdAddToStore : MixDryRun, StoreCommand
         ValidPathInfo info(store->makeFixedOutputPath(FileIngestionMethod::Recursive, narHash, *namePart));
         info.narHash = narHash;
         info.narSize = sink.s->size();
-        info.ca = makeFixedOutputCA(FileIngestionMethod::Recursive, info.narHash);
+        info.ca = std::optional { FixedOutputHash {
+            .method = FileIngestionMethod::Recursive,
+            .hash = info.narHash,
+        } };
 
         if (!dryRun) {
             auto source = StringSource { *sink.s };
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index 8b85caf82..eb93f56fc 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -50,7 +50,7 @@ BuildEnvironment readEnvironment(const Path & path)
         R"re((?:\$?'(?:[^'\\]|\\[abeEfnrtv\\'"?])*'))re";
 
     static std::string indexedArrayRegex =
-        R"re((?:\(( *\[[0-9]+]="(?:[^"\\]|\\.)*")**\)))re";
+        R"re((?:\(( *\[[0-9]+\]="(?:[^"\\]|\\.)*")*\)))re";
 
     static std::regex varRegex(
         "^(" + varNameRegex + ")=(" + simpleStringRegex + "|" + quotedStringRegex + "|" + indexedArrayRegex + ")\n");
@@ -135,13 +135,7 @@ StorePath getDerivationEnvironment(ref<Store> store, const StorePath & drvPath)
     drv.inputSrcs.insert(std::move(getEnvShPath));
     Hash h = hashDerivationModulo(*store, drv, true);
     auto shellOutPath = store->makeOutputPath("out", h, drvName);
-    drv.outputs.insert_or_assign("out", DerivationOutput {
-        .path = shellOutPath,
-        .hash = DerivationOutputHash {
-            .method = FileIngestionMethod::Flat,
-            .hash = Hash { },
-        },
-    });
+    drv.outputs.insert_or_assign("out", DerivationOutput { .path = shellOutPath });
     drv.env["out"] = store->printStorePath(shellOutPath);
     auto shellDrvPath2 = writeDerivation(store, drv, drvName);
 
diff --git a/src/nix/hash.cc b/src/nix/hash.cc
index f435192fc..b97c6d21f 100644
--- a/src/nix/hash.cc
+++ b/src/nix/hash.cc
@@ -1,5 +1,6 @@
 #include "command.hh"
 #include "hash.hh"
+#include "content-address.hh"
 #include "legacy.hh"
 #include "shared.hh"
 #include "references.hh"
diff --git a/src/nix/make-content-addressable.cc b/src/nix/make-content-addressable.cc
index 0ebb8f13b..fb36fc410 100644
--- a/src/nix/make-content-addressable.cc
+++ b/src/nix/make-content-addressable.cc
@@ -82,7 +82,10 @@ struct CmdMakeContentAddressable : StorePathsCommand, MixJSON
             if (hasSelfReference) info.references.insert(info.path);
             info.narHash = narHash;
             info.narSize = sink.s->size();
-            info.ca = makeFixedOutputCA(FileIngestionMethod::Recursive, info.narHash);
+            info.ca = FixedOutputHash {
+                .method = FileIngestionMethod::Recursive,
+                .hash = info.narHash,
+            };
 
             if (!json)
                 printInfo("rewrote '%s' to '%s'", pathS, store->printStorePath(info.path));
diff --git a/src/nix/path-info.cc b/src/nix/path-info.cc
index fb7bacc4c..b89a44f83 100644
--- a/src/nix/path-info.cc
+++ b/src/nix/path-info.cc
@@ -115,7 +115,7 @@ struct CmdPathInfo : StorePathsCommand, MixJSON
                     std::cout << '\t';
                     Strings ss;
                     if (info->ultimate) ss.push_back("ultimate");
-                    if (info->ca != "") ss.push_back("ca:" + info->ca);
+                    if (info->ca) ss.push_back("ca:" + renderContentAddress(*info->ca));
                     for (auto & sig : info->sigs) ss.push_back(sig);
                     std::cout << concatStringsSep(" ", ss);
                 }
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index d5661f7b7..ab8ae41e8 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -211,12 +211,12 @@ void NixRepl::mainLoop(const std::vector<std::string> & files)
                 // input without clearing the input so far.
                 continue;
             } else {
-              printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
+              printMsg(lvlError, e.msg());
             }
         } catch (Error & e) {
-            printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
+          printMsg(lvlError, e.msg());
         } catch (Interrupted & e) {
-            printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
+          printMsg(lvlError, e.msg());
         }
 
         // We handled the current input fully, so we should clear it
diff --git a/src/nix/search.cc b/src/nix/search.cc
index ba72c1e79..93c3f3f83 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -216,7 +216,7 @@ struct CmdSearch : SourceExprCommand, MixJSON
             } catch (AssertionError & e) {
             } catch (Error & e) {
                 if (!toplevel) {
-                    e.addPrefix(fmt("While evaluating the attribute '%s':\n", attrPath));
+                    e.addTrace(std::nullopt, "While evaluating the attribute '%s'", attrPath);
                     throw;
                 }
             }
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
index d1aba08e3..bb5e4529b 100644
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@@ -87,7 +87,7 @@ struct CmdVerify : StorePathsCommand
                 if (!noContents) {
 
                     std::unique_ptr<AbstractHashSink> hashSink;
-                    if (info->ca == "")
+                    if (!info->ca)
                         hashSink = std::make_unique<HashSink>(*info->narHash.type);
                     else
                         hashSink = std::make_unique<HashModuloSink>(*info->narHash.type, std::string(info->path.hashPart()));