Merge remote-tracking branch 'origin/master' into flakes

57 files changed, 746 insertions, 356 deletions

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 000000000..5ace4600a
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 447a6d43b..e16e6c62d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -12,6 +12,6 @@ jobs:
     - uses: actions/checkout@v2
       with:
         fetch-depth: 0
-    - uses: cachix/install-nix-action@v8
+    - uses: cachix/install-nix-action@v10
     #- run: nix flake check
     - run: nix-build -A checks.$(if [[ `uname` = Linux ]]; then echo x86_64-linux; else echo x86_64-darwin; fi)
diff --git a/mk/run_test.sh b/mk/run_test.sh
new file mode 100755
index 000000000..6af5b070a
--- /dev/null
+++ b/mk/run_test.sh
@@ -0,0 +1,28 @@
+#!/bin/sh
+
+set -u
+
+red=""
+green=""
+yellow=""
+normal=""
+
+post_run_msg="ran test $1..."
+if [ -t 1 ]; then
+    red=""
+    green=""
+    yellow=""
+    normal=""
+fi
+(cd $(dirname $1) && env ${TESTS_ENVIRONMENT} init.sh 2>/dev/null > /dev/null)
+log="$(cd $(dirname $1) && env ${TESTS_ENVIRONMENT} $(basename $1) 2>&1)"
+status=$?
+if [ $status -eq 0 ]; then
+  echo "$post_run_msg [${green}PASS$normal]"
+elif [ $status -eq 99 ]; then
+  echo "$post_run_msg [${yellow}SKIP$normal]"
+else
+  echo "$post_run_msg [${red}FAIL$normal]"
+  echo "$log" | sed 's/^/    /'
+  exit "$status"
+fi
diff --git a/mk/tests.mk b/mk/tests.mk
index 70c30661b..2e39bb694 100644
--- a/mk/tests.mk
+++ b/mk/tests.mk
@@ -1,45 +1,15 @@
 # Run program $1 as part of ‘make installcheck’.
+
+test-deps =
+
 define run-install-test
 
-  installcheck: $1
+  installcheck: $1.test
 
-  _installcheck-list += $1
+  .PHONY: $1.test
+  $1.test: $1 $(test-deps)
+	@env TEST_NAME=$(notdir $(basename $1)) TESTS_ENVIRONMENT="$(tests-environment)" mk/run_test.sh $1
 
 endef
 
-# Color code from https://unix.stackexchange.com/a/10065
-installcheck:
-	@total=0; failed=0; \
-	red=""; \
-	green=""; \
-	yellow=""; \
-	normal=""; \
-	if [ -t 1 ]; then \
-		red=""; \
-		green=""; \
-		yellow=""; \
-		normal=""; \
-	fi; \
-	for i in $(_installcheck-list); do \
-	  total=$$((total + 1)); \
-	  printf "running test $$i..."; \
-	  log="$$(cd $$(dirname $$i) && $(tests-environment) $$(basename $$i) 2>&1)"; \
-	  status=$$?; \
-	  if [ $$status -eq 0 ]; then \
-	    echo " [$${green}PASS$$normal]"; \
-	  elif [ $$status -eq 99 ]; then \
-	    echo " [$${yellow}SKIP$$normal]"; \
-	  else \
-	    echo " [$${red}FAIL$$normal]"; \
-	    echo "$$log" | sed 's/^/    /'; \
-	    failed=$$((failed + 1)); \
-	  fi; \
-	done; \
-	if [ "$$failed" != 0 ]; then \
-	  echo "$${red}$$failed out of $$total tests failed $$normal"; \
-	  exit 1; \
-	else \
-		echo "$${green}All tests succeeded$$normal"; \
-	fi
-
 .PHONY: check installcheck
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 ba600ad0c..5510c64e9 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -544,7 +544,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const
 {
     throw EvalError({
         .hint = hintfmt(s, s2),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -557,7 +557,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const
 {
     throw EvalError({
         .hint = hintfmt(s, s2, s3),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -566,7 +566,7 @@ 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
     });
 }
 
@@ -574,7 +574,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 {
     throw TypeError({
         .hint = hintfmt(s),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -587,7 +587,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 {
     throw TypeError({
         .hint = hintfmt(s, fun.showNamePos(), s2),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -595,7 +595,7 @@ LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s,
 {
     throw AssertionError({
         .hint = hintfmt(s, s1),
-        .nixCode = NixCode { .errPos = pos }
+        .errPos = pos
     });
 }
 
@@ -603,23 +603,18 @@ 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);
 }
 
 
@@ -838,7 +833,7 @@ void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
             throw Error("file '%s' must be an attribute set", path);
         eval(e, v);
     } catch (Error & e) {
-        addErrorPrefix(e, "while evaluating the file '%1%':\n", path2);
+        addErrorTrace(e, "while evaluating the file '%1%':", path2);
         throw;
     }
 
@@ -1088,8 +1083,8 @@ 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;
     }
 
@@ -1257,11 +1252,15 @@ 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
@@ -1536,7 +1535,7 @@ 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;
                 }
         }
@@ -1979,7 +1978,7 @@ 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 87bb42c11..8986952e3 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -253,7 +253,7 @@ 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/flake/flake.cc b/src/libexpr/flake/flake.cc
index 871af07b0..fd168b74f 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -123,7 +123,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
                         attr.name, showType(*attr.value));
             }
         } catch (Error & e) {
-            e.addPrefix(fmt("in flake attribute '%s' at '%s':\n", attr.name, *attr.pos));
+            e.addTrace(*attr.pos, hintfmt("in flake attribute '%s'", attr.name));
             throw;
         }
     }
@@ -132,7 +132,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
         try {
             input.ref = FlakeRef::fromAttrs(attrs);
         } catch (Error & e) {
-            e.addPrefix(fmt("in flake input at '%s':\n", pos));
+            e.addTrace(pos, hintfmt("in flake input"));
             throw;
         }
     else {
@@ -195,7 +195,7 @@ static Flake getFlake(
     Value vInfo;
     state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
 
-    expectType(state, tAttrs, vInfo, Pos(state.symbols.create(flakeFile), 0, 0));
+    expectType(state, tAttrs, vInfo, Pos(foFile, state.symbols.create(flakeFile), 0, 0));
 
     auto sEdition = state.symbols.create("edition"); // FIXME: remove soon
 
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 7b28b8509..24b21f7da 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 96c20b227..539a13f30 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -84,7 +84,7 @@ 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
         });
     }
 
@@ -165,7 +165,7 @@ 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
         });
     }
 
@@ -203,7 +203,7 @@ 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;
@@ -218,7 +218,7 @@ 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
         });
     }
 
@@ -227,13 +227,13 @@ 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;
     }
 }
@@ -373,7 +373,7 @@ 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);
 
@@ -387,7 +387,7 @@ 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);
 
@@ -409,7 +409,7 @@ 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);
 
@@ -459,7 +459,7 @@ 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;
     }
 }
@@ -544,14 +544,14 @@ 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;
     }
 
@@ -591,7 +591,7 @@ 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
                 });
         };
 
@@ -601,7 +601,7 @@ 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. */
@@ -611,14 +611,14 @@ 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
                 });
         };
 
@@ -684,8 +684,9 @@ 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;
         }
     }
@@ -733,20 +734,20 @@ 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) {
@@ -754,7 +755,7 @@ 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);
@@ -868,7 +869,7 @@ 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)
@@ -889,7 +890,7 @@ 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
         });
     }
 
@@ -935,7 +936,7 @@ 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)));
@@ -966,7 +967,7 @@ 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;
@@ -977,7 +978,7 @@ 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
             });
         }
 
@@ -997,7 +998,7 @@ 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
@@ -1016,7 +1017,7 @@ 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
         });
     }
 
@@ -1092,7 +1093,7 @@ 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));
     }
@@ -1163,7 +1164,7 @@ 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);
@@ -1172,7 +1173,7 @@ 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);
@@ -1195,7 +1196,7 @@ 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);
@@ -1209,13 +1210,13 @@ 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);
@@ -1276,7 +1277,7 @@ 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]++;
@@ -1359,7 +1360,7 @@ 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);
 
@@ -1369,7 +1370,7 @@ 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));
         }
@@ -1445,7 +1446,7 @@ 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) {
@@ -1501,7 +1502,7 @@ 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];
@@ -1531,7 +1532,7 @@ 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);
@@ -1676,7 +1677,7 @@ 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);
@@ -1838,7 +1839,7 @@ 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) {
@@ -1850,7 +1851,7 @@ 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);
@@ -1911,7 +1912,7 @@ 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);
@@ -1934,7 +1935,7 @@ 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
@@ -1980,12 +1981,12 @@ 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
             });
         }
     }
@@ -2053,12 +2054,12 @@ 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
             });
         }
     }
@@ -2092,7 +2093,7 @@ 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 a7c4aa1d0..5013e74f0 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 cf3d8ccc9..fc2a6a1c2 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 8c2ab62fe..6a796f3d3 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -78,7 +78,7 @@ 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::Input::fromAttrs(std::move(attrs));
@@ -124,14 +124,14 @@ 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/libmain/loggers.cc b/src/libmain/loggers.cc
index f62e9372c..0a7291780 100644
--- a/src/libmain/loggers.cc
+++ b/src/libmain/loggers.cc
@@ -27,7 +27,7 @@ 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 0c25897f8..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"); }
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/daemon.cc b/src/libstore/daemon.cc
index 842aef20c..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();
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 699abf353..101633869 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/local-store.cc b/src/libstore/local-store.cc
index 5b462c5b3..12a6a03c3 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -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;
     });
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index e17cc45ae..ff36cb00e 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -133,7 +133,7 @@ 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/path.hh b/src/libstore/path.hh
index 4f79843fe..e43a8b50c 100644
--- a/src/libstore/path.hh
+++ b/src/libstore/path.hh
@@ -62,6 +62,7 @@ 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";
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index b7cc7a5fc..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;
@@ -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());
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 e4a4ae11e..c0a8bc9f6 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());
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 25d78c297..b122e05d6 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -418,8 +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. */
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 4f63fa681..44ddac86a 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) \
@@ -174,7 +198,7 @@ public:
 
     template<typename... Args>
     SysError(const Args & ... args)
-      :Error("")
+        : Error("")
     {
         errNo = errno;
         auto hf = hintfmt(args...);
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index baea6412b..f954b2923 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -373,7 +373,7 @@ string printHashType(HashType ht)
     default:
         // illegal hash type enum value internally, as opposed to external input
         // which should be validated with nice error message.
-        abort();
+        assert(false);
     }
 }
 
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 6a6fb4ac3..ef22e9966 100644
--- a/src/libutil/tests/logging.cc
+++ b/src/libutil/tests/logging.cc
@@ -11,6 +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);
@@ -137,7 +144,6 @@ namespace nix {
      * logError
      * --------------------------------------------------------------------------*/
 
-
     TEST(logError, logErrorWithoutHintOrCode) {
         testing::internal::CaptureStderr();
 
@@ -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();
 
@@ -162,21 +168,16 @@ namespace nix {
                 .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",
-                }});
-
+                .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({
@@ -185,28 +186,23 @@ namespace nix {
                 .hint = hintfmt("this hint has %1% templated %2%!!",
                     "yellow",
                     "values"),
-                .nixCode = NixCode {
-                    .errPos = Pos(problem_file, 40, 13)
-                }});
+                .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");
 
     }
 
@@ -219,18 +215,18 @@ namespace nix {
 
         logWarning({
                 .name = "name",
-                .description = "error description",
+                .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();
 
@@ -240,16 +236,68 @@ namespace nix {
                 .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
-                }});
+                .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");
     }
 
     /* ----------------------------------------------------------------------------
@@ -289,4 +337,22 @@ namespace nix {
             "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/develop.cc b/src/nix/develop.cc
index 9a50fd47e..0bd862daa 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 = FixedOutputHash {
-            .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/flake.cc b/src/nix/flake.cc
index 847985ea3..027a9871e 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -234,7 +234,7 @@ struct CmdFlakeCheck : FlakeCommand
                 // FIXME: check meta attributes
                 return store->parseStorePath(drvInfo->queryDrvPath());
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the derivation '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the derivation '%s'", attrPath));
                 throw;
             }
         };
@@ -252,7 +252,7 @@ struct CmdFlakeCheck : FlakeCommand
                 }
                 #endif
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the app definition '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the app definition '%s'", attrPath));
                 throw;
             }
         };
@@ -268,7 +268,7 @@ struct CmdFlakeCheck : FlakeCommand
                 // FIXME: if we have a 'nixpkgs' input, use it to
                 // evaluate the overlay.
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the overlay '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the overlay '%s'", attrPath));
                 throw;
             }
         };
@@ -284,7 +284,7 @@ struct CmdFlakeCheck : FlakeCommand
                         try {
                             state->forceValue(*attr.value, *attr.pos);
                         } catch (Error & e) {
-                            e.addPrefix(fmt("while evaluating the option '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attr.name, *attr.pos));
+                            e.addTrace(*attr.pos, hintfmt("while evaluating the option '%s'", attr.name));
                             throw;
                         }
                 } else
@@ -292,7 +292,7 @@ struct CmdFlakeCheck : FlakeCommand
                 // FIXME: if we have a 'nixpkgs' input, use it to
                 // check the module.
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the NixOS module '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the NixOS module '%s'", attrPath));
                 throw;
             }
         };
@@ -314,7 +314,7 @@ struct CmdFlakeCheck : FlakeCommand
                 }
 
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the Hydra jobset '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the Hydra jobset '%s'", attrPath));
                 throw;
             }
         };
@@ -329,7 +329,7 @@ struct CmdFlakeCheck : FlakeCommand
                 if (!state->isDerivation(*vToplevel))
                     throw Error("attribute 'config.system.build.toplevel' is not a derivation");
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the NixOS configuration '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the NixOS configuration '%s'", attrPath));
                 throw;
             }
         };
@@ -363,7 +363,7 @@ struct CmdFlakeCheck : FlakeCommand
                         throw Error("template '%s' has unsupported attribute '%s'", attrPath, name);
                 }
             } catch (Error & e) {
-                e.addPrefix(fmt("while checking the template '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                e.addTrace(pos, hintfmt("while checking the template '%s'", attrPath));
                 throw;
             }
         };
@@ -494,7 +494,7 @@ struct CmdFlakeCheck : FlakeCommand
                             warn("unknown flake output '%s'", name);
 
                     } catch (Error & e) {
-                        e.addPrefix(fmt("while checking flake output '" ANSI_BOLD "%s" ANSI_NORMAL "':\n", name));
+                        e.addTrace(pos, hintfmt("while checking flake output '%s'", name));
                         throw;
                     }
                 });
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index c30ac2f79..8eb58f62a 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/tests/common.sh.in b/tests/common.sh.in
index 73fe77345..308126094 100644
--- a/tests/common.sh.in
+++ b/tests/common.sh.in
@@ -1,6 +1,6 @@
 set -e
 
-export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)
+export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)/${TEST_NAME:-default}
 export NIX_STORE_DIR
 if ! NIX_STORE_DIR=$(readlink -f $TEST_ROOT/store 2> /dev/null); then
     # Maybe the build directory is symlinked.
@@ -11,6 +11,7 @@ export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
 export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
 export NIX_STATE_DIR=$TEST_ROOT/var/nix
 export NIX_CONF_DIR=$TEST_ROOT/etc
+export NIX_DAEMON_SOCKET_PATH=$TEST_ROOT/daemon-socket
 unset NIX_USER_CONF_FILES
 export _NIX_TEST_SHARED=$TEST_ROOT/shared
 if [[ -n $NIX_STORE ]]; then
@@ -76,7 +77,7 @@ startDaemon() {
     rm -f $NIX_STATE_DIR/daemon-socket/socket
     nix-daemon &
     for ((i = 0; i < 30; i++)); do
-        if [ -e $NIX_STATE_DIR/daemon-socket/socket ]; then break; fi
+        if [ -e $NIX_DAEMON_SOCKET_PATH ]; then break; fi
         sleep 1
     done
     pidDaemon=$!
diff --git a/tests/gc-auto.sh b/tests/gc-auto.sh
index e593697a9..3add896c6 100644
--- a/tests/gc-auto.sh
+++ b/tests/gc-auto.sh
@@ -13,24 +13,32 @@ fake_free=$TEST_ROOT/fake-free
 export _NIX_TEST_FREE_SPACE_FILE=$fake_free
 echo 1100 > $fake_free
 
+fifoLock=$TEST_ROOT/fifoLock
+mkfifo "$fifoLock"
+
 expr=$(cat <<EOF
 with import ./config.nix; mkDerivation {
   name = "gc-A";
   buildCommand = ''
     set -x
     [[ \$(ls \$NIX_STORE/*-garbage? | wc -l) = 3 ]]
+
     mkdir \$out
     echo foo > \$out/bar
-    echo 1...
-    sleep 2
-    echo 200 > ${fake_free}.tmp1
+
+    # Pretend that we run out of space
+    echo 100 > ${fake_free}.tmp1
     mv ${fake_free}.tmp1 $fake_free
-    echo 2...
-    sleep 2
-    echo 3...
-    sleep 2
-    echo 4...
-    [[ \$(ls \$NIX_STORE/*-garbage? | wc -l) = 1 ]]
+
+    # Wait for the GC to run
+    for i in {1..20}; do
+        echo ''\${i}...
+        if [[ \$(ls \$NIX_STORE/*-garbage? | wc -l) = 1 ]]; then
+            exit 0
+        fi
+        sleep 1
+    done
+    exit 1
   '';
 }
 EOF
@@ -43,15 +51,9 @@ with import ./config.nix; mkDerivation {
     set -x
     mkdir \$out
     echo foo > \$out/bar
-    echo 1...
-    sleep 2
-    echo 200 > ${fake_free}.tmp2
-    mv ${fake_free}.tmp2 $fake_free
-    echo 2...
-    sleep 2
-    echo 3...
-    sleep 2
-    echo 4...
+
+    # Wait for the first build to finish
+    cat "$fifoLock"
   '';
 }
 EOF
@@ -59,12 +61,19 @@ EOF
 
 nix build --impure -v -o $TEST_ROOT/result-A -L --expr "$expr" \
     --min-free 1000 --max-free 2000 --min-free-check-interval 1 &
-pid=$!
+pid1=$!
 
 nix build --impure -v -o $TEST_ROOT/result-B -L --expr "$expr2" \
-    --min-free 1000 --max-free 2000 --min-free-check-interval 1
+    --min-free 1000 --max-free 2000 --min-free-check-interval 1 &
+pid2=$!
 
-wait "$pid"
+# Once the first build is done, unblock the second one.
+# If the first build fails, we need to postpone the failure to still allow
+# the second one to finish
+wait "$pid1" || FIRSTBUILDSTATUS=$?
+echo "unlock" > $fifoLock
+( exit ${FIRSTBUILDSTATUS:-0} )
+wait "$pid2"
 
 [[ foo = $(cat $TEST_ROOT/result-A/bar) ]]
 [[ foo = $(cat $TEST_ROOT/result-B/bar) ]]
diff --git a/tests/gc-concurrent.builder.sh b/tests/gc-concurrent.builder.sh
index 0cd67df3a..bb6dcd4cf 100644
--- a/tests/gc-concurrent.builder.sh
+++ b/tests/gc-concurrent.builder.sh
@@ -1,7 +1,10 @@
+echo "Build started" > "$lockFifo"
+
 mkdir $out
 echo $(cat $input1/foo)$(cat $input2/bar) > $out/foobar
 
-sleep 10
+# Wait for someone to write on the fifo
+cat "$lockFifo"
 
 # $out should not have been GC'ed while we were sleeping, but just in
 # case...
diff --git a/tests/gc-concurrent.nix b/tests/gc-concurrent.nix
index 21671ea2c..0aba1f983 100644
--- a/tests/gc-concurrent.nix
+++ b/tests/gc-concurrent.nix
@@ -1,5 +1,7 @@
 with import ./config.nix;
 
+{ lockFifo ? null }:
+
 rec {
 
   input1 = mkDerivation {
@@ -16,6 +18,7 @@ rec {
     name = "gc-concurrent";
     builder = ./gc-concurrent.builder.sh;
     inherit input1 input2;
+    inherit lockFifo;
   };
 
   test2 = mkDerivation {
diff --git a/tests/gc-concurrent.sh b/tests/gc-concurrent.sh
index d395930ca..2c6622c62 100644
--- a/tests/gc-concurrent.sh
+++ b/tests/gc-concurrent.sh
@@ -2,7 +2,10 @@ source common.sh
 
 clearStore
 
-drvPath1=$(nix-instantiate gc-concurrent.nix -A test1)
+lockFifo1=$TEST_ROOT/test1.fifo
+mkfifo "$lockFifo1"
+
+drvPath1=$(nix-instantiate gc-concurrent.nix -A test1 --argstr lockFifo "$lockFifo1")
 outPath1=$(nix-store -q $drvPath1)
 
 drvPath2=$(nix-instantiate gc-concurrent.nix -A test2)
@@ -22,19 +25,16 @@ ln -s $outPath3 "$NIX_STATE_DIR"/gcroots/foo2
 nix-store -rvv "$drvPath1" &
 pid1=$!
 
-# Start build #2 in the background after 10 seconds.
-(sleep 10 && nix-store -rvv "$drvPath2") &
-pid2=$!
+# Wait for the build of $drvPath1 to start
+cat $lockFifo1
 
 # Run the garbage collector while the build is running.
-sleep 6
 nix-collect-garbage
 
-# Wait for build #1/#2 to finish.
+# Unlock the build of $drvPath1
+echo "" > $lockFifo1
 echo waiting for pid $pid1 to finish...
 wait $pid1
-echo waiting for pid $pid2 to finish...
-wait $pid2
 
 # Check that the root of build #1 and its dependencies haven't been
 # deleted.  The should not be deleted by the GC because they were
@@ -42,8 +42,9 @@ wait $pid2
 cat $outPath1/foobar
 cat $outPath1/input-2/bar
 
-# Check that build #2 has succeeded.  It should succeed because the
-# derivation is a GC root.
+# Check that the build build $drvPath2 succeeds.
+# It should succeed because the derivation is a GC root.
+nix-store -rvv "$drvPath2"
 cat $outPath2/foobar
 
 rm -f "$NIX_STATE_DIR"/gcroots/foo*
diff --git a/tests/gc-concurrent2.builder.sh b/tests/gc-concurrent2.builder.sh
index 4bfb33103..4f6c58b96 100644
--- a/tests/gc-concurrent2.builder.sh
+++ b/tests/gc-concurrent2.builder.sh
@@ -3,5 +3,3 @@ echo $(cat $input1/foo)$(cat $input2/bar)xyzzy > $out/foobar
 
 # Check that the GC hasn't deleted the lock on our output.
 test -e "$out.lock"
-
-sleep 6
diff --git a/tests/init.sh b/tests/init.sh
index 90f1bdcb6..f9ced6b0d 100644
--- a/tests/init.sh
+++ b/tests/init.sh
@@ -18,6 +18,7 @@ build-users-group =
 keep-derivations = false
 sandbox = false
 experimental-features = nix-command flakes
+gc-reserved-space = 0
 flake-registry = $TEST_ROOT/registry.json
 include nix.conf.extra
 EOF
diff --git a/tests/local.mk b/tests/local.mk
index 77247f9f6..81366160b 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -41,4 +41,4 @@ tests-environment = NIX_REMOTE= $(bash) -e
 
 clean-files += $(d)/common.sh $(d)/config.nix
 
-installcheck: $(d)/common.sh $(d)/config.nix $(d)/plugins/libplugintest.$(SO_EXT)
+test-deps += tests/common.sh tests/config.nix tests/plugins/libplugintest.$(SO_EXT)
diff --git a/tests/misc.sh b/tests/misc.sh
index fd4908e25..a81c9dbb1 100644
--- a/tests/misc.sh
+++ b/tests/misc.sh
@@ -16,6 +16,11 @@ nix-env --foo 2>&1 | grep "no operation"
 nix-env -q --foo 2>&1 | grep "unknown flag"
 
 # Eval Errors.
-eval_res=$(nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 || true)
-echo $eval_res | grep "(string) (1:15)"
-echo $eval_res | grep "infinite recursion encountered"
+eval_arg_res=$(nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 || true)
+echo $eval_arg_res | grep "at: (1:15) from string"
+echo $eval_arg_res | grep "infinite recursion encountered"
+
+eval_stdin_res=$(echo 'let a = {} // a; in a.foo' | nix-instantiate --eval -E - 2>&1 || true)
+echo $eval_stdin_res | grep "at: (1:15) from stdin"
+echo $eval_stdin_res | grep "infinite recursion encountered"
+
diff --git a/tests/nix-shell.sh b/tests/nix-shell.sh
index 235e2a5ff..650904057 100644
--- a/tests/nix-shell.sh
+++ b/tests/nix-shell.sh
@@ -55,3 +55,10 @@ chmod a+rx $TEST_ROOT/shell.shebang.rb
 
 output=$($TEST_ROOT/shell.shebang.rb abc ruby)
 [ "$output" = '-e load("'"$TEST_ROOT"'/shell.shebang.rb") -- abc ruby' ]
+
+# Test 'nix develop'.
+nix develop -f shell.nix shellDrv -c bash -c '[[ -n $stdenv ]]'
+
+# Test 'nix print-dev-env'.
+source <(nix print-dev-env -f shell.nix shellDrv)
+[[ -n $stdenv ]]