Merge branch 'master' into caveman-LOCs

21 files changed, 424 insertions, 597 deletions

diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index b8eef9286..83854df49 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -6,11 +6,11 @@
 namespace nix {
 
 
-static Strings parseAttrPath(const string & s)
+static Strings parseAttrPath(std::string_view s)
 {
     Strings res;
     string cur;
-    string::const_iterator i = s.begin();
+    auto i = s.begin();
     while (i != s.end()) {
         if (*i == '.') {
             res.push_back(cur);
@@ -32,6 +32,15 @@ static Strings parseAttrPath(const string & s)
 }
 
 
+std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s)
+{
+    std::vector<Symbol> res;
+    for (auto & a : parseAttrPath(s))
+        res.push_back(state.symbols.create(a));
+    return res;
+}
+
+
 std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attrPath,
     Bindings & autoArgs, Value & vIn)
 {
@@ -62,7 +71,7 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
                 throw TypeError(
                     "the expression selected by the selection path '%1%' should be a set but is %2%",
                     attrPath,
-                    showType(*v)); 
+                    showType(*v));
             if (attr.empty())
                 throw Error("empty attribute name in selection path '%1%'", attrPath);
 
@@ -79,7 +88,7 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
                 throw TypeError(
                     "the expression selected by the selection path '%1%' should be a list but is %2%",
                     attrPath,
-                    showType(*v)); 
+                    showType(*v));
             if (attrIndex >= v->listSize())
                 throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", attrIndex, attrPath);
 
diff --git a/src/libexpr/attr-path.hh b/src/libexpr/attr-path.hh
index fce160da7..d9d74ab2d 100644
--- a/src/libexpr/attr-path.hh
+++ b/src/libexpr/attr-path.hh
@@ -16,4 +16,6 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 /* Heuristic to find the filename and lineno or a nix value. */
 Pos findDerivationFilename(EvalState & state, Value & v, std::string what);
 
+std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
+
 }
diff --git a/src/libexpr/attr-set.hh b/src/libexpr/attr-set.hh
index f5651891f..c601d09c2 100644
--- a/src/libexpr/attr-set.hh
+++ b/src/libexpr/attr-set.hh
@@ -76,11 +76,10 @@ public:
     {
         auto a = get(name);
         if (!a)
-            throw Error(
-                ErrorInfo { 
-                    .hint = hintfmt("attribute '%s' missing", name),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw Error({
+                .hint = hintfmt("attribute '%s' missing", name),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
         return *a;
     }
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index eee49e02e..3d544c903 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -9,11 +9,10 @@ namespace nix {
 
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s))
 {
-    throw EvalError(
-        ErrorInfo { 
-            .hint = hintfmt(s),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw EvalError({
+        .hint = hintfmt(s),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
@@ -24,11 +23,10 @@ LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
 
 LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const Value & v))
 {
-    throw TypeError(
-        ErrorInfo { 
-            .hint = hintfmt(s, showType(v)),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw TypeError({
+        .hint = hintfmt(s, showType(v)),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 7bf25ea17..b90a64357 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -161,12 +161,12 @@ const Value *getPrimOp(const Value &v) {
 }
 
 
-string showType(const Value & v)
+string showType(ValueType type)
 {
-    switch (v.type) {
+    switch (type) {
         case tInt: return "an integer";
-        case tBool: return "a boolean";
-        case tString: return v.string.context ? "a string with context" : "a string";
+        case tBool: return "a Boolean";
+        case tString: return "a string";
         case tPath: return "a path";
         case tNull: return "null";
         case tAttrs: return "a set";
@@ -175,14 +175,27 @@ string showType(const Value & v)
         case tApp: return "a function application";
         case tLambda: return "a function";
         case tBlackhole: return "a black hole";
+        case tPrimOp: return "a built-in function";
+        case tPrimOpApp: return "a partially applied built-in function";
+        case tExternal: return "an external value";
+        case tFloat: return "a float";
+    }
+    abort();
+}
+
+
+string showType(const Value & v)
+{
+    switch (v.type) {
+        case tString: return v.string.context ? "a string with context" : "a string";
         case tPrimOp:
             return fmt("the built-in function '%s'", string(v.primOp->name));
         case tPrimOpApp:
             return fmt("the partially applied built-in function '%s'", string(getPrimOp(v)->primOp->name));
         case tExternal: return v.external->showType();
-        case tFloat: return "a float";
+    default:
+        return showType(v.type);
     }
-    abort();
 }
 
 
@@ -323,6 +336,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sOutputHash(symbols.create("outputHash"))
     , sOutputHashAlgo(symbols.create("outputHashAlgo"))
     , sOutputHashMode(symbols.create("outputHashMode"))
+    , sRecurseForDerivations(symbols.create("recurseForDerivations"))
     , repair(NoRepair)
     , store(store)
     , baseEnv(allocEnv(128))
@@ -471,14 +485,21 @@ Value * EvalState::addConstant(const string & name, Value & v)
 Value * EvalState::addPrimOp(const string & name,
     size_t arity, PrimOpFun primOp)
 {
+    auto name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
+    Symbol sym = symbols.create(name2);
+
+    /* Hack to make constants lazy: turn them into a application of
+       the primop to a dummy value. */
     if (arity == 0) {
+        auto vPrimOp = allocValue();
+        vPrimOp->type = tPrimOp;
+        vPrimOp->primOp = new PrimOp(primOp, 1, sym);
         Value v;
-        primOp(*this, noPos, nullptr, v);
+        mkApp(v, *vPrimOp, *vPrimOp);
         return addConstant(name, v);
     }
+
     Value * v = allocValue();
-    string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
-    Symbol sym = symbols.create(name2);
     v->type = tPrimOp;
     v->primOp = new PrimOp(primOp, arity, sym);
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
@@ -506,11 +527,10 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
 
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2))
 {
-    throw EvalError(
-        ErrorInfo { 
-            .hint = hintfmt(s, s2),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw EvalError({
+        .hint = hintfmt(s, s2),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3))
@@ -520,30 +540,27 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, con
 
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2, const string & s3))
 {
-    throw EvalError(
-        ErrorInfo { 
-            .hint = hintfmt(s, s2, s3),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw EvalError({
+        .hint = hintfmt(s, s2, s3),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const Pos & p1, const char * s, const Symbol & sym, const Pos & p2))
 {
     // p1 is where the error occurred; p2 is a position mentioned in the message.
-    throw EvalError(
-        ErrorInfo { 
-            .hint = hintfmt(s, sym, p2),
-            .nixCode = NixCode { .errPos = p1 }
-        });
+    throw EvalError({
+        .hint = hintfmt(s, sym, p2),
+        .nixCode = NixCode { .errPos = p1 }
+    });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 {
-    throw TypeError(
-        ErrorInfo { 
-            .hint = hintfmt(s),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw TypeError({
+        .hint = hintfmt(s),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
@@ -553,29 +570,26 @@ LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
 
 LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const ExprLambda & fun, const Symbol & s2))
 {
-    throw TypeError(
-        ErrorInfo { 
-            .hint = hintfmt(s, fun.showNamePos(), s2),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw TypeError({
+        .hint = hintfmt(s, fun.showNamePos(), s2),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s, const string & s1))
 {
-    throw AssertionError(
-        ErrorInfo { 
-            .hint = hintfmt(s, s1),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw AssertionError({
+        .hint = hintfmt(s, s1),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInlineNoReturn(void throwUndefinedVarError(const Pos & pos, const char * s, const string & s1))
 {
-    throw UndefinedVarError(
-        ErrorInfo { 
-            .hint = hintfmt(s, s1),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw UndefinedVarError({
+        .hint = hintfmt(s, s1),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2))
@@ -1594,7 +1608,7 @@ string EvalState::forceStringNoCtx(Value & v, const Pos & pos)
     string s = forceString(v, pos);
     if (v.string.context) {
         if (pos)
-            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')", 
+            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')",
                 v.string.s, v.string.context[0]);
         else
             throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
@@ -1920,11 +1934,10 @@ void EvalState::printStats()
 
 string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    throw TypeError(
-        ErrorInfo { 
-            .hint = hintfmt("cannot coerce %1% to a string", showType()),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw TypeError({
+        .hint = hintfmt("cannot coerce %1% to a string", showType()),
+        .nixCode = NixCode { .errPos = pos }
+    });
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 8471c2f8b..0d52a7f63 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -18,7 +18,7 @@ namespace nix {
 
 class Store;
 class EvalState;
-struct StorePath;
+class StorePath;
 enum RepairFlag : bool;
 
 
@@ -74,7 +74,8 @@ public:
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
-        sOutputHash, sOutputHashAlgo, sOutputHashMode;
+        sOutputHash, sOutputHashAlgo, sOutputHashMode,
+        sRecurseForDerivations;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
@@ -324,6 +325,7 @@ private:
 
 
 /* Return a string representing the type of the value `v'. */
+string showType(ValueType type);
 string showType(const Value & v);
 
 /* Decode a context string ‘!<name>!<path>’ into a pair <path,
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index ca9c547fa..a4937e722 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -348,7 +348,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    should we recurse into it?  => Only if it has a
                    `recurseForDerivations = true' attribute. */
                 if (i->value->type == tAttrs) {
-                    Bindings::iterator j = i->value->attrs->find(state.symbols.create("recurseForDerivations"));
+                    Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
                     if (j != i->value->attrs->end() && state.forceBool(*j->value, *j->pos))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 }
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 85376a08f..f6e83926b 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -219,4 +219,3 @@ or          { return OR_KW; }
             }
 
 %%
-
diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index 917e8a1c7..9ed39e745 100644
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@@ -8,7 +8,7 @@ libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexe
 
 libexpr_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libmain -I src/libexpr
 
-libexpr_LIBS = libutil libstore libfetchers libnixrust
+libexpr_LIBS = libutil libstore libfetchers
 
 libexpr_LDFLAGS =
 ifneq ($(OS), FreeBSD)
diff --git a/src/libexpr/names.cc b/src/libexpr/names.cc
deleted file mode 100644
index d1c8a6101..000000000
--- a/src/libexpr/names.cc
+++ /dev/null
@@ -1,107 +0,0 @@
-#include "names.hh"
-#include "util.hh"
-
-
-namespace nix {
-
-
-DrvName::DrvName()
-{
-    name = "";
-}
-
-
-/* Parse a derivation name.  The `name' part of a derivation name is
-   everything up to but not including the first dash *not* followed by
-   a letter.  The `version' part is the rest (excluding the separating
-   dash).  E.g., `apache-httpd-2.0.48' is parsed to (`apache-httpd',
-   '2.0.48'). */
-DrvName::DrvName(std::string_view s) : hits(0)
-{
-    name = fullName = std::string(s);
-    for (unsigned int i = 0; i < s.size(); ++i) {
-        /* !!! isalpha/isdigit are affected by the locale. */
-        if (s[i] == '-' && i + 1 < s.size() && !isalpha(s[i + 1])) {
-            name = s.substr(0, i);
-            version = s.substr(i + 1);
-            break;
-        }
-    }
-}
-
-
-bool DrvName::matches(DrvName & n)
-{
-    if (name != "*") {
-        if (!regex) regex = std::unique_ptr<std::regex>(new std::regex(name, std::regex::extended));
-        if (!std::regex_match(n.name, *regex)) return false;
-    }
-    if (version != "" && version != n.version) return false;
-    return true;
-}
-
-
-string nextComponent(string::const_iterator & p,
-    const string::const_iterator end)
-{
-    /* Skip any dots and dashes (component separators). */
-    while (p != end && (*p == '.' || *p == '-')) ++p;
-
-    if (p == end) return "";
-
-    /* If the first character is a digit, consume the longest sequence
-       of digits.  Otherwise, consume the longest sequence of
-       non-digit, non-separator characters. */
-    string s;
-    if (isdigit(*p))
-        while (p != end && isdigit(*p)) s += *p++;
-    else
-        while (p != end && (!isdigit(*p) && *p != '.' && *p != '-'))
-            s += *p++;
-
-    return s;
-}
-
-
-static bool componentsLT(const string & c1, const string & c2)
-{
-    int n1, n2;
-    bool c1Num = string2Int(c1, n1), c2Num = string2Int(c2, n2);
-
-    if (c1Num && c2Num) return n1 < n2;
-    else if (c1 == "" && c2Num) return true;
-    else if (c1 == "pre" && c2 != "pre") return true;
-    else if (c2 == "pre") return false;
-    /* Assume that `2.3a' < `2.3.1'. */
-    else if (c2Num) return true;
-    else if (c1Num) return false;
-    else return c1 < c2;
-}
-
-
-int compareVersions(const string & v1, const string & v2)
-{
-    string::const_iterator p1 = v1.begin();
-    string::const_iterator p2 = v2.begin();
-
-    while (p1 != v1.end() || p2 != v2.end()) {
-        string c1 = nextComponent(p1, v1.end());
-        string c2 = nextComponent(p2, v2.end());
-        if (componentsLT(c1, c2)) return -1;
-        else if (componentsLT(c2, c1)) return 1;
-    }
-
-    return 0;
-}
-
-
-DrvNames drvNamesFromArgs(const Strings & opArgs)
-{
-    DrvNames result;
-    for (auto & i : opArgs)
-        result.push_back(DrvName(i));
-    return result;
-}
-
-
-}
diff --git a/src/libexpr/names.hh b/src/libexpr/names.hh
deleted file mode 100644
index 00e14b8c7..000000000
--- a/src/libexpr/names.hh
+++ /dev/null
@@ -1,32 +0,0 @@
-#pragma once
-
-#include <memory>
-
-#include "types.hh"
-#include <regex>
-
-namespace nix {
-
-struct DrvName
-{
-    string fullName;
-    string name;
-    string version;
-    unsigned int hits;
-
-    DrvName();
-    DrvName(std::string_view s);
-    bool matches(DrvName & n);
-
-private:
-    std::unique_ptr<std::regex> regex;
-};
-
-typedef list<DrvName> DrvNames;
-
-string nextComponent(string::const_iterator & p,
-    const string::const_iterator end);
-int compareVersions(const string & v1, const string & v2);
-DrvNames drvNamesFromArgs(const Strings & opArgs);
-
-}
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 6ab36dd35..d4e179008 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -282,12 +282,11 @@ void ExprVar::bindVars(const StaticEnv & env)
     /* Otherwise, the variable must be obtained from the nearest
        enclosing `with'.  If there is no `with', then we can issue an
        "undefined variable" error now. */
-    if (withLevel == -1) 
-        throw UndefinedVarError(
-            ErrorInfo {
-                .hint = hintfmt("undefined variable '%1%'", name),
-                .nixCode = NixCode { .errPos = pos }
-            });
+    if (withLevel == -1)
+        throw UndefinedVarError({
+            .hint = hintfmt("undefined variable '%1%'", name),
+            .nixCode = NixCode { .errPos = pos }
+        });
     fromWith = true;
     this->level = withLevel;
 }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 5a98b9149..b1bf9f67b 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -237,11 +237,10 @@ struct ExprLambda : Expr
         : pos(pos), arg(arg), matchAttrs(matchAttrs), formals(formals), body(body)
     {
         if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
-            throw ParseError(
-                ErrorInfo {
-                    .hint = hintfmt("duplicate formal function argument '%1%'", arg),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw ParseError({
+                .hint = hintfmt("duplicate formal function argument '%1%'", arg),
+                .nixCode = NixCode { .errPos = pos }
+            });
     };
     void setName(Symbol & name);
     string showNamePos() const;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index f8b5fb0d5..b746e70af 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -65,23 +65,20 @@ namespace nix {
 
 static void dupAttr(const AttrPath & attrPath, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(
-        ErrorInfo {
-            .hint = hintfmt("attribute '%1%' already defined at %2%",
-                showAttrPath(attrPath), prevPos),
-            .nixCode = NixCode { .errPos = pos },
-        });
+    throw ParseError({
+        .hint = hintfmt("attribute '%1%' already defined at %2%",
+            showAttrPath(attrPath), prevPos),
+        .nixCode = NixCode { .errPos = pos },
+    });
 }
 
 
 static void dupAttr(Symbol attr, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(
-        ErrorInfo {
-            .hint = hintfmt("attribute '%1%' already defined at %2%",
-                attr, prevPos),
-            .nixCode = NixCode { .errPos = pos },
-        });
+    throw ParseError({
+        .hint = hintfmt("attribute '%1%' already defined at %2%", attr, prevPos),
+        .nixCode = NixCode { .errPos = pos },
+    });
 }
 
 
@@ -149,12 +146,11 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
 static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
 {
     if (!formals->argNames.insert(formal.name).second)
-        throw ParseError(
-            ErrorInfo {
-                .hint = hintfmt("duplicate formal function argument '%1%'",
-                    formal.name),
-                .nixCode = NixCode { .errPos = pos },
-            });
+        throw ParseError({
+            .hint = hintfmt("duplicate formal function argument '%1%'",
+                formal.name),
+            .nixCode = NixCode { .errPos = pos },
+        });
     formals->formals.push_front(formal);
 }
 
@@ -262,10 +258,10 @@ static inline Pos makeCurPos(const YYLTYPE & loc, ParseData * data)
 
 void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * error)
 {
-    data->error = ErrorInfo { 
+    data->error = {
         .hint = hintfmt(error),
         .nixCode = NixCode { .errPos = makeCurPos(*loc, data) }
-        };
+    };
 }
 
 
@@ -342,11 +338,10 @@ expr_function
     { $$ = new ExprWith(CUR_POS, $2, $4); }
   | LET binds IN expr_function
     { if (!$2->dynamicAttrs.empty())
-        throw ParseError(
-            ErrorInfo {
-                .hint = hintfmt("dynamic attributes not allowed in let"),
-                .nixCode = NixCode { .errPos = CUR_POS },
-            });
+        throw ParseError({
+            .hint = hintfmt("dynamic attributes not allowed in let"),
+            .nixCode = NixCode { .errPos = CUR_POS },
+        });
       $$ = new ExprLet($2, $4);
     }
   | expr_if
@@ -423,11 +418,10 @@ expr_simple
   | URI {
       static bool noURLLiterals = settings.isExperimentalFeatureEnabled("no-url-literals");
       if (noURLLiterals)
-          throw ParseError(
-              ErrorInfo { 
-                  .hint = hintfmt("URL literals are disabled"),
-                  .nixCode = NixCode { .errPos = CUR_POS }
-              });
+          throw ParseError({
+              .hint = hintfmt("URL literals are disabled"),
+              .nixCode = NixCode { .errPos = CUR_POS }
+          });
       $$ = new ExprString(data->symbols.create($1));
   }
   | '(' expr ')' { $$ = $2; }
@@ -497,11 +491,10 @@ attrs
           $$->push_back(AttrName(str->s));
           delete str;
       } else
-          throw ParseError(
-              ErrorInfo {
-                  .hint = hintfmt("dynamic attributes not allowed in inherit"),
-                  .nixCode = NixCode { .errPos = makeCurPos(@2, data) },
-              });
+          throw ParseError({
+              .hint = hintfmt("dynamic attributes not allowed in inherit"),
+              .nixCode = NixCode { .errPos = makeCurPos(@2, data) },
+          });
     }
   | { $$ = new AttrPath; }
   ;
@@ -707,11 +700,13 @@ Path EvalState::findFile(SearchPath & searchPath, const string & path, const Pos
         Path res = r.second + suffix;
         if (pathExists(res)) return canonPath(res);
     }
-    throw ThrownError(
-        ErrorInfo { 
-            .hint = hintfmt("file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)", path),
-            .nixCode = NixCode { .errPos = pos }
-        });
+    throw ThrownError({
+        .hint = hintfmt(evalSettings.pureEval
+            ? "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 }
+    });
 }
 
 
@@ -727,10 +722,9 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
             res = { true, store->toRealPath(fetchers::downloadTarball(
                         store, resolveUri(elem.second), "source", false).storePath) };
         } catch (FileTransferError & e) {
-            logWarning(
-                ErrorInfo { 
-                    .name = "Entry download",
-                    .hint = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
+            logWarning({
+                .name = "Entry download",
+                .hint = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
             });
             res = { false, "" };
         }
@@ -739,10 +733,9 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
         if (pathExists(path))
             res = { true, path };
         else {
-            logWarning(
-                ErrorInfo { 
-                    .name = "Entry not found",
-                    .hint = hintfmt("warning: Nix search path entry '%1%' does not exist, ignoring", elem.second)
+            logWarning({
+                .name = "Entry not found",
+                .hint = hintfmt("warning: Nix search path entry '%1%' does not exist, ignoring", elem.second)
             });
             res = { false, "" };
         }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 7886bf80b..2f1a41a64 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -50,20 +50,20 @@ void EvalState::realiseContext(const PathSet & context)
     std::vector<StorePathWithOutputs> drvs;
 
     for (auto & i : context) {
-        std::pair<string, string> decoded = decodeContext(i);
-        auto ctx = store->parseStorePath(decoded.first);
+        auto [ctxS, outputName] = decodeContext(i);
+        auto ctx = store->parseStorePath(ctxS);
         if (!store->isValidPath(ctx))
             throw InvalidPathError(store->printStorePath(ctx));
-        if (!decoded.second.empty() && ctx.isDerivation()) {
-            drvs.push_back(StorePathWithOutputs{ctx.clone(), {decoded.second}});
+        if (!outputName.empty() && ctx.isDerivation()) {
+            drvs.push_back(StorePathWithOutputs{ctx, {outputName}});
 
             /* Add the output of this derivation to the allowed
                paths. */
             if (allowedPaths) {
-                auto drv = store->derivationFromPath(store->parseStorePath(decoded.first));
-                DerivationOutputs::iterator i = drv.outputs.find(decoded.second);
+                auto drv = store->derivationFromPath(ctx);
+                DerivationOutputs::iterator i = drv.outputs.find(outputName);
                 if (i == drv.outputs.end())
-                    throw Error("derivation '%s' does not have an output named '%s'", decoded.first, decoded.second);
+                    throw Error("derivation '%s' does not have an output named '%s'", ctxS, outputName);
                 allowedPaths->insert(store->printStorePath(i->second.path));
             }
         }
@@ -79,6 +79,7 @@ void EvalState::realiseContext(const PathSet & context)
     StorePathSet willBuild, willSubstitute, unknown;
     unsigned long long downloadSize, narSize;
     store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
+
     store->buildPaths(drvs);
 }
 
@@ -93,12 +94,10 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("cannot import '%1%', since path '%2%' is not valid",
-                        path, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("cannot import '%1%', since path '%2%' is not valid", path, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
 
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));
@@ -174,13 +173,12 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt(
-                    "cannot import '%1%', since path '%2%' is not valid",
-                    path, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt(
+                "cannot import '%1%', since path '%2%' is not valid",
+                path, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
 
     path = state.checkSourcePath(path);
@@ -215,11 +213,10 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("at least one argument to 'exec' required"),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("at least one argument to 'exec' required"),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
     PathSet context;
     auto program = state.coerceToString(pos, *elems[0], context, false, false);
@@ -230,12 +227,12 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
-                    program, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });}
+        throw EvalError({
+            .hint = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
+                program, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
+    }
 
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
@@ -386,11 +383,10 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator startSet =
         args[0]->attrs->find(state.symbols.create("startSet"));
     if (startSet == args[0]->attrs->end())
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("attribute 'startSet' required"),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("attribute 'startSet' required"),
+            .nixCode = NixCode { .errPos = pos }
+        });
     state.forceList(*startSet->value, pos);
 
     ValueList workSet;
@@ -401,11 +397,10 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator op =
         args[0]->attrs->find(state.symbols.create("operator"));
     if (op == args[0]->attrs->end())
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("attribute 'operator' required"),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("attribute 'operator' required"),
+            .nixCode = NixCode { .errPos = pos }
+        });
     state.forceValue(*op->value, pos);
 
     /* Construct the closure by applying the operator to element of
@@ -424,11 +419,10 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         Bindings::iterator key =
             e->attrs->find(state.symbols.create("key"));
         if (key == e->attrs->end())
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("attribute 'key' required"),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("attribute 'key' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
         state.forceValue(*key->value, pos);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -560,11 +554,10 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     /* Figure out the name first (for stack backtraces). */
     Bindings::iterator attr = args[0]->attrs->find(state.sName);
     if (attr == args[0]->attrs->end())
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("required attribute 'name' missing"),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("required attribute 'name' missing"),
+            .nixCode = NixCode { .errPos = pos }
+        });
     string drvName;
     Pos & posDrvName(*attr->pos);
     try {
@@ -607,42 +600,38 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         auto handleHashMode = [&](const std::string & s) {
             if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
             else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
-            else 
-                throw EvalError(
-                    ErrorInfo { 
-                        .hint = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
-                        .nixCode = NixCode { .errPos = posDrvName }
-                    });
+            else
+                throw EvalError({
+                    .hint = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
+                    .nixCode = NixCode { .errPos = posDrvName }
+                });
         };
 
         auto handleOutputs = [&](const Strings & ss) {
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    throw EvalError(
-                        ErrorInfo { 
-                            .hint = hintfmt("duplicate derivation output '%1%'", j),
-                            .nixCode = NixCode { .errPos = posDrvName }
-                        });
+                    throw EvalError({
+                        .hint = hintfmt("duplicate derivation output '%1%'", j),
+                        .nixCode = NixCode { .errPos = posDrvName }
+                    });
                 /* !!! Check whether j is a valid attribute
                    name. */
                 /* Derivations cannot be named ‘drv’, because
                    then we'd have an attribute ‘drvPath’ in
                    the resulting set. */
                 if (j == "drv")
-                    throw EvalError(
-                        ErrorInfo { 
-                            .hint = hintfmt("invalid derivation output name 'drv'" ),
-                            .nixCode = NixCode { .errPos = posDrvName }
-                        });
+                    throw EvalError({
+                        .hint = hintfmt("invalid derivation output name 'drv'" ),
+                        .nixCode = NixCode { .errPos = posDrvName }
+                    });
                 outputs.insert(j);
             }
             if (outputs.empty())
-                throw EvalError(
-                    ErrorInfo { 
-                        .hint = hintfmt("derivation cannot have an empty set of outputs"),
-                        .nixCode = NixCode { .errPos = posDrvName }
-                    });
+                throw EvalError({
+                    .hint = hintfmt("derivation cannot have an empty set of outputs"),
+                    .nixCode = NixCode { .errPos = posDrvName }
+                });
         };
 
         try {
@@ -735,9 +724,9 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             StorePathSet refs;
             state.store->computeFSClosure(state.store->parseStorePath(std::string_view(path).substr(1)), refs);
             for (auto & j : refs) {
-                drv.inputSrcs.insert(j.clone());
+                drv.inputSrcs.insert(j);
                 if (j.isDerivation())
-                    drv.inputDrvs[j.clone()] = state.store->queryDerivationOutputNames(j);
+                    drv.inputDrvs[j] = state.store->readDerivation(j).outputNames();
             }
         }
 
@@ -754,38 +743,35 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("required attribute 'builder' missing"),
-                .nixCode = NixCode { .errPos = posDrvName }
-            });
+        throw EvalError({
+            .hint = hintfmt("required attribute 'builder' missing"),
+            .nixCode = NixCode { .errPos = posDrvName }
+        });
 
     if (drv.platform == "")
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("required attribute 'system' missing"),
-                .nixCode = NixCode { .errPos = posDrvName }
-            });
+        throw EvalError({
+            .hint = hintfmt("required attribute 'system' missing"),
+            .nixCode = NixCode { .errPos = posDrvName }
+        });
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
-                .nixCode = NixCode { .errPos = posDrvName }
-            });
+        throw EvalError({
+            .hint = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
+            .nixCode = NixCode { .errPos = posDrvName }
+        });
 
     if (outputHash) {
         /* Handle fixed-output derivations. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            throw Error(
-                ErrorInfo { 
-                    .hint = hintfmt("multiple outputs are not supported in fixed-output derivations"),
-                    .nixCode = NixCode { .errPos = posDrvName }
-                });
+            throw Error({
+                .hint = hintfmt("multiple outputs are not supported in fixed-output derivations"),
+                .nixCode = NixCode { .errPos = posDrvName }
+            });
 
         HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
-        Hash h(*outputHash, ht);
+
+        Hash h = newHashAllowEmpty(*outputHash, ht);
 
         auto outPath = state.store->makeFixedOutputPath(ingestionMethod, h, drvName);
         if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
@@ -807,7 +793,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         for (auto & i : outputs) {
             if (!jsonObject) drv.env[i] = "";
             drv.outputs.insert_or_assign(i,
-                DerivationOutput(StorePath::dummy.clone(), "", ""));
+                DerivationOutput { StorePath::dummy, "", "" });
         }
 
         Hash h = hashDerivationModulo(*state.store, Derivation(drv), true);
@@ -816,7 +802,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             auto outPath = state.store->makeOutputPath(i, h, drvName);
             if (!jsonObject) drv.env[i] = state.store->printStorePath(outPath);
             drv.outputs.insert_or_assign(i,
-                DerivationOutput(std::move(outPath), "", ""));
+                DerivationOutput { std::move(outPath), "", "" });
         }
     }
 
@@ -829,7 +815,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     /* Optimisation, but required in read-only mode! because in that
        case we don't actually write store derivations, so we can't
        read them later. */
-    drvHashes.insert_or_assign(drvPath.clone(),
+    drvHashes.insert_or_assign(drvPath,
         hashDerivationModulo(*state.store, Derivation(drv), false));
 
     state.mkAttrs(v, 1 + drv.outputs.size());
@@ -886,11 +872,10 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
        e.g. nix-push does the right thing. */
     if (!state.store->isStorePath(path)) path = canonPath(path, true);
     if (!state.store->isInStore(path))
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("path '%1%' is not in the Nix store", path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("path '%1%' is not in the Nix store", path),
+            .nixCode = NixCode { .errPos = pos }
+        });
     Path path2 = state.store->toStorePath(path);
     if (!settings.readOnlyMode)
         state.store->ensurePath(state.store->parseStorePath(path2));
@@ -906,13 +891,12 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt(
-                    "cannot check the existence of '%1%', since path '%2%' is not valid", 
-                    path, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt(
+                "cannot check the existence of '%1%', since path '%2%' is not valid",
+                path, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
 
     try {
@@ -955,13 +939,11 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid"
-                     , path, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });
- }
+        throw EvalError({
+            .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid", path, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
+    }
     string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
     if (s.find((char) 0) != string::npos)
         throw Error("the contents of the file '%1%' cannot be represented as a Nix string", path);
@@ -988,11 +970,10 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
 
         i = v2.attrs->find(state.symbols.create("path"));
         if (i == v2.attrs->end())
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("attribute 'path' missing"),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("attribute 'path' missing"),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
         PathSet context;
         string path = state.coerceToString(pos, *i->value, context, false, false);
@@ -1000,12 +981,10 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         try {
             state.realiseContext(context);
         } catch (InvalidPathError & e) {
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("cannot find '%1%', since path '%2%' is not valid",
-                        path, e.path),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
         }
 
         searchPath.emplace_back(prefix, path);
@@ -1022,11 +1001,10 @@ static void prim_hashFile(EvalState & state, const Pos & pos, Value * * args, Va
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-      throw Error(
-          ErrorInfo {
-              .hint = hintfmt("unknown hash type '%1%'", type),
-              .nixCode = NixCode { .errPos = pos }
-          });
+      throw Error({
+          .hint = hintfmt("unknown hash type '%1%'", type),
+          .nixCode = NixCode { .errPos = pos }
+      });
 
     PathSet context; // discarded
     Path p = state.coerceToPath(pos, *args[1], context);
@@ -1042,12 +1020,10 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
     try {
         state.realiseContext(ctx);
     } catch (InvalidPathError & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid",
-                        path, e.path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid", path, e.path),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
 
     DirEntries entries = readDirectory(state.checkSourcePath(path));
@@ -1117,15 +1093,13 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt(
-                        "in 'toFile': the file named '%1%' must not contain a reference "
-                        "to a derivation but contains (%2%)",
-                        name,
-                        path),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError( {
+                .hint = hintfmt(
+                    "in 'toFile': the file named '%1%' must not contain a reference "
+                    "to a derivation but contains (%2%)",
+                    name, path),
+                .nixCode = NixCode { .errPos = pos }
+            });
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1193,21 +1167,19 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
     PathSet context;
     Path path = state.coerceToPath(pos, *args[1], context);
     if (!context.empty())
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("string '%1%' cannot refer to other paths", path),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("string '%1%' cannot refer to other paths", path),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     state.forceValue(*args[0], pos);
     if (args[0]->type != tLambda)
-        throw TypeError(
-            ErrorInfo { 
-                .hint = hintfmt(
-                    "first argument in call to 'filterSource' is not a function but %1%", 
-                    showType(*args[0])),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw TypeError({
+            .hint = hintfmt(
+                "first argument in call to 'filterSource' is not a function but %1%",
+                showType(*args[0])),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, Hash(), v);
 }
@@ -1227,12 +1199,10 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
             PathSet context;
             path = state.coerceToPath(*attr.pos, *attr.value, context);
             if (!context.empty())
-                throw EvalError(
-                    ErrorInfo { 
-                        .hint = hintfmt("string '%1%' cannot refer to other paths", 
-                            path),
-                        .nixCode = NixCode { .errPos = *attr.pos }
-                    });
+                throw EvalError({
+                    .hint = hintfmt("string '%1%' cannot refer to other paths", path),
+                    .nixCode = NixCode { .errPos = *attr.pos }
+                });
         } else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, *attr.pos);
         else if (n == "filter") {
@@ -1241,21 +1211,18 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         } else if (n == "recursive")
             method = FileIngestionMethod { state.forceBool(*attr.value, *attr.pos) };
         else if (n == "sha256")
-            expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+            expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
         else
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt("unsupported argument '%1%' to 'addPath'",
-                        attr.name),
-                    .nixCode = NixCode { .errPos = *attr.pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("unsupported argument '%1%' to 'addPath'", attr.name),
+                .nixCode = NixCode { .errPos = *attr.pos }
+            });
     }
     if (path.empty())
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("'path' required"),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("'path' required"),
+            .nixCode = NixCode { .errPos = pos }
+        });
     if (name.empty())
         name = baseNameOf(path);
 
@@ -1313,11 +1280,10 @@ void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
     // !!! Should we create a symbol here or just do a lookup?
     Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
     if (i == args[1]->attrs->end())
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("attribute '%1%' missing", attr),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("attribute '%1%' missing", attr),
+            .nixCode = NixCode { .errPos = pos }
+        });
     // !!! add to stack trace?
     if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
     state.forceValue(*i->value, pos);
@@ -1397,22 +1363,20 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
 
         Bindings::iterator j = v2.attrs->find(state.sName);
         if (j == v2.attrs->end())
-            throw TypeError(
-                ErrorInfo { 
-                    .hint = hintfmt("'name' attribute missing in a call to 'listToAttrs'"), 
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw TypeError({
+                .hint = hintfmt("'name' attribute missing in a call to 'listToAttrs'"),
+                .nixCode = NixCode { .errPos = pos }
+            });
         string name = state.forceStringNoCtx(*j->value, pos);
 
         Symbol sym = state.symbols.create(name);
         if (seen.insert(sym).second) {
             Bindings::iterator j2 = v2.attrs->find(state.symbols.create(state.sValue));
             if (j2 == v2.attrs->end())
-                throw TypeError(
-                    ErrorInfo { 
-                        .hint = hintfmt("'value' attribute missing in a call to 'listToAttrs'"), 
-                        .nixCode = NixCode { .errPos = pos }
-                    });
+                throw TypeError({
+                    .hint = hintfmt("'value' attribute missing in a call to 'listToAttrs'"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
             v.attrs->push_back(Attr(sym, j2->value, j2->pos));
         }
     }
@@ -1485,11 +1449,10 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
 {
     state.forceValue(*args[0], pos);
     if (args[0]->type != tLambda)
-        throw TypeError(
-            ErrorInfo { 
-                .hint = hintfmt("'functionArgs' requires a function"), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw TypeError({
+            .hint = hintfmt("'functionArgs' requires a function"),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     if (!args[0]->lambda.fun->matchAttrs) {
         state.mkAttrs(v, 0);
@@ -1542,11 +1505,10 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
 {
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
-        throw Error(
-            ErrorInfo { 
-                .hint = hintfmt("list index %1% is out of bounds", n), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw Error({
+            .hint = hintfmt("list index %1% is out of bounds", n),
+            .nixCode = NixCode { .errPos = pos }
+        });
     state.forceValue(*list.listElems()[n], pos);
     v = *list.listElems()[n];
 }
@@ -1573,11 +1535,10 @@ static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error(
-            ErrorInfo { 
-                .hint = hintfmt("'tail' called on an empty list"), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw Error({
+            .hint = hintfmt("'tail' called on an empty list"),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
@@ -1719,12 +1680,10 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("cannot create list of size %1%", len), 
-                .nixCode = NixCode { .errPos = pos }
-            });
-
+        throw EvalError({
+            .hint = hintfmt("cannot create list of size %1%", len),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     state.mkList(v, len);
 
@@ -1882,12 +1841,11 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[1], pos);
 
     NixFloat f2 = state.forceFloat(*args[1], pos);
-    if (f2 == 0) 
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("division by zero"), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+    if (f2 == 0)
+        throw EvalError({
+            .hint = hintfmt("division by zero"),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     if (args[0]->type == tFloat || args[1]->type == tFloat) {
         mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
@@ -1896,11 +1854,10 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
         NixInt i2 = state.forceInt(*args[1], pos);
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt("overflow in integer division"), 
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("overflow in integer division"),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
         mkInt(v, i1 / i2);
     }
@@ -1957,12 +1914,11 @@ static void prim_substring(EvalState & state, const Pos & pos, Value * * args, V
     PathSet context;
     string s = state.coerceToString(pos, *args[2], context);
 
-    if (start < 0) 
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("negative start position in 'substring'"), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+    if (start < 0)
+        throw EvalError({
+            .hint = hintfmt("negative start position in 'substring'"),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     mkString(v, (unsigned int) start >= s.size() ? "" : string(s, start, len), context);
 }
@@ -1982,11 +1938,10 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-        throw Error(
-            ErrorInfo { 
-                .hint = hintfmt("unknown hash type '%1%'", type), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw Error({
+            .hint = hintfmt("unknown hash type '%1%'", type),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     PathSet context; // discarded
     string s = state.forceString(*args[1], context, pos);
@@ -2029,17 +1984,15 @@ void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
     } catch (std::regex_error &e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
+                .nixCode = NixCode { .errPos = pos }
+            });
         } else {
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("invalid regular expression '%s'", re),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("invalid regular expression '%s'", re),
+                .nixCode = NixCode { .errPos = pos }
+            });
         }
     }
 }
@@ -2104,17 +2057,15 @@ static void prim_split(EvalState & state, const Pos & pos, Value * * args, Value
     } catch (std::regex_error &e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("memory limit exceeded by regular expression '%s'", re),
+                .nixCode = NixCode { .errPos = pos }
+            });
         } else {
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("invalid regular expression '%s'", re),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("invalid regular expression '%s'", re),
+                .nixCode = NixCode { .errPos = pos }
+            });
         }
     }
 }
@@ -2145,11 +2096,10 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     state.forceList(*args[0], pos);
     state.forceList(*args[1], pos);
     if (args[0]->listSize() != args[1]->listSize())
-        throw EvalError(
-            ErrorInfo { 
-                .hint = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"), 
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
+            .nixCode = NixCode { .errPos = pos }
+        });
 
     vector<string> from;
     from.reserve(args[0]->listSize());
@@ -2252,10 +2202,11 @@ static void prim_splitVersion(EvalState & state, const Pos & pos, Value * * args
 RegisterPrimOp::PrimOps * RegisterPrimOp::primOps;
 
 
-RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
+RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun,
+    std::optional<std::string> requiredFeature)
 {
     if (!primOps) primOps = new PrimOps;
-    primOps->emplace_back(name, arity, fun);
+    primOps->push_back({name, arity, fun, requiredFeature});
 }
 
 
@@ -2447,7 +2398,8 @@ void EvalState::createBaseEnv()
 
     if (RegisterPrimOp::primOps)
         for (auto & primOp : *RegisterPrimOp::primOps)
-            addPrimOp(std::get<0>(primOp), std::get<1>(primOp), std::get<2>(primOp));
+            if (!primOp.requiredFeature || settings.isExperimentalFeatureEnabled(*primOp.requiredFeature))
+                addPrimOp(primOp.name, primOp.arity, primOp.primOp);
 
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */
diff --git a/src/libexpr/primops.hh b/src/libexpr/primops.hh
index 05d0792ef..75c460ecf 100644
--- a/src/libexpr/primops.hh
+++ b/src/libexpr/primops.hh
@@ -7,12 +7,25 @@ namespace nix {
 
 struct RegisterPrimOp
 {
-    typedef std::vector<std::tuple<std::string, size_t, PrimOpFun>> PrimOps;
+    struct Info
+    {
+        std::string name;
+        size_t arity;
+        PrimOpFun primOp;
+        std::optional<std::string> requiredFeature;
+    };
+
+    typedef std::vector<Info> PrimOps;
     static PrimOps * primOps;
+
     /* You can register a constant by passing an arity of 0. fun
        will get called during EvalState initialization, so there
        may be primops not yet added and builtins is not yet sorted. */
-    RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun);
+    RegisterPrimOp(
+        std::string name,
+        size_t arity,
+        PrimOpFun fun,
+        std::optional<std::string> requiredFeature = {});
 };
 
 /* These primops are disabled without enableNativeCode, but plugins
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 7f895fc01..efa2e9576 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -146,11 +146,10 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
     auto sAllOutputs = state.symbols.create("allOutputs");
     for (auto & i : *args[1]->attrs) {
         if (!state.store->isStorePath(i.name))
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt("Context key '%s' is not a store path", i.name),
-                    .nixCode = NixCode { .errPos = *i.pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("Context key '%s' is not a store path", i.name),
+                .nixCode = NixCode { .errPos = *i.pos }
+            });
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(i.name));
         state.forceAttrs(*i.value, *i.pos);
@@ -164,11 +163,10 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (iter != i.value->attrs->end()) {
             if (state.forceBool(*iter->value, *iter->pos)) {
                 if (!isDerivation(i.name)) {
-                    throw EvalError(
-                        ErrorInfo { 
-                            .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 }
-                        });
+                    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 }
+                    });
                 }
                 context.insert("=" + string(i.name));
             }
@@ -178,11 +176,10 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (iter != i.value->attrs->end()) {
             state.forceList(*iter->value, *iter->pos);
             if (iter->value->listSize() && !isDerivation(i.name)) {
-                throw EvalError(
-                    ErrorInfo { 
-                        .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 }
-                    });
+                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 }
+                });
             }
             for (unsigned int n = 0; n < iter->value->listSize(); ++n) {
                 auto name = state.forceStringNoCtx(*iter->value->listElems()[n], *iter->pos);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 52826b56c..dd7229a3d 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -35,19 +35,17 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
             else if (n == "submodules")
                 fetchSubmodules = state.forceBool(*attr.value, *attr.pos);
             else
-                throw EvalError(
-                    ErrorInfo { 
-                        .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
-                        .nixCode = NixCode { .errPos = *attr.pos }
-                    });
+                throw EvalError({
+                    .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
+                    .nixCode = NixCode { .errPos = *attr.pos }
+                });
         }
 
         if (url.empty())
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt("'url' argument required"),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("'url' argument required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index bb008ba6b..9bace8f89 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -38,19 +38,17 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError(
-                    ErrorInfo { 
-                        .hint = hintfmt("unsupported argument '%s' to 'fetchMercurial'", attr.name),
-                        .nixCode = NixCode { .errPos = *attr.pos }
-                    });
+                throw EvalError({
+                    .hint = hintfmt("unsupported argument '%s' to 'fetchMercurial'", attr.name),
+                    .nixCode = NixCode { .errPos = *attr.pos }
+                });
         }
 
         if (url.empty())
-            throw EvalError(
-                ErrorInfo { 
-                    .hint = hintfmt("'url' argument required"),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("'url' argument required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index af3b61d6a..9be93710a 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -23,7 +23,7 @@ void emitTreeAttrs(
 
     assert(tree.info.narHash);
     mkString(*state.allocAttr(v, state.symbols.create("narHash")),
-        tree.info.narHash.to_string(SRI));
+        tree.info.narHash.to_string(SRI, true));
 
     if (input->getRev()) {
         mkString(*state.allocAttr(v, state.symbols.create("rev")), input->getRev()->gitRev());
@@ -66,11 +66,10 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
         }
 
         if (!attrs.count("type"))
-            throw Error(
-                ErrorInfo {
-                    .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
-                    .nixCode = NixCode { .errPos = pos }
-                    });
+            throw Error({
+                .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
         input = fetchers::inputFromAttrs(attrs);
     } else
@@ -107,24 +106,21 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             if (n == "url")
                 url = state.forceStringNoCtx(*attr.value, *attr.pos);
             else if (n == "sha256")
-                expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError(
-                    ErrorInfo {
-                        .hint = hintfmt("unsupported argument '%s' to '%s'",
-                            attr.name, who),
-                        .nixCode = NixCode { .errPos = *attr.pos }
-                    });
+                throw EvalError({
+                    .hint = hintfmt("unsupported argument '%s' to '%s'", attr.name, who),
+                    .nixCode = NixCode { .errPos = *attr.pos }
+                });
             }
 
         if (!url)
-            throw EvalError(
-                ErrorInfo {
-                    .hint = hintfmt("'url' argument required"),
-                    .nixCode = NixCode { .errPos = pos }
-                });
+            throw EvalError({
+                .hint = hintfmt("'url' argument required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
@@ -151,7 +147,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             : hashFile(htSHA256, path);
         if (hash != *expectedHash)
             throw Error((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
-                *url, expectedHash->to_string(), hash.to_string());
+                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true));
     }
 
     if (state.allowedPaths)
diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index 948069401..7615d1379 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -81,11 +81,10 @@ static void prim_fromTOML(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         visit(v, parser(tomlStream).parse());
     } catch (std::runtime_error & e) {
-        throw EvalError(
-            ErrorInfo {
-                .hint = hintfmt("while parsing a TOML string: %s", e.what()),
-                .nixCode = NixCode { .errPos = pos }
-            });
+        throw EvalError({
+            .hint = hintfmt("while parsing a TOML string: %s", e.what()),
+            .nixCode = NixCode { .errPos = pos }
+        });
     }
 }