Merge branch 'master' into nix-repl-flakes

14 files changed, 739 insertions, 310 deletions

diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 6a2e775d0..d77b25898 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -576,14 +576,14 @@ std::string AttrCursor::getString()
                 debug("using cached string attribute '%s'", getAttrPathStr());
                 return s->first;
             } else
-                throw TypeError("'%s' is not a string", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 
     return v.type() == nString ? v.string.s : v.path;
 }
@@ -607,7 +607,7 @@ string_t AttrCursor::getStringWithContext()
                     return *s;
                 }
             } else
-                throw TypeError("'%s' is not a string", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
         }
     }
 
@@ -618,7 +618,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path, {}};
     else
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 }
 
 bool AttrCursor::getBool()
@@ -631,14 +631,14 @@ bool AttrCursor::getBool()
                 debug("using cached Boolean attribute '%s'", getAttrPathStr());
                 return *b;
             } else
-                throw TypeError("'%s' is not a Boolean", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nBool)
-        throw TypeError("'%s' is not a Boolean", getAttrPathStr());
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
 
     return v.boolean;
 }
@@ -708,14 +708,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
                 debug("using cached attrset attribute '%s'", getAttrPathStr());
                 return *attrs;
             } else
-                throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nAttrs)
-        throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+        root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
 
     std::vector<Symbol> attrs;
     for (auto & attr : *getValue().attrs)
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 7f01d08e3..f2f4ba725 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -4,7 +4,6 @@
 
 namespace nix {
 
-
 /* Note: Various places expect the allocated memory to be zeroed. */
 [[gnu::always_inline]]
 inline void * allocBytes(size_t n)
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 8d67691f0..40462afdf 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -18,6 +18,7 @@
 #include <sys/resource.h>
 #include <iostream>
 #include <fstream>
+#include <functional>
 
 #include <sys/resource.h>
 
@@ -36,7 +37,6 @@
 
 namespace nix {
 
-
 static char * allocString(size_t size)
 {
     char * t;
@@ -459,10 +459,14 @@ EvalState::EvalState(
     , sKey(symbols.create("key"))
     , sPath(symbols.create("path"))
     , sPrefix(symbols.create("prefix"))
+    , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
     , store(store)
     , buildStore(buildStore ? buildStore : store)
+    , debugRepl(0)
+    , debugStop(false)
+    , debugQuit(false)
     , regexCache(makeRegexCache())
 #if HAVE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
@@ -472,7 +476,7 @@ EvalState::EvalState(
     , env1AllocCache(std::make_shared<void *>(nullptr))
 #endif
     , baseEnv(allocEnv(128))
-    , staticBaseEnv(false, 0)
+    , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
 {
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
@@ -530,7 +534,7 @@ void EvalState::allowPath(const StorePath & storePath)
         allowedPaths->insert(store->toRealPath(storePath));
 }
 
-void EvalState::allowAndSetStorePathString(const StorePath &storePath, Value & v)
+void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value & v)
 {
     allowPath(storePath);
 
@@ -638,7 +642,7 @@ Value * EvalState::addConstant(const std::string & name, Value & v)
 
 void EvalState::addConstant(const std::string & name, Value * v)
 {
-    staticBaseEnv.vars.emplace_back(symbols.create(name), baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     auto name2 = name.substr(0, 2) == "__" ? name.substr(2) : name;
     baseEnv.values[0]->attrs->push_back(Attr(symbols.create(name2), v));
@@ -663,7 +667,7 @@ Value * EvalState::addPrimOp(const std::string & name,
 
     Value * v = allocValue();
     v->mkPrimOp(new PrimOp { .fun = primOp, .arity = arity, .name = name2 });
-    staticBaseEnv.vars.emplace_back(symbols.create(name), baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(sym, v));
     return v;
@@ -689,7 +693,7 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
 
     Value * v = allocValue();
     v->mkPrimOp(new PrimOp(primOp));
-    staticBaseEnv.vars.emplace_back(envName, baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(envName, baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(symbols.create(primOp.name), v));
     return v;
@@ -719,128 +723,284 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
 }
 
 
+// just for the current level of StaticEnv, not the whole chain.
+void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
+{
+    std::cout << ANSI_MAGENTA;
+    for (auto & i : se.vars)
+        std::cout << st[i.first] << " ";
+    std::cout << ANSI_NORMAL;
+    std::cout << std::endl;
+}
+
+// just for the current level of Env, not the whole chain.
+void printWithBindings(const SymbolTable & st, const Env & env)
+{
+    if (env.type == Env::HasWithAttrs) {
+        std::cout << "with: ";
+        std::cout << ANSI_MAGENTA;
+        Bindings::iterator j = env.values[0]->attrs->begin();
+        while (j != env.values[0]->attrs->end()) {
+            std::cout << st[j->name] << " ";
+            ++j;
+        }
+        std::cout << ANSI_NORMAL;
+        std::cout << std::endl;
+    }
+}
+
+void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, int lvl)
+{
+    std::cout << "Env level " << lvl << std::endl;
+
+    if (se.up && env.up) {
+        std::cout << "static: ";
+        printStaticEnvBindings(st, se);
+        printWithBindings(st, env);
+        std::cout << std::endl;
+        printEnvBindings(st, *se.up, *env.up, ++lvl);
+    } else {
+        std::cout << ANSI_MAGENTA;
+        // for the top level, don't print the double underscore ones;
+        // they are in builtins.
+        for (auto & i : se.vars)
+            if (!hasPrefix(st[i.first], "__"))
+                std::cout << st[i.first] << " ";
+        std::cout << ANSI_NORMAL;
+        std::cout << std::endl;
+        printWithBindings(st, env);  // probably nothing there for the top level.
+        std::cout << std::endl;
+
+    }
+}
+
+void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env)
+{
+    // just print the names for now
+    auto se = es.getStaticEnv(expr);
+    if (se)
+        printEnvBindings(es.symbols, *se, env, 0);
+}
+
+void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, ValMap & vm)
+{
+    // add bindings for the next level up first, so that the bindings for this level
+    // override the higher levels.
+    // The top level bindings (builtins) are skipped since they are added for us by initEnv()
+    if (env.up && se.up) {
+        mapStaticEnvBindings(st, *se.up, *env.up, vm);
+
+        if (env.type == Env::HasWithAttrs) {
+            // add 'with' bindings.
+            Bindings::iterator j = env.values[0]->attrs->begin();
+            while (j != env.values[0]->attrs->end()) {
+                vm[st[j->name]] = j->value;
+                ++j;
+            }
+        } else {
+            // iterate through staticenv bindings and add them.
+            for (auto & i : se.vars)
+                vm[st[i.first]] = env.values[i.second];
+        }
+    }
+}
+
+std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env)
+{
+    auto vm = std::make_unique<ValMap>();
+    mapStaticEnvBindings(st, se, env, *vm);
+    return vm;
+}
+
+void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr & expr)
+{
+    // double check we've got the debugRepl function pointer.
+    if (!debugRepl)
+        return;
+
+    auto dts =
+        error && expr.getPos()
+        ? std::make_unique<DebugTraceStacker>(
+            *this,
+            DebugTrace {
+                .pos = error->info().errPos ? *error->info().errPos : positions[expr.getPos()],
+                .expr = expr,
+                .env = env,
+                .hint = error->info().msg,
+                .isError = true
+            })
+        : nullptr;
+
+    if (error)
+        printError("%s\n\n" ANSI_BOLD "Starting REPL to allow you to inspect the current state of the evaluator.\n" ANSI_NORMAL, error->what());
+
+    auto se = getStaticEnv(expr);
+    if (se) {
+        auto vm = mapStaticEnvBindings(symbols, *se.get(), env);
+        (debugRepl)(ref<EvalState>(shared_from_this()), *vm);
+    }
+}
+
 /* Every "format" object (even temporary) takes up a few hundred bytes
    of stack space, which is a real killer in the recursive
    evaluator.  So here are some helper functions for throwing
    exceptions. */
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, Env & env, Expr & expr)
 {
-    throw EvalError({
+    debugThrow(EvalError({
         .msg = hintfmt(s),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s)
 {
-    throw TypeError({
-        .msg = hintfmt(s, showType(v)),
+    debugThrowLastTrace(EvalError({
+        .msg = hintfmt(s),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const char * s, const std::string & s2) const
+void EvalState::throwEvalError(const char * s, const std::string & s2)
 {
-    throw EvalError(s, s2);
+    debugThrowLastTrace(EvalError(s, s2));
 }
 
 void EvalState::throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const std::string & s2) const
+    const std::string & s2, Env & env, Expr & expr)
 {
-    throw EvalError(ErrorInfo {
+    debugThrow(EvalError(ErrorInfo{
         .msg = hintfmt(s, s2),
         .errPos = positions[pos],
         .suggestions = suggestions,
-    });
+    }), env, expr);
 }
 
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2)
 {
-    throw EvalError(ErrorInfo {
+    debugThrowLastTrace(EvalError({
         .msg = hintfmt(s, s2),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const char * s, const std::string & s2, const std::string & s3) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2, Env & env, Expr & expr)
 {
-    throw EvalError(s, s2, s3);
+    debugThrow(EvalError({
+        .msg = hintfmt(s, s2),
+        .errPos = positions[pos]
+    }), env, expr);
+}
+
+void EvalState::throwEvalError(const char * s, const std::string & s2,
+    const std::string & s3)
+{
+    debugThrowLastTrace(EvalError({
+        .msg = hintfmt(s, s2),
+        .errPos = positions[noPos]
+    }));
 }
 
 void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3) const
+    const std::string & s3)
 {
-    throw EvalError({
-        .msg = hintfmt(s, s2, s3),
+    debugThrowLastTrace(EvalError({
+        .msg = hintfmt(s, s2),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
+    const std::string & s3, Env & env, Expr & expr)
+{
+    debugThrow(EvalError({
+        .msg = hintfmt(s, s2),
+        .errPos = positions[pos]
+    }), env, expr);
+}
+
+void EvalState::throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2, Env & env, Expr & expr)
 {
     // p1 is where the error occurred; p2 is a position mentioned in the message.
-    throw EvalError({
+    debugThrow(EvalError({
         .msg = hintfmt(s, symbols[sym], positions[p2]),
         .errPos = positions[p1]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwTypeError(const PosIdx pos, const char * s) const
+void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v)
 {
-    throw TypeError({
+    debugThrowLastTrace(TypeError({
+        .msg = hintfmt(s, showType(v)),
+        .errPos = positions[pos]
+    }));
+}
+
+void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v, Env & env, Expr & expr)
+{
+    debugThrow(TypeError({
+        .msg = hintfmt(s, showType(v)),
+        .errPos = positions[pos]
+    }), env, expr);
+}
+
+void EvalState::throwTypeError(const PosIdx pos, const char * s)
+{
+    debugThrowLastTrace(TypeError({
         .msg = hintfmt(s),
         .errPos = positions[pos]
-    });
+    }));
 }
 
 void EvalState::throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun,
-    const Symbol s2) const
+    const Symbol s2, Env & env, Expr &expr)
 {
-    throw TypeError({
+    debugThrow(TypeError({
         .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
 void EvalState::throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const ExprLambda & fun, const Symbol s2) const
+    const ExprLambda & fun, const Symbol s2, Env & env, Expr &expr)
 {
-    throw TypeError(ErrorInfo {
+    debugThrow(TypeError(ErrorInfo {
         .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
         .errPos = positions[pos],
         .suggestions = suggestions,
-    });
+    }), env, expr);
 }
 
-
-void EvalState::throwTypeError(const char * s, const Value & v) const
+void EvalState::throwTypeError(const char * s, const Value & v, Env & env, Expr &expr)
 {
-    throw TypeError(s, showType(v));
+    debugThrow(TypeError({
+        .msg = hintfmt(s, showType(v)),
+        .errPos = positions[expr.getPos()],
+    }), env, expr);
 }
 
-void EvalState::throwAssertionError(const PosIdx pos, const char * s, const std::string & s1) const
+void EvalState::throwAssertionError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw AssertionError({
+    debugThrow(AssertionError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1) const
+void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw UndefinedVarError({
+    debugThrow(UndefinedVarError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1) const
+void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw MissingArgumentError({
+    debugThrow(MissingArgumentError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
@@ -853,6 +1013,32 @@ void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const
     e.addTrace(positions[pos], s, s2);
 }
 
+static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
+    EvalState & state,
+    Expr & expr,
+    Env & env,
+    std::optional<ErrPos> pos,
+    const char * s,
+    const std::string & s2)
+{
+    return std::make_unique<DebugTraceStacker>(state,
+        DebugTrace {
+            .pos = pos,
+            .expr = expr,
+            .env = env,
+            .hint = hintfmt(s, s2),
+            .isError = false
+        });
+}
+
+DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
+    : evalState(evalState)
+    , trace(std::move(t))
+{
+    evalState.debugTraces.push_front(trace);
+    if (evalState.debugStop && evalState.debugRepl)
+        evalState.runDebugRepl(nullptr, trace.env, trace.expr);
+}
 
 void Value::mkString(std::string_view s)
 {
@@ -911,12 +1097,11 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
             return j->value;
         }
         if (!env->prevWith)
-            throwUndefinedVarError(var.pos, "undefined variable '%1%'", symbols[var.name]);
+            throwUndefinedVarError(var.pos, "undefined variable '%1%'", symbols[var.name], *env, const_cast<ExprVar&>(var));
         for (size_t l = env->prevWith; l; --l, env = env->up) ;
     }
 }
 
-
 void EvalState::mkList(Value & v, size_t size)
 {
     v.mkList(size);
@@ -1049,6 +1234,15 @@ void EvalState::cacheFile(
     fileParseCache[resolvedPath] = e;
 
     try {
+        auto dts = debugRepl
+            ? makeDebugTraceStacker(
+                *this,
+                *e,
+                this->baseEnv,
+                e->getPos() ? std::optional(ErrPos(positions[e->getPos()])) : std::nullopt,
+                "while evaluating the file '%1%':", resolvedPath)
+            : nullptr;
+
         // Enforce that 'flake.nix' is a direct attrset, not a
         // computation.
         if (mustBeTrivial &&
@@ -1076,7 +1270,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e)
     Value v;
     e->eval(*this, env, v);
     if (v.type() != nBool)
-        throwTypeError("value is %1% while a Boolean was expected", v);
+        throwTypeError(noPos, "value is %1% while a Boolean was expected", v, env, *e);
     return v.boolean;
 }
 
@@ -1086,7 +1280,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos)
     Value v;
     e->eval(*this, env, v);
     if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v);
+        throwTypeError(pos, "value is %1% while a Boolean was expected", v, env, *e);
     return v.boolean;
 }
 
@@ -1095,7 +1289,7 @@ inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
 {
     e->eval(*this, env, v);
     if (v.type() != nAttrs)
-        throwTypeError("value is %1% while a set was expected", v);
+        throwTypeError(noPos, "value is %1% while a set was expected", v, env, *e);
 }
 
 
@@ -1200,7 +1394,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         auto nameSym = state.symbols.create(nameVal.string.s);
         Bindings::iterator j = v.attrs->find(nameSym);
         if (j != v.attrs->end())
-            state.throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, j->pos);
+            state.throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, j->pos, env, *this);
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1274,6 +1468,15 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     e->eval(state, env, vTmp);
 
     try {
+        auto dts = state.debugRepl
+            ? makeDebugTraceStacker(
+                state,
+                *this,
+                env,
+                state.positions[pos2],
+                "while evaluating the attribute '%1%'",
+                showAttrPath(state, env, attrPath))
+            : nullptr;
 
         for (auto & i : attrPath) {
             state.nrLookups++;
@@ -1296,7 +1499,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     state.throwEvalError(
                         pos,
                         Suggestions::bestMatches(allAttrNames, state.symbols[name]),
-                        "attribute '%1%' missing", state.symbols[name]);
+                        "attribute '%1%' missing", state.symbols[name], env, *this);
                 }
             }
             vAttrs = j->value;
@@ -1387,7 +1590,6 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
             if (!lambda.hasFormals())
                 env2.values[displ++] = args[0];
-
             else {
                 forceAttrs(*args[0], pos);
 
@@ -1402,7 +1604,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                     auto j = args[0]->attrs->get(i.name);
                     if (!j) {
                         if (!i.def) throwTypeError(pos, "%1% called without required argument '%2%'",
-                            lambda, i.name);
+                            lambda, i.name, *fun.lambda.env, lambda);
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
                     } else {
                         attrsUsed++;
@@ -1424,8 +1626,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                                 pos,
                                 Suggestions::bestMatches(formalNames, symbols[i.name]),
                                 "%1% called with unexpected argument '%2%'",
-                                lambda,
-                                i.name);
+                                lambda, i.name, *fun.lambda.env, lambda);
                         }
                     abort(); // can't happen
                 }
@@ -1436,6 +1637,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
             /* Evaluate the body. */
             try {
+                auto dts = debugRepl
+                    ? makeDebugTraceStacker(
+                        *this, *lambda.body, env2, positions[lambda.pos],
+                        "while evaluating %s",
+                        lambda.name
+                        ? concatStrings("'", symbols[lambda.name], "'")
+                        : "anonymous lambda")
+                    : nullptr;
+
                 lambda.body->eval(*this, env2, vCur);
             } catch (Error & e) {
                 if (loggerSettings.showTrace.get()) {
@@ -1590,8 +1800,8 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 Nix attempted to evaluate a function as a top level expression; in
 this case it must have its arguments supplied either by default
 values, or passed explicitly with '--arg' or '--argstr'. See
-https://nixos.org/manual/nix/stable/expressions/language-constructs.html#functions.)", symbols[i.name]);
-
+https://nixos.org/manual/nix/stable/expressions/language-constructs.html#functions.)", symbols[i.name],                 
+                *fun.lambda.env, *fun.lambda.fun);
             }
         }
     }
@@ -1623,7 +1833,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos)) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        state.throwAssertionError(pos, "assertion '%1%' failed", out.str());
+        state.throwAssertionError(pos, "assertion '%1%' failed", out.str(), env, *this);
     }
     body->eval(state, env, v);
 }
@@ -1800,14 +2010,14 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
                 nf = n;
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp));
+                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp), env, *this);
         } else if (firstType == nFloat) {
             if (vTmp.type() == nInt) {
                 nf += vTmp.integer;
             } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp));
+                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp), env, *this);
         } else {
             if (s.empty()) s.reserve(es->size());
             /* skip canonization of first path, which would only be not
@@ -1827,7 +2037,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         v.mkFloat(nf);
     else if (firstType == nPath) {
         if (!context.empty())
-            state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path");
+            state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path", env, *this);
         v.mkPath(canonPath(str()));
     } else
         v.mkStringMove(c_str(), context);
@@ -1854,6 +2064,12 @@ void EvalState::forceValueDeep(Value & v)
         if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)
                 try {
+                    // If the value is a thunk, we're evaling. Otherwise no trace necessary.
+                    auto dts = debugRepl && i.value->isThunk()
+                        ? makeDebugTraceStacker(*this, *i.value->thunk.expr, *i.value->thunk.env, positions[i.pos],
+                            "while evaluating the attribute '%1%'", symbols[i.name])
+                        : nullptr;
+
                     recurse(*i.value);
                 } catch (Error & e) {
                     addErrorTrace(e, i.pos, "while evaluating the attribute '%1%'", symbols[i.name]);
@@ -1876,6 +2092,7 @@ NixInt EvalState::forceInt(Value & v, const PosIdx pos)
     forceValue(v, pos);
     if (v.type() != nInt)
         throwTypeError(pos, "value is %1% while an integer was expected", v);
+
     return v.integer;
 }
 
@@ -1918,10 +2135,7 @@ std::string_view EvalState::forceString(Value & v, const PosIdx pos)
 {
     forceValue(v, pos);
     if (v.type() != nString) {
-        if (pos)
-            throwTypeError(pos, "value is %1% while a string was expected", v);
-        else
-            throwTypeError("value is %1% while a string was expected", v);
+        throwTypeError(pos, "value is %1% while a string was expected", v);
     }
     return v.string.s;
 }
@@ -2038,7 +2252,8 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (maybeString)
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end()) throwTypeError(pos, "cannot coerce a set to a string");
+        if (i == v.attrs->end())
+            throwTypeError(pos, "cannot coerce a set to a string");
         return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
     }
 
@@ -2046,7 +2261,6 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
 
     if (coerceMore) {
-
         /* Note that `false' is represented as an empty string for
            shell scripting convenience, just like `null'. */
         if (v.type() == nBool && v.boolean) return "1";
@@ -2191,7 +2405,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             return v1.fpoint == v2.fpoint;
 
         default:
-            throwEvalError("cannot compare %1% with %2%", showType(v1), showType(v2));
+            throwEvalError("cannot compare %1% with %2%",
+                showType(v1),
+                showType(v2));
     }
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 774bc17bb..7b8732169 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -13,7 +13,6 @@
 #include <unordered_map>
 #include <mutex>
 
-
 namespace nix {
 
 
@@ -25,7 +24,6 @@ enum RepairFlag : bool;
 
 typedef void (* PrimOpFun) (EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
-
 struct PrimOp
 {
     PrimOpFun fun;
@@ -35,6 +33,11 @@ struct PrimOp
     const char * doc = nullptr;
 };
 
+#if HAVE_BOEHMGC
+    typedef std::map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *> > > ValMap;
+#else
+    typedef std::map<std::string, Value *> ValMap;
+#endif
 
 struct Env
 {
@@ -44,6 +47,10 @@ struct Env
     Value * values[0];
 };
 
+void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env);
+void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, int lvl = 0);
+
+std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env);
 
 void copyContext(const Value & v, PathSet & context);
 
@@ -70,8 +77,17 @@ struct RegexCache;
 
 std::shared_ptr<RegexCache> makeRegexCache();
 
+struct DebugTrace {
+    std::optional<ErrPos> pos;
+    const Expr & expr;
+    const Env & env;
+    hintformat hint;
+    bool isError;
+};
+
+void debugError(Error * e, Env & env, Expr & expr);
 
-class EvalState
+class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
     SymbolTable symbols;
@@ -87,7 +103,8 @@ public:
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
-        sPrefix;
+        sPrefix,
+        sOutputSpecified;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
@@ -109,12 +126,55 @@ public:
     RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
+    /* Debugger */
+    void (* debugRepl)(ref<EvalState> es, const ValMap & extraEnv);
+    bool debugStop;
+    bool debugQuit;
+    std::list<DebugTrace> debugTraces;
+    std::map<const Expr*, const std::shared_ptr<const StaticEnv>> exprEnvs;
+    const std::shared_ptr<const StaticEnv> getStaticEnv(const Expr & expr) const
+    {
+        auto i = exprEnvs.find(&expr);
+        if (i != exprEnvs.end())
+            return i->second;
+        else
+            return std::shared_ptr<const StaticEnv>();;
+    }
+
+    void runDebugRepl(const Error * error, const Env & env, const Expr & expr);
+
+    template<class E>
+    [[gnu::noinline, gnu::noreturn]]
+    void debugThrow(E && error, const Env & env, const Expr & expr)
+    {
+        if (debugRepl)
+            runDebugRepl(&error, env, expr);
+
+        throw error;
+    }
+
+    template<class E>
+    [[gnu::noinline, gnu::noreturn]]
+    void debugThrowLastTrace(E && e)
+    {
+        // Call this in the situation where Expr and Env are inaccessible.
+        // The debugger will start in the last context that's in the
+        // DebugTrace stack.
+        if (debugRepl && !debugTraces.empty()) {
+            const DebugTrace & last = debugTraces.front();
+            runDebugRepl(&e, last.env, last.expr);
+        }
+
+        throw e;
+    }
+
+
 private:
     SrcToStore srcToStore;
 
     /* A cache from path names to parse trees. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *> > > FileParseCache;
+    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *>>> FileParseCache;
 #else
     typedef std::map<Path, Expr *> FileParseCache;
 #endif
@@ -122,7 +182,7 @@ private:
 
     /* A cache from path names to values. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value> > > FileEvalCache;
+    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value>>> FileEvalCache;
 #else
     typedef std::map<Path, Value> FileEvalCache;
 #endif
@@ -185,10 +245,10 @@ public:
 
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
-    Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);
+    Expr * parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv);
+    Expr * parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv);
     Expr * parseExprFromString(std::string s, const Path & basePath);
 
     Expr * parseStdin();
@@ -198,7 +258,7 @@ public:
        trivial (i.e. doesn't require arbitrary computation). */
     void evalFile(const Path & path, Value & v, bool mustBeTrivial = false);
 
-    /* Like `cacheFile`, but with an already parsed expression. */
+    /* Like `evalFile`, but with an already parsed expression. */
     void cacheFile(
         const Path & path,
         const Path & resolvedPath,
@@ -255,37 +315,68 @@ public:
     std::string_view forceStringNoCtx(Value & v, const PosIdx pos = noPos);
 
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s) const;
+    void throwEvalError(const PosIdx pos, const char * s);
+    [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const PosIdx pos, const char * s,
+        Env & env, Expr & expr);
+    [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const char * s, const std::string & s2);
+    [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2);
+    [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const char * s, const std::string & s2,
+        Env & env, Expr & expr);
+    [[gnu::noinline, gnu::noreturn]]
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v) const;
+    void throwEvalError(const char * s, const std::string & s2, const std::string & s3,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2) const;
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-        const std::string & s2) const;
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2) const;
+    void throwEvalError(const char * s, const std::string & s2, const std::string & s3);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2, const std::string & s3) const;
+    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s, const std::string & s2,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3) const;
+    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2,
+        Env & env, Expr & expr);
+
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2) const;
+    void throwTypeError(const PosIdx pos, const char * s, const Value & v);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s) const;
+    void throwTypeError(const PosIdx pos, const char * s, const Value & v,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2) const;
+    void throwTypeError(const PosIdx pos, const char * s);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-        const ExprLambda & fun, const Symbol s2) const;
+    void throwTypeError(const PosIdx pos, const char * s,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const char * s, const Value & v) const;
+    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwAssertionError(const PosIdx pos, const char * s, const std::string & s1) const;
+    void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s, const ExprLambda & fun, const Symbol s2,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1) const;
+    void throwTypeError(const char * s, const Value & v,
+        Env & env, Expr & expr);
+
     [[gnu::noinline, gnu::noreturn]]
-    void throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1) const;
+    void throwAssertionError(const PosIdx pos, const char * s, const std::string & s1,
+        Env & env, Expr & expr);
+
+    [[gnu::noinline, gnu::noreturn]]
+    void throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1,
+        Env & env, Expr & expr);
+
+    [[gnu::noinline, gnu::noreturn]]
+    void throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1,
+        Env & env, Expr & expr);
 
     [[gnu::noinline]]
     void addErrorTrace(Error & e, const char * s, const std::string & s2) const;
@@ -325,7 +416,7 @@ public:
     Env & baseEnv;
 
     /* The same, but used during parsing to resolve variables. */
-    StaticEnv staticBaseEnv; // !!! should be private
+    std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
 
 private:
 
@@ -366,7 +457,7 @@ private:
     friend struct ExprLet;
 
     Expr * parse(char * text, size_t length, FileOrigin origin, const PathView path,
-        const PathView basePath, StaticEnv & staticEnv);
+        const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
 
@@ -461,6 +552,16 @@ private:
     friend struct Value;
 };
 
+struct DebugTraceStacker {
+    DebugTraceStacker(EvalState & evalState, DebugTrace t);
+    ~DebugTraceStacker()
+    {
+        // assert(evalState.debugTraces.front() == trace);
+        evalState.debugTraces.pop_front();
+    }
+    EvalState & evalState;
+    DebugTrace trace;
+};
 
 /* Return a string representing the type of the value `v'. */
 std::string_view showType(ValueType type);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index d616b3921..346741dd5 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -132,23 +132,36 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         } else
             outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
     }
+
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
 
-    /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
-    const Value * outTI = queryMeta("outputsToInstall");
-    if (!outTI) return outputs;
-    const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
-        /* ^ this shows during `nix-env -i` right under the bad derivation */
-    if (!outTI->isList()) throw errMsg;
-    Outputs result;
-    for (auto elem : outTI->listItems()) {
-        if (elem->type() != nString) throw errMsg;
-        auto out = outputs.find(elem->string.s);
-        if (out == outputs.end()) throw errMsg;
+    Bindings::iterator i;
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
+        Outputs result;
+        auto out = outputs.find(queryOutputName());
+        if (out == outputs.end())
+            throw Error("derivation does not have output '%s'", queryOutputName());
         result.insert(*out);
+        return result;
+    }
+
+    else {
+        /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+        const Value * outTI = queryMeta("outputsToInstall");
+        if (!outTI) return outputs;
+        const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+            /* ^ this shows during `nix-env -i` right under the bad derivation */
+        if (!outTI->isList()) throw errMsg;
+        Outputs result;
+        for (auto elem : outTI->listItems()) {
+            if (elem->type() != nString) throw errMsg;
+            auto out = outputs.find(elem->string.s);
+            if (out == outputs.end()) throw errMsg;
+            result.insert(*out);
+        }
+        return result;
     }
-    return result;
 }
 
 
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 7cc1abef2..bbd2d3c47 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -73,7 +73,7 @@ public:
 
 
 #if HAVE_BOEHMGC
-typedef std::list<DrvInfo, traceable_allocator<DrvInfo> > DrvInfos;
+typedef std::list<DrvInfo, traceable_allocator<DrvInfo>> DrvInfos;
 #else
 typedef std::list<DrvInfo> DrvInfos;
 #endif
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 4c28b976e..462b3b602 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -198,7 +198,7 @@ or          { return OR_KW; }
                    (...|\$[^\{\"\\]|\\.|\$\\.)+ would have triggered.
                    This is technically invalid, but we leave the problem to the
                    parser who fails with exact location. */
-                return STR;
+                return EOF;
               }
 
 \'\'(\ *\n)?     { PUSH_STATE(IND_STRING); return IND_STRING_OPEN; }
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index c529fdc89..7c623a07d 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -6,10 +6,8 @@
 
 #include <cstdlib>
 
-
 namespace nix {
 
-
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -294,35 +292,46 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 
 /* Computing levels/displacements for variables. */
 
-void Expr::bindVars(const EvalState & es, const StaticEnv & env)
+void Expr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
     abort();
 }
 
-void ExprInt::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprInt::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprFloat::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprFloat::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprString::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprString::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprPath::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprPath::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprVar::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     /* Check whether the variable appears in the environment.  If so,
        set its level and displacement. */
     const StaticEnv * curEnv;
     Level level;
     int withLevel = -1;
-    for (curEnv = &env, level = 0; curEnv; curEnv = curEnv->up, level++) {
+    for (curEnv = env.get(), level = 0; curEnv; curEnv = curEnv->up, level++) {
         if (curEnv->isWith) {
             if (withLevel == -1) withLevel = level;
         } else {
@@ -348,8 +357,11 @@ void ExprVar::bindVars(const EvalState & es, const StaticEnv & env)
     this->level = withLevel;
 }
 
-void ExprSelect::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
     if (def) def->bindVars(es, env);
     for (auto & i : attrPath)
@@ -357,64 +369,78 @@ void ExprSelect::bindVars(const EvalState & es, const StaticEnv & env)
             i.expr->bindVars(es, env);
 }
 
-void ExprOpHasAttr::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprOpHasAttr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
     for (auto & i : attrPath)
         if (!i.symbol)
             i.expr->bindVars(es, env);
 }
 
-void ExprAttrs::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    const StaticEnv * dynamicEnv = &env;
-    StaticEnv newEnv(false, &env, recursive ? attrs.size() : 0);
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 
     if (recursive) {
-        dynamicEnv = &newEnv;
+        auto newEnv = std::make_shared<StaticEnv>(false, env.get(), recursive ? attrs.size() : 0);
 
         Displacement displ = 0;
         for (auto & i : attrs)
-            newEnv.vars.emplace_back(i.first, i.second.displ = displ++);
+            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
 
         // No need to sort newEnv since attrs is in sorted order.
 
         for (auto & i : attrs)
             i.second.e->bindVars(es, i.second.inherited ? env : newEnv);
-    }
 
-    else
+        for (auto & i : dynamicAttrs) {
+            i.nameExpr->bindVars(es, newEnv);
+            i.valueExpr->bindVars(es, newEnv);
+        }
+    }
+    else {
         for (auto & i : attrs)
             i.second.e->bindVars(es, env);
 
-    for (auto & i : dynamicAttrs) {
-        i.nameExpr->bindVars(es, *dynamicEnv);
-        i.valueExpr->bindVars(es, *dynamicEnv);
+        for (auto & i : dynamicAttrs) {
+            i.nameExpr->bindVars(es, env);
+            i.valueExpr->bindVars(es, env);
+        }
     }
 }
 
-void ExprList::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprList::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     for (auto & i : elems)
         i->bindVars(es, env);
 }
 
-void ExprLambda::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    StaticEnv newEnv(
-        false, &env,
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
+    auto newEnv = std::make_shared<StaticEnv>(
+        false, env.get(),
         (hasFormals() ? formals->formals.size() : 0) +
         (!arg ? 0 : 1));
 
     Displacement displ = 0;
 
-    if (arg) newEnv.vars.emplace_back(arg, displ++);
+    if (arg) newEnv->vars.emplace_back(arg, displ++);
 
     if (hasFormals()) {
         for (auto & i : formals->formals)
-            newEnv.vars.emplace_back(i.name, displ++);
+            newEnv->vars.emplace_back(i.name, displ++);
 
-        newEnv.sort();
+        newEnv->sort();
 
         for (auto & i : formals->formals)
             if (i.def) i.def->bindVars(es, newEnv);
@@ -423,20 +449,26 @@ void ExprLambda::bindVars(const EvalState & es, const StaticEnv & env)
     body->bindVars(es, newEnv);
 }
 
-void ExprCall::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     fun->bindVars(es, env);
     for (auto e : args)
         e->bindVars(es, env);
 }
 
-void ExprLet::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    StaticEnv newEnv(false, &env, attrs->attrs.size());
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
+    auto newEnv = std::make_shared<StaticEnv>(false, env.get(), attrs->attrs.size());
 
     Displacement displ = 0;
     for (auto & i : attrs->attrs)
-        newEnv.vars.emplace_back(i.first, i.second.displ = displ++);
+        newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
 
     // No need to sort newEnv since attrs->attrs is in sorted order.
 
@@ -446,51 +478,71 @@ void ExprLet::bindVars(const EvalState & es, const StaticEnv & env)
     body->bindVars(es, newEnv);
 }
 
-void ExprWith::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     /* Does this `with' have an enclosing `with'?  If so, record its
        level so that `lookupVar' can look up variables in the previous
        `with' if this one doesn't contain the desired attribute. */
     const StaticEnv * curEnv;
     Level level;
     prevWith = 0;
-    for (curEnv = &env, level = 1; curEnv; curEnv = curEnv->up, level++)
+    for (curEnv = env.get(), level = 1; curEnv; curEnv = curEnv->up, level++)
         if (curEnv->isWith) {
             prevWith = level;
             break;
         }
 
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     attrs->bindVars(es, env);
-    StaticEnv newEnv(true, &env);
+    auto newEnv = std::make_shared<StaticEnv>(true, env.get());
     body->bindVars(es, newEnv);
 }
 
-void ExprIf::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprIf::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     cond->bindVars(es, env);
     then->bindVars(es, env);
     else_->bindVars(es, env);
 }
 
-void ExprAssert::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprAssert::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     cond->bindVars(es, env);
     body->bindVars(es, env);
 }
 
-void ExprOpNot::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprOpNot::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
 }
 
-void ExprConcatStrings::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprConcatStrings::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     for (auto & i : *this->es)
         i.second->bindVars(es, env);
 }
 
-void ExprPos::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprPos::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 5df69e000..8813c61a9 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -22,7 +22,6 @@ MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 
-
 /* Position objects. */
 
 struct Pos
@@ -143,24 +142,25 @@ struct Expr
 {
     virtual ~Expr() { };
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
-    virtual void bindVars(const EvalState & es, const StaticEnv & env);
+    virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
     virtual void eval(EvalState & state, Env & env, Value & v);
     virtual Value * maybeThunk(EvalState & state, Env & env);
     virtual void setName(Symbol name);
+    virtual PosIdx getPos() const { return noPos; }
 };
 
 #define COMMON_METHODS \
     void show(const SymbolTable & symbols, std::ostream & str) const;    \
     void eval(EvalState & state, Env & env, Value & v); \
-    void bindVars(const EvalState & es, const StaticEnv & env);
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
 
 struct ExprInt : Expr
 {
     NixInt n;
     Value v;
     ExprInt(NixInt n) : n(n) { v.mkInt(n); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprFloat : Expr
@@ -168,8 +168,8 @@ struct ExprFloat : Expr
     NixFloat nf;
     Value v;
     ExprFloat(NixFloat nf) : nf(nf) { v.mkFloat(nf); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprString : Expr
@@ -177,8 +177,8 @@ struct ExprString : Expr
     std::string s;
     Value v;
     ExprString(std::string s) : s(std::move(s)) { v.mkString(this->s.data()); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprPath : Expr
@@ -186,8 +186,8 @@ struct ExprPath : Expr
     std::string s;
     Value v;
     ExprPath(std::string s) : s(std::move(s)) { v.mkPath(this->s.c_str()); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 typedef uint32_t Level;
@@ -213,8 +213,9 @@ struct ExprVar : Expr
 
     ExprVar(Symbol name) : name(name) { };
     ExprVar(const PosIdx & pos, Symbol name) : pos(pos), name(name) { };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    PosIdx getPos() const override { return pos; }
+    COMMON_METHODS
 };
 
 struct ExprSelect : Expr
@@ -224,6 +225,7 @@ struct ExprSelect : Expr
     AttrPath attrPath;
     ExprSelect(const PosIdx & pos, Expr * e, const AttrPath & attrPath, Expr * def) : pos(pos), e(e), def(def), attrPath(attrPath) { };
     ExprSelect(const PosIdx & pos, Expr * e, Symbol name) : pos(pos), e(e), def(0) { attrPath.push_back(AttrName(name)); };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -232,6 +234,7 @@ struct ExprOpHasAttr : Expr
     Expr * e;
     AttrPath attrPath;
     ExprOpHasAttr(Expr * e, const AttrPath & attrPath) : e(e), attrPath(attrPath) { };
+    PosIdx getPos() const override { return e->getPos(); }
     COMMON_METHODS
 };
 
@@ -260,6 +263,7 @@ struct ExprAttrs : Expr
     DynamicAttrDefs dynamicAttrs;
     ExprAttrs(const PosIdx &pos) : recursive(false), pos(pos) { };
     ExprAttrs() : recursive(false) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -268,6 +272,11 @@ struct ExprList : Expr
     std::vector<Expr *> elems;
     ExprList() { };
     COMMON_METHODS
+
+    PosIdx getPos() const override
+    {
+        return elems.empty() ? noPos : elems.front()->getPos();
+    }
 };
 
 struct Formal
@@ -320,6 +329,7 @@ struct ExprLambda : Expr
     void setName(Symbol name);
     std::string showNamePos(const EvalState & state) const;
     inline bool hasFormals() const { return formals != nullptr; }
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -331,6 +341,7 @@ struct ExprCall : Expr
     ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args)
         : fun(fun), args(args), pos(pos)
     { }
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -348,6 +359,7 @@ struct ExprWith : Expr
     Expr * attrs, * body;
     size_t prevWith;
     ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -356,6 +368,7 @@ struct ExprIf : Expr
     PosIdx pos;
     Expr * cond, * then, * else_;
     ExprIf(const PosIdx & pos, Expr * cond, Expr * then, Expr * else_) : pos(pos), cond(cond), then(then), else_(else_) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -364,6 +377,7 @@ struct ExprAssert : Expr
     PosIdx pos;
     Expr * cond, * body;
     ExprAssert(const PosIdx & pos, Expr * cond, Expr * body) : pos(pos), cond(cond), body(body) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -385,11 +399,12 @@ struct ExprOpNot : Expr
         { \
             str << "("; e1->show(symbols, str); str << " " s " "; e2->show(symbols, str); str << ")"; \
         } \
-        void bindVars(const EvalState & es, const StaticEnv & env)    \
+        void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)    \
         { \
             e1->bindVars(es, env); e2->bindVars(es, env);    \
         } \
         void eval(EvalState & state, Env & env, Value & v); \
+        PosIdx getPos() const override { return pos; } \
     };
 
 MakeBinOp(ExprOpEq, "==")
@@ -404,9 +419,10 @@ struct ExprConcatStrings : Expr
 {
     PosIdx pos;
     bool forceString;
-    std::vector<std::pair<PosIdx, Expr *> > * es;
-    ExprConcatStrings(const PosIdx & pos, bool forceString, std::vector<std::pair<PosIdx, Expr *> > * es)
+    std::vector<std::pair<PosIdx, Expr *>> * es;
+    ExprConcatStrings(const PosIdx & pos, bool forceString, std::vector<std::pair<PosIdx, Expr *>> * es)
         : pos(pos), forceString(forceString), es(es) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -414,6 +430,7 @@ struct ExprPos : Expr
 {
     PosIdx pos;
     ExprPos(const PosIdx & pos) : pos(pos) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index be0598b75..8cbc2da4d 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -193,7 +193,7 @@ static Formals * toFormals(ParseData & data, ParserFormals * formals,
 
 
 static Expr * stripIndentation(const PosIdx pos, SymbolTable & symbols,
-    std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken> > > & es)
+    std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken>>> & es)
 {
     if (es.empty()) return new ExprString("");
 
@@ -233,7 +233,7 @@ static Expr * stripIndentation(const PosIdx pos, SymbolTable & symbols,
     }
 
     /* Strip spaces from each line. */
-    auto * es2 = new std::vector<std::pair<PosIdx, Expr *> >;
+    auto * es2 = new std::vector<std::pair<PosIdx, Expr *>>;
     atStartOfLine = true;
     size_t curDropped = 0;
     size_t n = es.size();
@@ -320,8 +320,8 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
   StringToken uri;
   StringToken str;
   std::vector<nix::AttrName> * attrNames;
-  std::vector<std::pair<nix::PosIdx, nix::Expr *> > * string_parts;
-  std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, StringToken> > > * ind_string_parts;
+  std::vector<std::pair<nix::PosIdx, nix::Expr *>> * string_parts;
+  std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, StringToken>>> * ind_string_parts;
 }
 
 %type <e> start expr expr_function expr_if expr_op
@@ -415,7 +415,7 @@ expr_op
   | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(CUR_POS, $1, $3); }
   | expr_op '?' attrpath { $$ = new ExprOpHasAttr($1, *$3); }
   | expr_op '+' expr_op
-    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<PosIdx, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
+    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<PosIdx, Expr *>>({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
   | expr_op '-' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
   | expr_op '*' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
   | expr_op '/' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__div")), {$1, $3}); }
@@ -503,9 +503,9 @@ string_parts_interpolated
   : string_parts_interpolated STR
   { $$ = $1; $1->emplace_back(makeCurPos(@2, data), new ExprString(std::string($2))); }
   | string_parts_interpolated DOLLAR_CURLY expr '}' { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $3); }
-  | DOLLAR_CURLY expr '}' { $$ = new std::vector<std::pair<PosIdx, Expr *> >; $$->emplace_back(makeCurPos(@1, data), $2); }
+  | DOLLAR_CURLY expr '}' { $$ = new std::vector<std::pair<PosIdx, Expr *>>; $$->emplace_back(makeCurPos(@1, data), $2); }
   | STR DOLLAR_CURLY expr '}' {
-      $$ = new std::vector<std::pair<PosIdx, Expr *> >;
+      $$ = new std::vector<std::pair<PosIdx, Expr *>>;
       $$->emplace_back(makeCurPos(@1, data), new ExprString(std::string($1)));
       $$->emplace_back(makeCurPos(@2, data), $3);
     }
@@ -528,7 +528,7 @@ path_start
 ind_string_parts
   : ind_string_parts IND_STR { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $2); }
   | ind_string_parts DOLLAR_CURLY expr '}' { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $3); }
-  | { $$ = new std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken> > >; }
+  | { $$ = new std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken>>>; }
   ;
 
 binds
@@ -643,7 +643,7 @@ namespace nix {
 
 
 Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
-    const PathView path, const PathView basePath, StaticEnv & staticEnv)
+    const PathView path, const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
     std::string file;
@@ -706,7 +706,7 @@ Expr * EvalState::parseExprFromFile(const Path & path)
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv)
 {
     auto buffer = readFile(path);
     // readFile should have left some extra space for terminators
@@ -715,7 +715,7 @@ Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     s.append("\0\0", 2);
     return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
@@ -782,13 +782,13 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
     if (hasPrefix(path, "nix/"))
         return concatStrings(corepkgsPrefix, path.substr(4));
 
-    throw ThrownError({
+    debugThrowLastTrace(ThrownError({
         .msg = 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),
         .errPos = positions[pos]
-    });
+    }));
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index fe4d0fc5f..eea274301 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -46,7 +46,7 @@ StringMap EvalState::realiseContext(const PathSet & context)
         auto [ctx, outputName] = decodeContext(*store, i);
         auto ctxS = store->printStorePath(ctx);
         if (!store->isValidPath(ctx))
-            throw InvalidPathError(store->printStorePath(ctx));
+            debugThrowLastTrace(InvalidPathError(store->printStorePath(ctx)));
         if (!outputName.empty() && ctx.isDerivation()) {
             drvs.push_back({ctx, {outputName}});
         } else {
@@ -57,9 +57,9 @@ StringMap EvalState::realiseContext(const PathSet & context)
     if (drvs.empty()) return {};
 
     if (!evalSettings.enableImportFromDerivation)
-        throw Error(
+        debugThrowLastTrace(Error(
             "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
-            store->printStorePath(drvs.begin()->drvPath));
+            store->printStorePath(drvs.begin()->drvPath)));
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
@@ -72,8 +72,8 @@ StringMap EvalState::realiseContext(const PathSet & context)
         for (auto & outputName : outputs) {
             auto outputPath = get(outputPaths, outputName);
             if (!outputPath)
-                throw Error("derivation '%s' does not have an output named '%s'",
-                        store->printStorePath(drvPath), outputName);
+                debugThrowLastTrace(Error("derivation '%s' does not have an output named '%s'",
+                    store->printStorePath(drvPath), outputName));
             res.insert_or_assign(
                 downstreamPlaceholder(*store, drvPath, outputName),
                 store->printStorePath(*outputPath)
@@ -216,11 +216,11 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             Env * env = &state.allocEnv(vScope->attrs->size());
             env->up = &state.baseEnv;
 
-            StaticEnv staticEnv(false, &state.staticBaseEnv, vScope->attrs->size());
+            auto staticEnv = std::make_shared<StaticEnv>(false, state.staticBaseEnv.get(), vScope->attrs->size());
 
             unsigned int displ = 0;
             for (auto & attr : *vScope->attrs) {
-                staticEnv.vars.emplace_back(attr.name, displ);
+                staticEnv->vars.emplace_back(attr.name, displ);
                 env->values[displ++] = attr.value;
             }
 
@@ -319,17 +319,16 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
-        throw EvalError("could not open '%1%': %2%", path, dlerror());
+        state.debugThrowLastTrace(EvalError("could not open '%1%': %2%", path, dlerror()));
 
     dlerror();
     ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
     if(!func) {
         char *message = dlerror();
         if (message)
-            throw EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message);
+            state.debugThrowLastTrace(EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message));
         else
-            throw EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected",
-                sym, path);
+            state.debugThrowLastTrace(EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected", sym, path));
     }
 
     (func)(state, v);
@@ -344,12 +343,11 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     state.forceList(*args[0], pos);
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
-    if (count == 0) {
-        throw EvalError({
+    if (count == 0)
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("at least one argument to 'exec' required"),
             .errPos = state.positions[pos]
-        });
-    }
+        }));
     PathSet context;
     auto program = state.coerceToString(pos, *elems[0], context, false, false).toOwned();
     Strings commandArgs;
@@ -359,11 +357,11 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
     } catch (InvalidPathError & e) {
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
                 program, e.path),
             .errPos = state.positions[pos]
-        });
+        }));
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -547,7 +545,7 @@ struct CompareValues
         if (v1->type() == nInt && v2->type() == nFloat)
             return v1->integer < v2->fpoint;
         if (v1->type() != v2->type())
-            throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+            state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
         switch (v1->type()) {
             case nInt:
                 return v1->integer < v2->integer;
@@ -569,14 +567,14 @@ struct CompareValues
                     }
                 }
             default:
-                throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+                state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
         }
     }
 };
 
 
 #if HAVE_BOEHMGC
-typedef std::list<Value *, gc_allocator<Value *> > ValueList;
+typedef std::list<Value *, gc_allocator<Value *>> ValueList;
 #else
 typedef std::list<Value *> ValueList;
 #endif
@@ -599,10 +597,10 @@ static Bindings::iterator getAttr(
 
         auto aPos = attrSet->pos;
         if (!aPos) {
-            throw TypeError({
+            state.debugThrowLastTrace(TypeError({
                 .msg = errorMsg,
                 .errPos = state.positions[pos],
-            });
+            }));
         } else {
             auto e = TypeError({
                 .msg = errorMsg,
@@ -612,7 +610,7 @@ static Bindings::iterator getAttr(
             // Adding another trace for the function name to make it clear
             // which call received wrong arguments.
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", funcName));
-            throw e;
+            state.debugThrowLastTrace(e);
         }
     }
 
@@ -666,10 +664,10 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
         Bindings::iterator key =
             e->attrs->find(state.sKey);
         if (key == e->attrs->end())
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'key' required"),
                 .errPos = state.positions[pos]
-            });
+            }));
         state.forceValue(*key->value, pos);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -725,6 +723,41 @@ static RegisterPrimOp primop_genericClosure(RegisterPrimOp::Info {
     .fun = prim_genericClosure,
 });
 
+
+static RegisterPrimOp primop_break({
+    .name = "break",
+    .args = {"v"},
+    .doc = R"(
+      In debug mode (enabled using `--debugger`), pause Nix expression evaluation and enter the REPL.
+      Otherwise, return the argument `v`.
+    )",
+    .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
+    {
+        if (state.debugRepl && !state.debugTraces.empty()) {
+            auto error = Error(ErrorInfo {
+                .level = lvlInfo,
+                .msg = hintfmt("breakpoint reached"),
+                .errPos = state.positions[pos],
+            });
+
+            auto & dt = state.debugTraces.front();
+            state.runDebugRepl(&error, dt.env, dt.expr);
+
+            if (state.debugQuit) {
+                // If the user elects to quit the repl, throw an exception.
+                throw Error(ErrorInfo{
+                    .level = lvlInfo,
+                    .msg = hintfmt("quit the debugger"),
+                    .errPos = state.positions[noPos],
+                });
+            }
+        }
+
+        // Return the value we were passed.
+        v = *args[0];
+    }
+});
+
 static RegisterPrimOp primop_abort({
     .name = "abort",
     .args = {"s"},
@@ -735,7 +768,7 @@ static RegisterPrimOp primop_abort({
     {
         PathSet context;
         auto s = state.coerceToString(pos, *args[0], context).toOwned();
-        throw Abort("evaluation aborted with the following error message: '%1%'", s);
+        state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
     }
 });
 
@@ -753,7 +786,7 @@ static RegisterPrimOp primop_throw({
     {
       PathSet context;
       auto s = state.coerceToString(pos, *args[0], context).toOwned();
-      throw ThrownError(s);
+      state.debugThrowLastTrace(ThrownError(s));
     }
 });
 
@@ -1008,37 +1041,37 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
             else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
             else
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
                     .errPos = state.positions[posDrvName]
-                });
+                }));
         };
 
         auto handleOutputs = [&](const Strings & ss) {
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    throw EvalError({
+                    state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("duplicate derivation output '%1%'", j),
                         .errPos = state.positions[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({
+                    state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("invalid derivation output name 'drv'" ),
                         .errPos = state.positions[posDrvName]
-                    });
+                    }));
                 outputs.insert(j);
             }
             if (outputs.empty())
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("derivation cannot have an empty set of outputs"),
                     .errPos = state.positions[posDrvName]
-                });
+                }));
         };
 
         try {
@@ -1163,23 +1196,23 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'builder' missing"),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     if (drv.platform == "")
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'system' missing"),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     if (outputHash) {
         /* Handle fixed-output derivations.
@@ -1187,10 +1220,10 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
            Ignore `__contentAddressed` because fixed output derivations are
            already content addressed. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            throw Error({
+            state.debugThrowLastTrace(Error({
                 .msg = hintfmt("multiple outputs are not supported in fixed-output derivations"),
                 .errPos = state.positions[posDrvName]
-            });
+            }));
 
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
 
@@ -1358,10 +1391,10 @@ static RegisterPrimOp primop_toPath({
 static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     if (evalSettings.pureEval)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'%s' is not allowed in pure evaluation mode", "builtins.storePath"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     PathSet context;
     Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
@@ -1370,10 +1403,10 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
        e.g. nix-push does the right thing. */
     if (!state.store->isStorePath(path)) path = canonPath(path, true);
     if (!state.store->isInStore(path))
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("path '%1%' is not in the Nix store", path),
             .errPos = state.positions[pos]
-        });
+        }));
     auto path2 = state.store->toStorePath(path).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
@@ -1476,7 +1509,7 @@ static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto path = realisePath(state, pos, *args[0]);
     auto s = readFile(path);
     if (s.find((char) 0) != std::string::npos)
-        throw Error("the contents of the file '%1%' cannot be represented as a Nix string", path);
+        state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
     StorePathSet refs;
     if (state.store->isInStore(path)) {
         try {
@@ -1528,13 +1561,12 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
             auto rewrites = state.realiseContext(context);
             path = rewriteStrings(path, rewrites);
         } catch (InvalidPathError & e) {
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
                 .errPos = state.positions[pos]
-            });
+            }));
         }
 
-
         searchPath.emplace_back(prefix, path);
     }
 
@@ -1555,10 +1587,10 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto type = state.forceStringNoCtx(*args[0], pos);
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("unknown hash type '%1%'", type),
             .errPos = state.positions[pos]
-        });
+        }));
 
     auto path = realisePath(state, pos, *args[1]);
 
@@ -1795,13 +1827,13 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError( {
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt(
                     "in 'toFile': the file named '%1%' must not contain a reference "
                     "to a derivation but contains (%2%)",
                     name, path),
                 .errPos = state.positions[pos]
-            });
+            }));
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1959,7 +1991,7 @@ static void addPath(
                 ? state.store->computeStorePathForPath(name, path, method, htSHA256, filter).first
                 : state.store->addToStore(name, path, method, htSHA256, filter, state.repair, refs);
             if (expectedHash && expectedStorePath != dstPath)
-                throw Error("store path mismatch in (possibly filtered) path added from '%s'", path);
+                state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
             state.allowAndSetStorePathString(dstPath, v);
         } else
             state.allowAndSetStorePathString(*expectedStorePath, v);
@@ -1977,12 +2009,12 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
 
     state.forceValue(*args[0], pos);
     if (args[0]->type() != nFunction)
-        throw TypeError({
+        state.debugThrowLastTrace(TypeError({
             .msg = hintfmt(
                 "first argument in call to 'filterSource' is not a function but %1%",
                 showType(*args[0])),
             .errPos = state.positions[pos]
-        });
+        }));
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
@@ -2066,16 +2098,16 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos), htSHA256);
         else
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("unsupported argument '%1%' to 'addPath'", state.symbols[attr.name]),
                 .errPos = state.positions[attr.pos]
-            });
+            }));
     }
     if (path.empty())
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'path' required"),
             .errPos = state.positions[pos]
-        });
+        }));
     if (name.empty())
         name = baseNameOf(path);
 
@@ -2447,10 +2479,10 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
         return;
     }
     if (!args[0]->isLambda())
-        throw TypeError({
+        state.debugThrowLastTrace(TypeError({
             .msg = hintfmt("'functionArgs' requires a function"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     if (!args[0]->lambda.fun->hasFormals()) {
         v.mkAttrs(&state.emptyBindings);
@@ -2538,7 +2570,7 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
                 attrsSeen[attr.name].first++;
         } catch (TypeError & e) {
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "zipAttrsWith"));
-            throw;
+            state.debugThrowLastTrace(e);
         }
     }
 
@@ -2625,10 +2657,10 @@ static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Val
 {
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("list index %1% is out of bounds", n),
             .errPos = state.positions[pos]
-        });
+        }));
     state.forceValue(*list.listElems()[n], pos);
     v = *list.listElems()[n];
 }
@@ -2673,10 +2705,10 @@ static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("'tail' called on an empty list"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
@@ -2911,10 +2943,10 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("cannot create list of size %1%", len),
             .errPos = state.positions[pos]
-        });
+        }));
 
     state.mkList(v, len);
 
@@ -3123,7 +3155,7 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
             state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)));
         } catch (TypeError &e) {
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "concatMap"));
-            throw;
+            state.debugThrowLastTrace(e);
         }
         len += lists[n].listSize();
     }
@@ -3218,10 +3250,10 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
 
     NixFloat f2 = state.forceFloat(*args[1], pos);
     if (f2 == 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("division by zero"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
         v.mkFloat(state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
@@ -3230,10 +3262,10 @@ static void prim_div(EvalState & state, const PosIdx 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({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("overflow in integer division"),
                 .errPos = state.positions[pos]
-            });
+            }));
 
         v.mkInt(i1 / i2);
     }
@@ -3361,10 +3393,10 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
     auto s = state.coerceToString(pos, *args[2], context);
 
     if (start < 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("negative start position in 'substring'"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     v.mkString((unsigned int) start >= s->size() ? "" : s->substr(start, len), context);
 }
@@ -3412,10 +3444,10 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args,
     auto type = state.forceStringNoCtx(*args[0], pos);
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("unknown hash type '%1%'", type),
             .errPos = state.positions[pos]
-        });
+        }));
 
     PathSet context; // discarded
     auto s = state.forceString(*args[1], context, pos);
@@ -3482,19 +3514,18 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
                 (v.listElems()[i] = state.allocValue())->mkString(match[i + 1].str());
         }
 
-    } catch (std::regex_error &e) {
+    } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
-        } else {
-            throw EvalError({
+            }));
+        } else
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("invalid regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
-        }
+            }));
     }
 }
 
@@ -3587,19 +3618,18 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         assert(idx == 2 * len + 1);
 
-    } catch (std::regex_error &e) {
+    } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
-        } else {
-            throw EvalError({
+            }));
+        } else
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("invalid regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
-        }
+            }));
     }
 }
 
@@ -3675,10 +3705,10 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     state.forceList(*args[0], pos);
     state.forceList(*args[1], pos);
     if (args[0]->listSize() != args[1]->listSize())
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
@@ -3931,7 +3961,7 @@ void EvalState::createBaseEnv()
        because attribute lookups expect it to be sorted. */
     baseEnv.values[0]->attrs->sort();
 
-    staticBaseEnv.sort();
+    staticBaseEnv->sort();
 
     /* Note: we have to initialize the 'derivation' constant *after*
        building baseEnv/staticBaseEnv because it uses 'builtins'. */
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index d7c3c9918..e5eeea520 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -108,16 +108,16 @@ static void fetchTree(
 
         if (auto aType = args[0]->attrs->get(state.sType)) {
             if (type)
-                throw Error({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unexpected attribute 'type'"),
                     .errPos = state.positions[pos]
-                });
+                }));
             type = state.forceStringNoCtx(*aType->value, aType->pos);
         } else if (!type)
-            throw Error({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
                 .errPos = state.positions[pos]
-            });
+            }));
 
         attrs.emplace("type", type.value());
 
@@ -138,16 +138,16 @@ static void fetchTree(
             else if (attr.value->type() == nInt)
                 attrs.emplace(state.symbols[attr.name], uint64_t(attr.value->integer));
             else
-                throw TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
-                    state.symbols[attr.name], showType(*attr.value));
+                state.debugThrowLastTrace(TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
+                    state.symbols[attr.name], showType(*attr.value)));
         }
 
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
-                throw Error({
-                    .msg = hintfmt("attribute 'name' isn't supported in call to 'fetchTree'"),
+                state.debugThrowLastTrace(EvalError({
+                    .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
                     .errPos = state.positions[pos]
-                });
+                }));
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
@@ -167,7 +167,7 @@ static void fetchTree(
         input = lookupInRegistries(state.store, input).first;
 
     if (evalSettings.pureEval && !input.isLocked())
-        throw Error("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]);
+        state.debugThrowLastTrace(EvalError("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]));
 
     auto [tree, input2] = input.fetch(state.store);
 
@@ -206,17 +206,17 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, attr.pos);
             else
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unsupported argument '%s' to '%s'", n, who),
                     .errPos = state.positions[attr.pos]
-                });
-            }
+                }));
+        }
 
         if (!url)
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("'url' argument required"),
                 .errPos = state.positions[pos]
-            });
+            }));
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
@@ -228,7 +228,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
         name = baseNameOf(*url);
 
     if (evalSettings.pureEval && !expectedHash)
-        throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);
+        state.debugThrowLastTrace(EvalError("in pure evaluation mode, '%s' requires a 'sha256' argument", who));
 
     // early exit if pinned and already in the store
     if (expectedHash && expectedHash->type == htSHA256) {
@@ -255,8 +255,8 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
             ? state.store->queryPathInfo(storePath)->narHash
             : hashFile(htSHA256, state.store->toRealPath(storePath));
         if (hash != *expectedHash)
-            throw Error((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  specified: %s\n  got:       %s",
-                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true));
+            state.debugThrowLastTrace(EvalError((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  specified: %s\n  got:       %s",
+                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true)));
     }
 
     state.allowAndSetStorePathString(storePath, v);
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index 68235ad11..03504db61 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -85,6 +85,7 @@ void printValueAsJSON(EvalState & state, bool strict,
                 .errPos = state.positions[v.determinePos(pos)]
             });
             e.addTrace(state.positions[pos], hintfmt("message for the trace"));
+            state.debugThrowLastTrace(e);
             throw e;
     }
 }
@@ -99,7 +100,7 @@ void printValueAsJSON(EvalState & state, bool strict,
 void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
     JSONPlaceholder & out, PathSet & context) const
 {
-    throw TypeError("cannot convert %1% to JSON", showType());
+    state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
 }
 
 
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 58a8a56a0..2008df74d 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -404,9 +404,9 @@ public:
 
 
 #if HAVE_BOEHMGC
-typedef std::vector<Value *, traceable_allocator<Value *> > ValueVector;
-typedef std::map<Symbol, Value *, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Value *> > > ValueMap;
-typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector> > > ValueVectorMap;
+typedef std::vector<Value *, traceable_allocator<Value *>> ValueVector;
+typedef std::map<Symbol, Value *, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Value *>>> ValueMap;
+typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector>>> ValueVectorMap;
 #else
 typedef std::vector<Value *> ValueVector;
 typedef std::map<Symbol, Value *> ValueMap;