libexpr: Support structured error classes

While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.

(cherry picked from commit c6a89c1a1659b31694c0fbcd21d78a6dd521c732)
Change-Id: Iced91ba4e00ca9e801518071fb43798936cbd05a

40 files changed, 645 insertions, 540 deletions

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 80b252c31..8ff8d04c4 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -427,8 +427,6 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             // Quietly ignore parse errors.
         } catch (EvalError & e) {
             // Quietly ignore evaluation errors.
-        } catch (UndefinedVarError & e) {
-            // Quietly ignore undefined variable errors.
         } catch (BadURL & e) {
             // Quietly ignore BadURL flake-related errors.
         }
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index ab654c1b0..f4da99a9a 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -66,10 +66,10 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
         if (!attrIndex) {
 
             if (v->type() != nAttrs)
-                throw TypeError(
+                state.error<TypeError>(
                     "the expression selected by the selection path '%1%' should be a set but is %2%",
                     attrPath,
-                    showType(*v));
+                    showType(*v)).debugThrow();
             if (attr.empty())
                 throw Error("empty attribute name in selection path '%1%'", attrPath);
 
@@ -89,10 +89,10 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
         else {
 
             if (!v->isList())
-                throw TypeError(
+                state.error<TypeError>(
                     "the expression selected by the selection path '%1%' should be a list but is %2%",
                     attrPath,
-                    showType(*v));
+                    showType(*v)).debugThrow();
             if (*attrIndex >= v->listSize())
                 throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", *attrIndex, attrPath);
 
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 2c9aa5532..20dd9e0bc 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -490,7 +490,7 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
                         if (forceErrors)
                             debug("reevaluating failed cached attribute '%s'", getAttrPathStr(name));
                         else
-                            throw CachedEvalError("cached failure of attribute '%s'", getAttrPathStr(name));
+                            throw CachedEvalError(root->state, "cached failure of attribute '%s'", getAttrPathStr(name));
                     } else
                         return std::make_shared<AttrCursor>(root,
                             std::make_pair(shared_from_this(), name), nullptr, std::move(attr));
@@ -499,7 +499,7 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
                 // evaluate to see whether 'name' exists
             } else
                 return nullptr;
-                //throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+                //error<TypeError>("'%s' is not an attribute set", getAttrPathStr()).debugThrow();
         }
     }
 
@@ -507,7 +507,7 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
 
     if (v.type() != nAttrs)
         return nullptr;
-        //throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+        //error<TypeError>("'%s' is not an attribute set", getAttrPathStr()).debugThrow();
 
     auto attr = v.attrs->get(name);
 
@@ -573,14 +573,14 @@ std::string AttrCursor::getString()
                 debug("using cached string attribute '%s'", getAttrPathStr());
                 return s->first;
             } else
-                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
+                root->state.error<TypeError>("'%s' is not a string", getAttrPathStr()).debugThrow();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
 
     return v.type() == nString ? v.string.s : v.path().to_string();
 }
@@ -615,7 +615,7 @@ string_t AttrCursor::getStringWithContext()
                     return *s;
                 }
             } else
-                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
+                root->state.error<TypeError>("'%s' is not a string", getAttrPathStr()).debugThrow();
         }
     }
 
@@ -629,7 +629,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path().to_string(), {}};
     else
-        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
 }
 
 bool AttrCursor::getBool()
@@ -642,14 +642,14 @@ bool AttrCursor::getBool()
                 debug("using cached Boolean attribute '%s'", getAttrPathStr());
                 return *b;
             } else
-                root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
+                root->state.error<TypeError>("'%s' is not a Boolean", getAttrPathStr()).debugThrow();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nBool)
-        root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
+        root->state.error<TypeError>("'%s' is not a Boolean", getAttrPathStr()).debugThrow();
 
     return v.boolean;
 }
@@ -664,14 +664,14 @@ NixInt AttrCursor::getInt()
                 debug("using cached integer attribute '%s'", getAttrPathStr());
                 return i->x;
             } else
-                throw TypeError("'%s' is not an integer", getAttrPathStr());
+                root->state.error<TypeError>("'%s' is not an integer", getAttrPathStr()).debugThrow();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nInt)
-        throw TypeError("'%s' is not an integer", getAttrPathStr());
+        root->state.error<TypeError>("'%s' is not an integer", getAttrPathStr()).debugThrow();
 
     return v.integer;
 }
@@ -686,7 +686,7 @@ std::vector<std::string> AttrCursor::getListOfStrings()
                 debug("using cached list of strings attribute '%s'", getAttrPathStr());
                 return *l;
             } else
-                throw TypeError("'%s' is not a list of strings", getAttrPathStr());
+                root->state.error<TypeError>("'%s' is not a list of strings", getAttrPathStr()).debugThrow();
         }
     }
 
@@ -696,7 +696,7 @@ std::vector<std::string> AttrCursor::getListOfStrings()
     root->state.forceValue(v, noPos);
 
     if (v.type() != nList)
-        throw TypeError("'%s' is not a list", getAttrPathStr());
+        root->state.error<TypeError>("'%s' is not a list", getAttrPathStr()).debugThrow();
 
     std::vector<std::string> res;
 
@@ -719,14 +719,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
                 debug("using cached attrset attribute '%s'", getAttrPathStr());
                 return *attrs;
             } else
-                root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
+                root->state.error<TypeError>("'%s' is not an attribute set", getAttrPathStr()).debugThrow();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nAttrs)
-        root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
+        root->state.error<TypeError>("'%s' is not an attribute set", getAttrPathStr()).debugThrow();
 
     std::vector<Symbol> attrs;
     for (auto & attr : *getValue().attrs)
diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
index 46c4999c8..ad6cd74fb 100644
--- a/src/libexpr/eval-cache.hh
+++ b/src/libexpr/eval-cache.hh
@@ -10,8 +10,6 @@
 
 namespace nix::eval_cache {
 
-MakeError(CachedEvalError, EvalError);
-
 struct AttrDb;
 class AttrCursor;
 
diff --git a/src/libexpr/eval-error.cc b/src/libexpr/eval-error.cc
new file mode 100644
index 000000000..b9411cbf4
--- /dev/null
+++ b/src/libexpr/eval-error.cc
@@ -0,0 +1,113 @@
+#include "eval-error.hh"
+#include "eval.hh"
+#include "value.hh"
+
+namespace nix {
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::withExitStatus(unsigned int exitStatus)
+{
+    error.withExitStatus(exitStatus);
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::atPos(PosIdx pos)
+{
+    error.err.pos = error.state.positions[pos];
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::atPos(Value & value, PosIdx fallback)
+{
+    return atPos(value.determinePos(fallback));
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::withTrace(PosIdx pos, const std::string_view text)
+{
+    error.err.traces.push_front(
+        Trace{.pos = error.state.positions[pos], .hint = hintfmt(std::string(text)), .frame = false});
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrameTrace(PosIdx pos, const std::string_view text)
+{
+    error.err.traces.push_front(
+        Trace{.pos = error.state.positions[pos], .hint = hintformat(std::string(text)), .frame = true});
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::withSuggestions(Suggestions & s)
+{
+    error.err.suggestions = s;
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrame(const Env & env, const Expr & expr)
+{
+    // NOTE: This is abusing side-effects.
+    // TODO: check compatibility with nested debugger calls.
+    // TODO: What side-effects??
+    error.state.debugTraces.push_front(DebugTrace{
+        .pos = error.state.positions[expr.getPos()],
+        .expr = expr,
+        .env = env,
+        .hint = hintformat("Fake frame for debugging purposes"),
+        .isError = true});
+    return *this;
+}
+
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::addTrace(PosIdx pos, hintformat hint, bool frame)
+{
+    error.addTrace(error.state.positions[pos], hint, frame);
+    return *this;
+}
+
+template<class T>
+template<typename... Args>
+EvalErrorBuilder<T> &
+EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs)
+{
+
+    addTrace(error.state.positions[pos], hintfmt(std::string(formatString), formatArgs...));
+    return *this;
+}
+
+template<class T>
+void EvalErrorBuilder<T>::debugThrow()
+{
+    if (error.state.debugRepl && !error.state.debugTraces.empty()) {
+        const DebugTrace & last = error.state.debugTraces.front();
+        const Env * env = &last.env;
+        const Expr * expr = &last.expr;
+        error.state.runDebugRepl(&error, *env, *expr);
+    }
+
+    // `EvalState` is the only class that can construct an `EvalErrorBuilder`,
+    // and it does so in dynamic storage. This is the final method called on
+    // any such instancve and must delete itself before throwing the underlying
+    // error.
+    auto error = std::move(this->error);
+    delete this;
+
+    throw error;
+}
+
+template class EvalErrorBuilder<EvalError>;
+template class EvalErrorBuilder<AssertionError>;
+template class EvalErrorBuilder<ThrownError>;
+template class EvalErrorBuilder<Abort>;
+template class EvalErrorBuilder<TypeError>;
+template class EvalErrorBuilder<UndefinedVarError>;
+template class EvalErrorBuilder<MissingArgumentError>;
+template class EvalErrorBuilder<InfiniteRecursionError>;
+template class EvalErrorBuilder<CachedEvalError>;
+template class EvalErrorBuilder<InvalidPathError>;
+
+}
diff --git a/src/libexpr/eval-error.hh b/src/libexpr/eval-error.hh
new file mode 100644
index 000000000..1faf76781
--- /dev/null
+++ b/src/libexpr/eval-error.hh
@@ -0,0 +1,119 @@
+#pragma once
+
+#include <algorithm>
+
+#include "error.hh"
+#include "pos-idx.hh"
+
+namespace nix {
+
+struct Env;
+struct Expr;
+struct Value;
+
+class EvalState;
+template<class T>
+class EvalErrorBuilder;
+
+class EvalError : public Error
+{
+    template<class T>
+    friend class EvalErrorBuilder;
+public:
+    EvalState & state;
+
+    EvalError(EvalState & state, ErrorInfo && errorInfo)
+        : Error(errorInfo)
+        , state(state)
+    {
+    }
+
+    template<typename... Args>
+    explicit EvalError(EvalState & state, const std::string & formatString, const Args &... formatArgs)
+        : Error(formatString, formatArgs...)
+        , state(state)
+    {
+    }
+};
+
+MakeError(ParseError, Error);
+MakeError(AssertionError, EvalError);
+MakeError(ThrownError, AssertionError);
+MakeError(Abort, EvalError);
+MakeError(TypeError, EvalError);
+MakeError(UndefinedVarError, EvalError);
+MakeError(MissingArgumentError, EvalError);
+MakeError(RestrictedPathError, Error);
+MakeError(CachedEvalError, EvalError);
+MakeError(InfiniteRecursionError, EvalError);
+
+struct InvalidPathError : public EvalError
+{
+public:
+    Path path;
+    InvalidPathError(EvalState & state, const Path & path)
+        : EvalError(state, "path '%s' is not valid", path)
+    {
+    }
+};
+
+template<class T>
+class EvalErrorBuilder final
+{
+    friend class EvalState;
+
+    template<typename... Args>
+    explicit EvalErrorBuilder(EvalState & state, const Args &... args)
+        : error(T(state, args...))
+    {
+    }
+
+public:
+    T error;
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withExitStatus(unsigned int exitStatus);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & atPos(PosIdx pos);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & atPos(Value & value, PosIdx fallback = noPos);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withTrace(PosIdx pos, const std::string_view text);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrameTrace(PosIdx pos, const std::string_view text);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withSuggestions(Suggestions & s);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrame(const Env & e, const Expr & ex);
+
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, hintformat hint, bool frame = false);
+
+    template<typename... Args>
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> &
+    addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs);
+
+    [[gnu::noinline, gnu::noreturn]] void debugThrow();
+};
+
+/**
+ * The size needed to allocate any `EvalErrorBuilder<T>`.
+ *
+ * The list of classes here needs to be kept in sync with the list of `template
+ * class` declarations in `eval-error.cc`.
+ *
+ * This is used by `EvalState` to preallocate a buffer of sufficient size for
+ * any `EvalErrorBuilder<T>` to avoid allocating while evaluating Nix code.
+ */
+constexpr size_t EVAL_ERROR_BUILDER_SIZE = std::max({
+    sizeof(EvalErrorBuilder<EvalError>),
+    sizeof(EvalErrorBuilder<AssertionError>),
+    sizeof(EvalErrorBuilder<ThrownError>),
+    sizeof(EvalErrorBuilder<Abort>),
+    sizeof(EvalErrorBuilder<TypeError>),
+    sizeof(EvalErrorBuilder<UndefinedVarError>),
+    sizeof(EvalErrorBuilder<MissingArgumentError>),
+    sizeof(EvalErrorBuilder<InfiniteRecursionError>),
+    sizeof(EvalErrorBuilder<CachedEvalError>),
+    sizeof(EvalErrorBuilder<InvalidPathError>),
+});
+
+}
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 42cb68bbe..03320c7c9 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -3,6 +3,7 @@
 
 #include "print.hh"
 #include "eval.hh"
+#include "eval-error.hh"
 
 namespace nix {
 
@@ -115,10 +116,11 @@ inline void EvalState::forceAttrs(Value & v, Callable getPos, std::string_view e
     PosIdx pos = getPos();
     forceValue(v, pos);
     if (v.type() != nAttrs) {
-        error("expected a set but found %1%: %2%",
-              showType(v),
-              ValuePrinter(*this, v, errorPrintOptions))
-            .withTrace(pos, errorCtx).debugThrow<TypeError>();
+        error<TypeError>(
+            "expected a set but found %1%: %2%",
+            showType(v),
+            ValuePrinter(*this, v, errorPrintOptions)
+        ).withTrace(pos, errorCtx).debugThrow();
     }
 }
 
@@ -128,10 +130,11 @@ inline void EvalState::forceList(Value & v, const PosIdx pos, std::string_view e
 {
     forceValue(v, pos);
     if (!v.isList()) {
-        error("expected a list but found %1%: %2%",
-              showType(v),
-              ValuePrinter(*this, v, errorPrintOptions))
-            .withTrace(pos, errorCtx).debugThrow<TypeError>();
+        error<TypeError>(
+            "expected a list but found %1%: %2%",
+            showType(v),
+            ValuePrinter(*this, v, errorPrintOptions)
+        ).withTrace(pos, errorCtx).debugThrow();
     }
 }
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 98bf086ec..fac67950b 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -333,46 +333,6 @@ void initGC()
     gcInitialised = true;
 }
 
-
-ErrorBuilder & ErrorBuilder::atPos(PosIdx pos)
-{
-    info.errPos = state.positions[pos];
-    return *this;
-}
-
-ErrorBuilder & ErrorBuilder::withTrace(PosIdx pos, const std::string_view text)
-{
-    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = false });
-    return *this;
-}
-
-ErrorBuilder & ErrorBuilder::withFrameTrace(PosIdx pos, const std::string_view text)
-{
-    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = true });
-    return *this;
-}
-
-ErrorBuilder & ErrorBuilder::withSuggestions(Suggestions & s)
-{
-    info.suggestions = s;
-    return *this;
-}
-
-ErrorBuilder & ErrorBuilder::withFrame(const Env & env, const Expr & expr)
-{
-    // NOTE: This is abusing side-effects.
-    // TODO: check compatibility with nested debugger calls.
-    state.debugTraces.push_front(DebugTrace {
-        .pos = nullptr,
-        .expr = expr,
-        .env = env,
-        .hint = hintformat("Fake frame for debugging purposes"),
-        .isError = true
-    });
-    return *this;
-}
-
-
 EvalState::EvalState(
     const SearchPath & _searchPath,
     ref<Store> store,
@@ -819,7 +779,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
         ? std::make_unique<DebugTraceStacker>(
             *this,
             DebugTrace {
-                .pos = error->info().errPos ? error->info().errPos : positions[expr.getPos()],
+                .pos = error->info().pos ? error->info().pos : positions[expr.getPos()],
                 .expr = expr,
                 .env = env,
                 .hint = error->info().msg,
@@ -936,7 +896,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
             return j->value;
         }
         if (!fromWith->parentWith)
-            error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
+            error<UndefinedVarError>("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow();
         for (size_t l = fromWith->prevWith; l; --l, env = env->up) ;
         fromWith = fromWith->parentWith;
     }
@@ -1162,7 +1122,7 @@ void EvalState::cacheFile(
         // computation.
         if (mustBeTrivial &&
             !(dynamic_cast<ExprAttrs *>(e)))
-            error("file '%s' must be an attribute set", path).debugThrow<EvalError>();
+            error<EvalError>("file '%s' must be an attribute set", path).debugThrow();
         eval(e, v);
     } catch (Error & e) {
         addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
@@ -1186,10 +1146,11 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos, std::stri
         Value v;
         e->eval(*this, env, v);
         if (v.type() != nBool)
-            error("expected a Boolean but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .withFrame(env, *e).debugThrow<TypeError>();
+            error<TypeError>(
+                 "expected a Boolean but found %1%: %2%",
+                 showType(v),
+                 ValuePrinter(*this, v, errorPrintOptions)
+             ).atPos(pos).withFrame(env, *e).debugThrow();
         return v.boolean;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -1203,10 +1164,11 @@ inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const PosIdx po
     try {
         e->eval(*this, env, v);
         if (v.type() != nAttrs)
-            error("expected a set but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .withFrame(env, *e).debugThrow<TypeError>();
+            error<TypeError>(
+                "expected a set but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).withFrame(env, *e).debugThrow();
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
@@ -1315,7 +1277,7 @@ 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.error("dynamic attribute '%1%' already defined at %2%", state.symbols[nameSym], state.positions[j->pos]).atPos(i.pos).withFrame(env, *this).debugThrow<EvalError>();
+            state.error<EvalError>("dynamic attribute '%1%' already defined at %2%", state.symbols[nameSym], state.positions[j->pos]).atPos(i.pos).withFrame(env, *this).debugThrow();
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1440,8 +1402,8 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     for (auto & attr : *vAttrs->attrs)
                         allAttrNames.insert(state.symbols[attr.name]);
                     auto suggestions = Suggestions::bestMatches(allAttrNames, state.symbols[name]);
-                    state.error("attribute '%1%' missing", state.symbols[name])
-                        .atPos(pos).withSuggestions(suggestions).withFrame(env, *this).debugThrow<EvalError>();
+                    state.error<EvalError>("attribute '%1%' missing", state.symbols[name])
+                        .atPos(pos).withSuggestions(suggestions).withFrame(env, *this).debugThrow();
                 }
             }
             vAttrs = j->value;
@@ -1514,7 +1476,7 @@ public:
 void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos)
 {
     if (callDepth > evalSettings.maxCallDepth)
-        error("stack overflow; max-call-depth exceeded").atPos(pos).template debugThrow<EvalError>();
+        error<EvalError>("stack overflow; max-call-depth exceeded").atPos(pos).debugThrow();
     CallDepth _level(callDepth);
 
     auto trace = evalSettings.traceFunctionCalls
@@ -1572,13 +1534,13 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                     auto j = args[0]->attrs->get(i.name);
                     if (!j) {
                         if (!i.def) {
-                            error("function '%1%' called without required argument '%2%'",
+                            error<TypeError>("function '%1%' called without required argument '%2%'",
                                              (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
                                              symbols[i.name])
                                     .atPos(lambda.pos)
                                     .withTrace(pos, "from call site")
                                     .withFrame(*fun.lambda.env, lambda)
-                                    .debugThrow<TypeError>();
+                                    .debugThrow();
                         }
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
                     } else {
@@ -1598,14 +1560,14 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                             for (auto & formal : lambda.formals->formals)
                                 formalNames.insert(symbols[formal.name]);
                             auto suggestions = Suggestions::bestMatches(formalNames, symbols[i.name]);
-                            error("function '%1%' called with unexpected argument '%2%'",
+                            error<TypeError>("function '%1%' called with unexpected argument '%2%'",
                                              (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
                                              symbols[i.name])
                                 .atPos(lambda.pos)
                                 .withTrace(pos, "from call site")
                                 .withSuggestions(suggestions)
                                 .withFrame(*fun.lambda.env, lambda)
-                                .debugThrow<TypeError>();
+                                .debugThrow();
                         }
                     abort(); // can't happen
                 }
@@ -1737,11 +1699,12 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
         }
 
         else
-            error("attempt to call something which is not a function but %1%: %2%",
+            error<TypeError>(
+                    "attempt to call something which is not a function but %1%: %2%",
                     showType(vCur),
                     ValuePrinter(*this, vCur, errorPrintOptions))
                 .atPos(pos)
-                .debugThrow<TypeError>();
+                .debugThrow();
     }
 
     vRes = vCur;
@@ -1823,12 +1786,12 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
             if (j != args.end()) {
                 attrs.insert(*j);
             } else if (!i.def) {
-                error(R"(cannot evaluate a function that has an argument without a value ('%1%')
+                error<MissingArgumentError>(R"(cannot evaluate a function that has an argument without a value ('%1%')
 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/language/constructs.html#functions.)", symbols[i.name])
-                    .atPos(i.pos).withFrame(*fun.lambda.env, *fun.lambda.fun).debugThrow<MissingArgumentError>();
+                    .atPos(i.pos).withFrame(*fun.lambda.env, *fun.lambda.fun).debugThrow();
             }
         }
     }
@@ -1859,7 +1822,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        state.error("assertion '%1%' failed", out.str()).atPos(pos).withFrame(env, *this).debugThrow<AssertionError>();
+        state.error<AssertionError>("assertion '%1%' failed", out.str()).atPos(pos).withFrame(env, *this).debugThrow();
     }
     body->eval(state, env, v);
 }
@@ -2037,14 +2000,14 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
                 nf = n;
                 nf += vTmp.fpoint;
             } else
-                state.error("cannot add %1% to an integer", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
+                state.error<EvalError>("cannot add %1% to an integer", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow();
         } else if (firstType == nFloat) {
             if (vTmp.type() == nInt) {
                 nf += vTmp.integer;
             } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint;
             } else
-                state.error("cannot add %1% to a float", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
+                state.error<EvalError>("cannot add %1% to a float", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow();
         } else {
             if (s.empty()) s.reserve(es->size());
             /* skip canonization of first path, which would only be not
@@ -2066,7 +2029,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         v.mkFloat(nf);
     else if (firstType == nPath) {
         if (!context.empty())
-            state.error("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow<EvalError>();
+            state.error<EvalError>("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow();
         v.mkPath(CanonPath(canonPath(str())));
     } else
         v.mkStringMove(c_str(), context);
@@ -2081,8 +2044,9 @@ void ExprPos::eval(EvalState & state, Env & env, Value & v)
 
 void ExprBlackHole::eval(EvalState & state, Env & env, Value & v)
 {
-    state.error("infinite recursion encountered")
-        .debugThrow<InfiniteRecursionError>();
+    state.error<InfiniteRecursionError>("infinite recursion encountered")
+        .atPos(v.determinePos(noPos))
+        .debugThrow();
 }
 
 // always force this to be separate, otherwise forceValue may inline it and take
@@ -2096,7 +2060,7 @@ void EvalState::tryFixupBlackHolePos(Value & v, PosIdx pos)
     try {
         std::rethrow_exception(e);
     } catch (InfiniteRecursionError & e) {
-        e.err.errPos = positions[pos];
+        e.atPos(positions[pos]);
     } catch (...) {
     }
 }
@@ -2144,15 +2108,18 @@ NixInt EvalState::forceInt(Value & v, const PosIdx pos, std::string_view errorCt
     try {
         forceValue(v, pos);
         if (v.type() != nInt)
-            error("expected an integer but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .debugThrow<TypeError>();
+            error<TypeError>(
+                "expected an integer but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).atPos(pos).debugThrow();
         return v.integer;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
     }
+
+    return v.integer;
 }
 
 
@@ -2163,10 +2130,11 @@ NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view err
         if (v.type() == nInt)
             return v.integer;
         else if (v.type() != nFloat)
-            error("expected a float but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .debugThrow<TypeError>();
+            error<TypeError>(
+                "expected a float but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).atPos(pos).debugThrow();
         return v.fpoint;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -2180,15 +2148,18 @@ bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx
     try {
         forceValue(v, pos);
         if (v.type() != nBool)
-            error("expected a Boolean but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .debugThrow<TypeError>();
+            error<TypeError>(
+                "expected a Boolean but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).atPos(pos).debugThrow();
         return v.boolean;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
     }
+
+    return v.boolean;
 }
 
 
@@ -2203,10 +2174,11 @@ void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view erro
     try {
         forceValue(v, pos);
         if (v.type() != nFunction && !isFunctor(v))
-            error("expected a function but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .debugThrow<TypeError>();
+            error<TypeError>(
+                "expected a function but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).atPos(pos).debugThrow();
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
@@ -2219,10 +2191,11 @@ std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string
     try {
         forceValue(v, pos);
         if (v.type() != nString)
-            error("expected a string but found %1%: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
-                .debugThrow<TypeError>();
+            error<TypeError>(
+                "expected a string but found %1%: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            ).atPos(pos).debugThrow();
         return v.string.s;
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
@@ -2251,7 +2224,7 @@ std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos, std::s
 {
     auto s = forceString(v, pos, errorCtx);
     if (v.string.context) {
-        error("the string '%1%' is not allowed to refer to a store path (such as '%2%')", v.string.s, v.string.context[0]).withTrace(pos, errorCtx).debugThrow<EvalError>();
+        error<EvalError>("the string '%1%' is not allowed to refer to a store path (such as '%2%')", v.string.s, v.string.context[0]).withTrace(pos, errorCtx).debugThrow();
     }
     return s;
 }
@@ -2316,11 +2289,13 @@ BackedStringView EvalState::coerceToString(
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
         if (i == v.attrs->end()) {
-            error("cannot coerce %1% to a string: %2%",
-                  showType(v),
-                  ValuePrinter(*this, v, errorPrintOptions))
+            error<TypeError>(
+                "cannot coerce %1% to a string: %2%",
+                showType(v),
+                ValuePrinter(*this, v, errorPrintOptions)
+            )
                 .withTrace(pos, errorCtx)
-                .debugThrow<TypeError>();
+                .debugThrow();
         }
         return coerceToString(pos, *i->value, context, errorCtx,
                               coerceMore, copyToStore, canonicalizePath);
@@ -2328,7 +2303,7 @@ BackedStringView EvalState::coerceToString(
 
     if (v.type() == nExternal) {
         try {
-            return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+            return v.external->coerceToString(*this, pos, context, coerceMore, copyToStore);
         } catch (Error & e) {
             e.addTrace(nullptr, errorCtx);
             throw;
@@ -2364,18 +2339,19 @@ BackedStringView EvalState::coerceToString(
         }
     }
 
-    error("cannot coerce %1% to a string: %2%",
-          showType(v),
-          ValuePrinter(*this, v, errorPrintOptions))
+    error<TypeError>("cannot coerce %1% to a string: %2%",
+        showType(v),
+        ValuePrinter(*this, v, errorPrintOptions)
+    )
         .withTrace(pos, errorCtx)
-        .debugThrow<TypeError>();
+        .debugThrow();
 }
 
 
 StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePath & path)
 {
     if (nix::isDerivation(path.path.abs()))
-        error("file names are not allowed to end in '%1%'", drvExtension).debugThrow<EvalError>();
+        error<EvalError>("file names are not allowed to end in '%1%'", drvExtension).debugThrow();
 
     auto i = srcToStore.find(path);
 
@@ -2400,7 +2376,7 @@ SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext
 {
     auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
-        error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
+        error<EvalError>("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow();
     return CanonPath(path);
 }
 
@@ -2410,7 +2386,7 @@ StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, NixStringCon
     auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
-    error("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
+    error<EvalError>("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow();
 }
 
 
@@ -2420,18 +2396,18 @@ std::pair<SingleDerivedPath, std::string_view> EvalState::coerceToSingleDerivedP
     auto s = forceString(v, context, pos, errorCtx);
     auto csize = context.size();
     if (csize != 1)
-        error(
+        error<EvalError>(
             "string '%s' has %d entries in its context. It should only have exactly one entry",
             s, csize)
-            .withTrace(pos, errorCtx).debugThrow<EvalError>();
+            .withTrace(pos, errorCtx).debugThrow();
     auto derivedPath = std::visit(overloaded {
         [&](NixStringContextElem::Opaque && o) -> SingleDerivedPath {
             return std::move(o);
         },
         [&](NixStringContextElem::DrvDeep &&) -> SingleDerivedPath {
-            error(
+            error<EvalError>(
                 "string '%s' has a context which refers to a complete source and binary closure. This is not supported at this time",
-                s).withTrace(pos, errorCtx).debugThrow<EvalError>();
+                s).withTrace(pos, errorCtx).debugThrow();
         },
         [&](NixStringContextElem::Built && b) -> SingleDerivedPath {
             return std::move(b);
@@ -2454,16 +2430,16 @@ SingleDerivedPath EvalState::coerceToSingleDerivedPath(const PosIdx pos, Value &
            error message. */
         std::visit(overloaded {
             [&](const SingleDerivedPath::Opaque & o) {
-                error(
+                error<EvalError>(
                     "path string '%s' has context with the different path '%s'",
                     s, sExpected)
-                    .withTrace(pos, errorCtx).debugThrow<EvalError>();
+                    .withTrace(pos, errorCtx).debugThrow();
             },
             [&](const SingleDerivedPath::Built & b) {
-                error(
+                error<EvalError>(
                     "string '%s' has context with the output '%s' from derivation '%s', but the string is not the right placeholder for this derivation output. It should be '%s'",
                     s, b.output, b.drvPath->to_string(*store), sExpected)
-                    .withTrace(pos, errorCtx).debugThrow<EvalError>();
+                    .withTrace(pos, errorCtx).debugThrow();
             }
         }, derivedPath.raw());
     }
@@ -2545,7 +2521,7 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
 
         case nThunk: // Must not be left by forceValue
         default:
-            error("cannot compare %1% with %2%", showType(v1), showType(v2)).withTrace(pos, errorCtx).debugThrow<EvalError>();
+            error<EvalError>("cannot compare %1% with %2%", showType(v1), showType(v2)).withTrace(pos, errorCtx).debugThrow();
     }
 }
 
@@ -2782,13 +2758,12 @@ SourcePath EvalState::findFile(const SearchPath & searchPath, const std::string_
     if (hasPrefix(path, "nix/"))
         return CanonPath(concatStrings(corepkgsPrefix, path.substr(4)));
 
-    debugThrow(ThrownError({
-        .msg = hintfmt(evalSettings.pureEval
+    error<ThrownError>(
+        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]
-    }), 0, 0);
+        path
+    ).atPos(pos).debugThrow();
 }
 
 
@@ -2858,11 +2833,11 @@ Expr * EvalState::parse(
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
+std::string ExternalValueBase::coerceToString(EvalState & state, const PosIdx & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
-    throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string: %2%", showType(), *this)
-    });
+    state.error<TypeError>(
+        "cannot coerce %1% to a string: %2%", showType(), *this
+    ).atPos(pos).debugThrow();
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 3f661e73e..830db4558 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -2,6 +2,7 @@
 ///@file
 
 #include "attr-set.hh"
+#include "eval-error.hh"
 #include "types.hh"
 #include "value.hh"
 #include "nixexpr.hh"
@@ -150,45 +151,6 @@ struct DebugTrace {
     bool isError;
 };
 
-void debugError(Error * e, Env & env, Expr & expr);
-
-class ErrorBuilder
-{
-    private:
-        EvalState & state;
-        ErrorInfo info;
-
-        ErrorBuilder(EvalState & s, ErrorInfo && i): state(s), info(i) { }
-
-    public:
-        template<typename... Args>
-        [[nodiscard, gnu::noinline]]
-        static ErrorBuilder * create(EvalState & s, const Args & ... args)
-        {
-            return new ErrorBuilder(s, ErrorInfo { .msg = hintfmt(args...) });
-        }
-
-        [[nodiscard, gnu::noinline]]
-        ErrorBuilder & atPos(PosIdx pos);
-
-        [[nodiscard, gnu::noinline]]
-        ErrorBuilder & withTrace(PosIdx pos, const std::string_view text);
-
-        [[nodiscard, gnu::noinline]]
-        ErrorBuilder & withFrameTrace(PosIdx pos, const std::string_view text);
-
-        [[nodiscard, gnu::noinline]]
-        ErrorBuilder & withSuggestions(Suggestions & s);
-
-        [[nodiscard, gnu::noinline]]
-        ErrorBuilder & withFrame(const Env & e, const Expr & ex);
-
-        template<class ErrorType>
-        [[gnu::noinline, gnu::noreturn]]
-        void debugThrow();
-};
-
-
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -262,39 +224,10 @@ public:
 
     void runDebugRepl(const Error * error, const Env & env, const Expr & expr);
 
-    template<class E>
-    [[gnu::noinline, gnu::noreturn]]
-    void debugThrowLastTrace(E && error)
-    {
-        debugThrow(error, nullptr, nullptr);
-    }
-
-    template<class E>
-    [[gnu::noinline, gnu::noreturn]]
-    void debugThrow(E && error, const Env * env, const Expr * expr)
-    {
-        if (debugRepl && ((env && expr) || !debugTraces.empty())) {
-            if (!env || !expr) {
-                const DebugTrace & last = debugTraces.front();
-                env = &last.env;
-                expr = &last.expr;
-            }
-            runDebugRepl(&error, *env, *expr);
-        }
-
-        throw std::move(error);
-    }
-
-    // This is dangerous, but gets in line with the idea that error creation and
-    // throwing should not allocate on the stack of hot functions.
-    // as long as errors are immediately thrown, it works.
-    ErrorBuilder * errorBuilder;
-
-    template<typename... Args>
+    template<class T, typename... Args>
     [[nodiscard, gnu::noinline]]
-    ErrorBuilder & error(const Args & ... args) {
-        errorBuilder = ErrorBuilder::create(*this, args...);
-        return *errorBuilder;
+    EvalErrorBuilder<T> & error(const Args & ... args) {
+        return *new EvalErrorBuilder<T>(*this, args...);
     }
 
 private:
@@ -840,23 +773,8 @@ std::string showType(const Value & v);
  */
 SourcePath resolveExprPath(SourcePath path);
 
-struct InvalidPathError : EvalError
-{
-    Path path;
-    InvalidPathError(const Path & path);
-#ifdef EXCEPTION_NEEDS_THROW_SPEC
-    ~InvalidPathError() throw () { };
-#endif
-};
-
 static const std::string corepkgsPrefix{"/__corepkgs__/"};
 
-template<class ErrorType>
-void ErrorBuilder::debugThrow()
-{
-    // NOTE: We always use the -LastTrace version as we push the new trace in withFrame()
-    state.debugThrowLastTrace(ErrorType(info));
-}
 
 }
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index e1f41a8a2..e45526a6c 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -140,8 +140,8 @@ static FlakeInput parseFlakeInput(EvalState & state,
                         attrs.emplace(state.symbols[attr.name], (long unsigned int)attr.value->integer);
                         break;
                     default:
-                        throw TypeError("flake input attribute '%s' is %s while a string, Boolean, or integer is expected",
-                            state.symbols[attr.name], showType(*attr.value));
+                        state.error<TypeError>("flake input attribute '%s' is %s while a string, Boolean, or integer is expected",
+                            state.symbols[attr.name], showType(*attr.value)).debugThrow();
                 }
                 #pragma GCC diagnostic pop
             }
@@ -288,15 +288,15 @@ static Flake getFlake(
                 std::vector<std::string> ss;
                 for (auto elem : setting.value->listItems()) {
                     if (elem->type() != nString)
-                        throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
-                            state.symbols[setting.name], showType(*setting.value));
+                        state.error<TypeError>("list element in flake configuration setting '%s' is %s while a string is expected",
+                            state.symbols[setting.name], showType(*setting.value)).debugThrow();
                     ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos, ""));
                 }
                 flake.config.settings.emplace(state.symbols[setting.name], ss);
             }
             else
-                throw TypeError("flake configuration setting '%s' is %s",
-                    state.symbols[setting.name], showType(*setting.value));
+                state.error<TypeError>("flake configuration setting '%s' is %s",
+                    state.symbols[setting.name], showType(*setting.value)).debugThrow();
         }
     }
 
@@ -857,11 +857,11 @@ static void prim_flakeRefToString(
             attrs.emplace(state.symbols[attr.name],
                           std::string(attr.value->str()));
         } else {
-            state.error(
+            state.error<EvalError>(
                 "flake reference attribute sets may only contain integers, Booleans, "
                 "and strings, but attribute '%s' is %s",
                 state.symbols[attr.name],
-                showType(*attr.value)).debugThrow<EvalError>();
+                showType(*attr.value)).debugThrow();
         }
     }
     auto flakeRef = FlakeRef::fromAttrs(attrs);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index e4ee4409c..495407c39 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -50,7 +50,7 @@ std::string DrvInfo::queryName() const
 {
     if (name == "" && attrs) {
         auto i = attrs->find(state->sName);
-        if (i == attrs->end()) throw TypeError("derivation name missing");
+        if (i == attrs->end()) state->error<TypeError>("derivation name missing").debugThrow();
         name = state->forceStringNoCtx(*i->value, noPos, "while evaluating the 'name' attribute of a derivation");
     }
     return name;
@@ -397,7 +397,8 @@ static void getDerivations(EvalState & state, Value & vIn,
         }
     }
 
-    else throw TypeError("expression does not evaluate to a derivation (or a set or list of those)");
+    else
+        state.error<TypeError>("expression does not evaluate to a derivation (or a set or list of those)").debugThrow();
 }
 
 
diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index 99a475ff9..2d12c47c5 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -1,4 +1,6 @@
 #include "json-to-value.hh"
+#include "value.hh"
+#include "eval.hh"
 
 #include <variant>
 #include <nlohmann/json.hpp>
@@ -159,7 +161,7 @@ public:
     }
 
     bool parse_error(std::size_t, const std::string&, const nlohmann::detail::exception& ex) {
-        throw JSONParseError(ex.what());
+        throw JSONParseError("%s", ex.what());
     }
 };
 
diff --git a/src/libexpr/json-to-value.hh b/src/libexpr/json-to-value.hh
index 3b8ec000f..3c8fa5cc0 100644
--- a/src/libexpr/json-to-value.hh
+++ b/src/libexpr/json-to-value.hh
@@ -1,13 +1,16 @@
 #pragma once
 ///@file
 
-#include "eval.hh"
+#include "error.hh"
 
 #include <string>
 
 namespace nix {
 
-MakeError(JSONParseError, EvalError);
+class EvalState;
+struct Value;
+
+MakeError(JSONParseError, Error);
 
 void parseJSON(EvalState & state, const std::string_view & s, Value & v);
 
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 6f7a9cd6b..fb06c3d69 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -152,9 +152,9 @@ or          { return OR_KW; }
               try {
                   yylval->n = boost::lexical_cast<int64_t>(yytext);
               } catch (const boost::bad_lexical_cast &) {
-                  throw ParseError({
+                  throw ParseError(ErrorInfo{
                       .msg = hintfmt("invalid integer '%1%'", yytext),
-                      .errPos = state->positions[CUR_POS],
+                      .pos = state->positions[CUR_POS],
                   });
               }
               return INT;
@@ -162,9 +162,9 @@ or          { return OR_KW; }
 {FLOAT}     { errno = 0;
               yylval->nf = strtod(yytext, 0);
               if (errno != 0)
-                  throw ParseError({
+                  throw ParseError(ErrorInfo{
                       .msg = hintfmt("invalid float '%1%'", yytext),
-                      .errPos = state->positions[CUR_POS],
+                      .pos = state->positions[CUR_POS],
                   });
               return FLOAT;
             }
@@ -291,9 +291,9 @@ or          { return OR_KW; }
 
 <INPATH_SLASH>{ANY} |
 <INPATH_SLASH><<EOF>> {
-  throw ParseError({
+  throw ParseError(ErrorInfo{
       .msg = hintfmt("path has a trailing slash"),
-      .errPos = state->positions[CUR_POS],
+      .pos = state->positions[CUR_POS],
   });
 }
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index eb5d0d3d4..3525ea1a3 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -294,10 +294,10 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
        enclosing `with'.  If there is no `with', then we can issue an
        "undefined variable" error now. */
     if (withLevel == -1)
-        throw UndefinedVarError({
-            .msg = hintfmt("undefined variable '%1%'", es.symbols[name]),
-            .errPos = es.positions[pos]
-        });
+        es.error<UndefinedVarError>(
+            "undefined variable '%1%'",
+            es.symbols[name]
+        ).atPos(pos).debugThrow();
     for (auto * e = env.get(); e && !fromWith; e = e->up)
         fromWith = e->isWith;
     this->level = withLevel;
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 84f66ba4c..6f34afaa7 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -9,29 +9,13 @@
 #include "error.hh"
 #include "chunked-vector.hh"
 #include "position.hh"
+#include "eval-error.hh"
 #include "pos-idx.hh"
 #include "pos-table.hh"
 
 namespace nix {
 
 
-MakeError(EvalError, Error);
-MakeError(ParseError, Error);
-MakeError(AssertionError, EvalError);
-MakeError(ThrownError, AssertionError);
-MakeError(Abort, EvalError);
-MakeError(TypeError, EvalError);
-MakeError(UndefinedVarError, Error);
-MakeError(MissingArgumentError, EvalError);
-MakeError(RestrictedPathError, Error);
-
-class InfiniteRecursionError : public EvalError
-{
-    friend class EvalState;
-public:
-    using EvalError::EvalError;
-};
-
 struct Env;
 struct Value;
 class EvalState;
diff --git a/src/libexpr/parser-state.hh b/src/libexpr/parser-state.hh
index ace2a109d..2fc072ca4 100644
--- a/src/libexpr/parser-state.hh
+++ b/src/libexpr/parser-state.hh
@@ -65,7 +65,7 @@ inline void ParserState::dupAttr(const AttrPath & attrPath, const PosIdx pos, co
     throw ParseError({
          .msg = hintfmt("attribute '%1%' already defined at %2%",
              showAttrPath(symbols, attrPath), positions[prevPos]),
-         .errPos = positions[pos]
+         .pos = positions[pos]
     });
 }
 
@@ -73,7 +73,7 @@ inline void ParserState::dupAttr(Symbol attr, const PosIdx pos, const PosIdx pre
 {
     throw ParseError({
         .msg = hintfmt("attribute '%1%' already defined at %2%", symbols[attr], positions[prevPos]),
-        .errPos = positions[pos]
+        .pos = positions[pos]
     });
 }
 
@@ -154,13 +154,13 @@ inline Formals * ParserState::validateFormals(Formals * formals, PosIdx pos, Sym
     if (duplicate)
         throw ParseError({
             .msg = hintfmt("duplicate formal function argument '%1%'", symbols[duplicate->first]),
-            .errPos = positions[duplicate->second]
+            .pos = positions[duplicate->second]
         });
 
     if (arg && formals->has(arg))
         throw ParseError({
             .msg = hintfmt("duplicate formal function argument '%1%'", symbols[arg]),
-            .errPos = positions[pos]
+            .pos = positions[pos]
         });
 
     return formals;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 982d112dc..fc83ee2dd 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -64,7 +64,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParserState * state, const char *
 {
     throw ParseError({
         .msg = hintfmt(error),
-        .errPos = state->positions[state->at(*loc)]
+        .pos = state->positions[state->at(*loc)]
     });
 }
 
@@ -153,7 +153,7 @@ expr_function
     { if (!$2->dynamicAttrs.empty())
         throw ParseError({
             .msg = hintfmt("dynamic attributes not allowed in let"),
-            .errPos = state->positions[CUR_POS]
+            .pos = state->positions[CUR_POS]
         });
       $$ = new ExprLet($2, $4);
     }
@@ -243,7 +243,7 @@ expr_simple
       if (noURLLiterals)
           throw ParseError({
               .msg = hintfmt("URL literals are disabled"),
-              .errPos = state->positions[CUR_POS]
+              .pos = state->positions[CUR_POS]
           });
       $$ = new ExprString(std::string($1));
   }
@@ -339,7 +339,7 @@ attrs
       } else
           throw ParseError({
               .msg = hintfmt("dynamic attributes not allowed in inherit"),
-              .errPos = state->positions[state->at(@2)]
+              .pos = state->positions[state->at(@2)]
           });
     }
   | { $$ = new AttrPath; }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 8d2b2895f..268d14d3d 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -37,10 +37,6 @@ namespace nix {
  * Miscellaneous
  *************************************************************/
 
-
-InvalidPathError::InvalidPathError(const Path & path) :
-    EvalError("path '%s' is not valid", path), path(path) {}
-
 StringMap EvalState::realiseContext(const NixStringContext & context)
 {
     std::vector<DerivedPath::Built> drvs;
@@ -49,7 +45,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
     for (auto & c : context) {
         auto ensureValid = [&](const StorePath & p) {
             if (!store->isValidPath(p))
-                debugThrowLastTrace(InvalidPathError(store->printStorePath(p)));
+                error<InvalidPathError>(store->printStorePath(p)).debugThrow();
         };
         std::visit(overloaded {
             [&](const NixStringContextElem::Built & b) {
@@ -76,9 +72,10 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
     if (drvs.empty()) return {};
 
     if (!evalSettings.enableImportFromDerivation)
-        debugThrowLastTrace(Error(
+        error<EvalError>(
             "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
-            drvs.begin()->to_string(*store)));
+            drvs.begin()->to_string(*store)
+        ).debugThrow();
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
@@ -345,16 +342,16 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 
     void *handle = dlopen(path.path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
-        state.debugThrowLastTrace(EvalError("could not open '%1%': %2%", path, dlerror()));
+        state.error<EvalError>("could not open '%1%': %2%", path, dlerror()).debugThrow();
 
     dlerror();
     ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
     if(!func) {
         char *message = dlerror();
         if (message)
-            state.debugThrowLastTrace(EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message));
+            state.error<EvalError>("could not load symbol '%1%' from '%2%': %3%", sym, path, message).debugThrow();
         else
-            state.debugThrowLastTrace(EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected", sym, path));
+            state.error<EvalError>("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected", sym, path).debugThrow();
     }
 
     (func)(state, v);
@@ -370,7 +367,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0)
-        state.error("at least one argument to 'exec' required").atPos(pos).debugThrow<EvalError>();
+        state.error<EvalError>("at least one argument to 'exec' required").atPos(pos).debugThrow();
     NixStringContext context;
     auto program = state.coerceToString(pos, *elems[0], context,
             "while evaluating the first element of the argument passed to builtins.exec",
@@ -385,7 +382,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
     } catch (InvalidPathError & e) {
-        state.error("cannot execute '%1%', since path '%2%' is not valid", program, e.path).atPos(pos).debugThrow<EvalError>();
+        state.error<EvalError>("cannot execute '%1%', since path '%2%' is not valid", program, e.path).atPos(pos).debugThrow();
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -587,7 +584,7 @@ struct CompareValues
             if (v1->type() == nInt && v2->type() == nFloat)
                 return v1->integer < v2->fpoint;
             if (v1->type() != v2->type())
-                state.error("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow<EvalError>();
+                state.error<EvalError>("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow();
             // Allow selecting a subset of enum values
             #pragma GCC diagnostic push
             #pragma GCC diagnostic ignored "-Wswitch-enum"
@@ -612,7 +609,7 @@ struct CompareValues
                         }
                     }
                 default:
-                    state.error("cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2)).debugThrow<EvalError>();
+                    state.error<EvalError>("cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2)).debugThrow();
             #pragma GCC diagnostic pop
             }
         } catch (Error & e) {
@@ -639,7 +636,7 @@ static Bindings::iterator getAttr(
 {
     Bindings::iterator value = attrSet->find(attrSym);
     if (value == attrSet->end()) {
-        state.error("attribute '%s' missing", state.symbols[attrSym]).withTrace(noPos, errorCtx).debugThrow<TypeError>();
+        state.error<TypeError>("attribute '%s' missing", state.symbols[attrSym]).withTrace(noPos, errorCtx).debugThrow();
     }
     return value;
 }
@@ -752,7 +749,7 @@ static RegisterPrimOp primop_break({
             auto error = Error(ErrorInfo {
                 .level = lvlInfo,
                 .msg = hintfmt("breakpoint reached"),
-                .errPos = state.positions[pos],
+                .pos = state.positions[pos],
             });
 
             auto & dt = state.debugTraces.front();
@@ -763,7 +760,7 @@ static RegisterPrimOp primop_break({
                 throw Error(ErrorInfo{
                     .level = lvlInfo,
                     .msg = hintfmt("quit the debugger"),
-                    .errPos = nullptr,
+                    .pos = nullptr,
                 });
             }
         }
@@ -784,7 +781,7 @@ static RegisterPrimOp primop_abort({
         NixStringContext context;
         auto s = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.abort").toOwned();
-        state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
+        state.error<Abort>("evaluation aborted with the following error message: '%1%'", s).debugThrow();
     }
 });
 
@@ -803,7 +800,7 @@ static RegisterPrimOp primop_throw({
       NixStringContext context;
       auto s = state.coerceToString(pos, *args[0], context,
               "while evaluating the error message passed to builtin.throw").toOwned();
-      state.debugThrowLastTrace(ThrownError(s));
+      state.error<ThrownError>(s).debugThrow();
     }
 });
 
@@ -1122,37 +1119,33 @@ drvName, Bindings * attrs, Value & v)
                 experimentalFeatureSettings.require(Xp::DynamicDerivations);
                 ingestionMethod = TextIngestionMethod {};
             } else
-                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
-                    .errPos = state.positions[noPos]
-                }));
+                state.error<EvalError>(
+                    "invalid value '%s' for 'outputHashMode' attribute", s
+                ).atPos(v).debugThrow();
         };
 
         auto handleOutputs = [&](const Strings & ss) {
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    state.debugThrowLastTrace(EvalError({
-                        .msg = hintfmt("duplicate derivation output '%1%'", j),
-                        .errPos = state.positions[noPos]
-                    }));
+                    state.error<EvalError>("duplicate derivation output '%1%'", j)
+                        .atPos(v)
+                        .debugThrow();
                 /* !!! 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")
-                    state.debugThrowLastTrace(EvalError({
-                        .msg = hintfmt("invalid derivation output name 'drv'" ),
-                        .errPos = state.positions[noPos]
-                    }));
+                    state.error<EvalError>("invalid derivation output name 'drv'")
+                        .atPos(v)
+                        .debugThrow();
                 outputs.insert(j);
             }
             if (outputs.empty())
-                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("derivation cannot have an empty set of outputs"),
-                    .errPos = state.positions[noPos]
-                }));
+                state.error<EvalError>("derivation cannot have an empty set of outputs")
+                    .atPos(v)
+                    .debugThrow();
         };
 
         try {
@@ -1275,16 +1268,14 @@ drvName, Bindings * attrs, Value & v)
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("required attribute 'builder' missing"),
-            .errPos = state.positions[noPos]
-        }));
+        state.error<EvalError>("required attribute 'builder' missing")
+            .atPos(v)
+            .debugThrow();
 
     if (drv.platform == "")
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("required attribute 'system' missing"),
-            .errPos = state.positions[noPos]
-        }));
+        state.error<EvalError>("required attribute 'system' missing")
+            .atPos(v)
+            .debugThrow();
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName) &&
@@ -1292,10 +1283,10 @@ drvName, Bindings * attrs, Value & v)
           outputs.size() == 1 &&
           *(outputs.begin()) == "out"))
     {
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("derivation names are allowed to end in '%s' only if they produce a single derivation file", drvExtension),
-            .errPos = state.positions[noPos]
-        }));
+        state.error<EvalError>(
+            "derivation names are allowed to end in '%s' only if they produce a single derivation file",
+            drvExtension
+        ).atPos(v).debugThrow();
     }
 
     if (outputHash) {
@@ -1304,10 +1295,9 @@ drvName, Bindings * attrs, Value & v)
            Ignore `__contentAddressed` because fixed output derivations are
            already content addressed. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            state.debugThrowLastTrace(Error({
-                .msg = hintfmt("multiple outputs are not supported in fixed-output derivations"),
-                .errPos = state.positions[noPos]
-            }));
+            state.error<EvalError>(
+                "multiple outputs are not supported in fixed-output derivations"
+            ).atPos(v).debugThrow();
 
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
 
@@ -1326,10 +1316,8 @@ drvName, Bindings * attrs, Value & v)
 
     else if (contentAddressed || isImpure) {
         if (contentAddressed && isImpure)
-            throw EvalError({
-                .msg = hintfmt("derivation cannot be both content-addressed and impure"),
-                .errPos = state.positions[noPos]
-            });
+            state.error<EvalError>("derivation cannot be both content-addressed and impure")
+                .atPos(v).debugThrow();
 
         auto ht = parseHashTypeOpt(outputHashAlgo).value_or(htSHA256);
         auto method = ingestionMethod.value_or(FileIngestionMethod::Recursive);
@@ -1370,10 +1358,10 @@ drvName, Bindings * attrs, Value & v)
             for (auto & i : outputs) {
                 auto h = get(hashModulo.hashes, i);
                 if (!h)
-                    throw AssertionError({
-                        .msg = hintfmt("derivation produced no hash for output '%s'", i),
-                        .errPos = state.positions[noPos],
-                    });
+                    state.error<AssertionError>(
+                        "derivation produced no hash for output '%s'",
+                        i
+                    ).atPos(v).debugThrow();
                 auto outPath = state.store->makeOutputPath(i, *h, drvName);
                 drv.env[i] = state.store->printStorePath(outPath);
                 drv.outputs.insert_or_assign(
@@ -1479,10 +1467,10 @@ static RegisterPrimOp primop_toPath({
 static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     if (evalSettings.pureEval)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("'%s' is not allowed in pure evaluation mode", "builtins.storePath"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>(
+            "'%s' is not allowed in pure evaluation mode",
+            "builtins.storePath"
+        ).atPos(pos).debugThrow();
 
     NixStringContext context;
     auto path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath")).path;
@@ -1492,10 +1480,8 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
     if (!state.store->isStorePath(path.abs()))
         path = CanonPath(canonPath(path.abs(), true));
     if (!state.store->isInStore(path.abs()))
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("path '%1%' is not in the Nix store", path),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("path '%1%' is not in the Nix store", path)
+            .atPos(pos).debugThrow();
     auto path2 = state.store->toStorePath(path.abs()).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
@@ -1623,7 +1609,10 @@ static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto path = realisePath(state, pos, *args[0]);
     auto s = path.readFile();
     if (s.find((char) 0) != std::string::npos)
-        state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
+        state.error<EvalError>(
+            "the contents of the file '%1%' cannot be represented as a Nix string",
+            path
+        ).atPos(pos).debugThrow();
     StorePathSet refs;
     if (state.store->isInStore(path.path.abs())) {
         try {
@@ -1680,10 +1669,11 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
             auto rewrites = state.realiseContext(context);
             path = rewriteStrings(path, rewrites);
         } catch (InvalidPathError & e) {
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>(
+                "cannot find '%1%', since path '%2%' is not valid",
+                path,
+                e.path
+            ).atPos(pos).debugThrow();
         }
 
         searchPath.elements.emplace_back(SearchPath::Elem {
@@ -1752,10 +1742,7 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto type = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.hashFile");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        state.debugThrowLastTrace(Error({
-            .msg = hintfmt("unknown hash type '%1%'", type),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("unknown hash type '%1%'", type).atPos(pos).debugThrow();
 
     auto path = realisePath(state, pos, *args[1]);
 
@@ -2075,13 +2062,12 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
         if (auto p = std::get_if<NixStringContextElem::Opaque>(&c.raw))
             refs.insert(p->path);
         else
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt(
-                    "in 'toFile': the file named '%1%' must not contain a reference "
-                    "to a derivation but contains (%2%)",
-                    name, c.to_string()),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>(
+                "files created by %1% may not reference derivations, but %2% references %3%",
+                "builtins.toFile",
+                name,
+                c.to_string()
+            ).atPos(pos).debugThrow();
     }
 
     auto storePath = settings.readOnlyMode
@@ -2240,7 +2226,10 @@ static void addPath(
             auto dstPath = fetchToStore(
                 *state.store, state.rootPath(CanonPath(path)), name, method, &filter, state.repair);
             if (expectedHash && expectedStorePath != dstPath)
-                state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
+                state.error<EvalError>(
+                    "store path mismatch in (possibly filtered) path added from '%s'",
+                    path
+                ).atPos(pos).debugThrow();
             state.allowAndSetStorePathString(dstPath, v);
         } else
             state.allowAndSetStorePathString(*expectedStorePath, v);
@@ -2339,16 +2328,15 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `sha256` attribute passed to builtins.path"), htSHA256);
         else
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("unsupported argument '%1%' to 'addPath'", state.symbols[attr.name]),
-                .errPos = state.positions[attr.pos]
-            }));
+            state.error<EvalError>(
+                "unsupported argument '%1%' to 'addPath'",
+                state.symbols[attr.name]
+            ).atPos(attr.pos).debugThrow();
     }
     if (!path)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("missing required 'path' attribute in the first argument to builtins.path"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>(
+            "missing required 'path' attribute in the first argument to builtins.path"
+        ).atPos(pos).debugThrow();
     if (name.empty())
         name = path->baseName();
 
@@ -2766,10 +2754,7 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
         return;
     }
     if (!args[0]->isLambda())
-        state.debugThrowLastTrace(TypeError({
-            .msg = hintfmt("'functionArgs' requires a function"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<TypeError>("'functionArgs' requires a function").atPos(pos).debugThrow();
 
     if (!args[0]->lambda.fun->hasFormals()) {
         v.mkAttrs(&state.emptyBindings);
@@ -2939,10 +2924,10 @@ static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Val
 {
     state.forceList(list, pos, "while evaluating the first argument passed to builtins.elemAt");
     if (n < 0 || (unsigned int) n >= list.listSize())
-        state.debugThrowLastTrace(Error({
-            .msg = hintfmt("list index %1% is out of bounds", n),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>(
+            "list index %1% is out of bounds",
+            n
+        ).atPos(pos).debugThrow();
     state.forceValue(*list.listElems()[n], pos);
     v = *list.listElems()[n];
 }
@@ -2987,10 +2972,7 @@ static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value
 {
     state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.tail");
     if (args[0]->listSize() == 0)
-        state.debugThrowLastTrace(Error({
-            .msg = hintfmt("'tail' called on an empty list"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("'tail' called on an empty list").atPos(pos).debugThrow();
 
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
@@ -3240,7 +3222,7 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     auto len = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.genList");
 
     if (len < 0)
-        state.error("cannot create list of size %1%", len).debugThrow<EvalError>();
+        state.error<EvalError>("cannot create list of size %1%", len).atPos(pos).debugThrow();
 
     // More strict than striclty (!) necessary, but acceptable
     // as evaluating map without accessing any values makes little sense.
@@ -3557,10 +3539,7 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
 
     NixFloat f2 = state.forceFloat(*args[1], pos, "while evaluating the second operand of the division");
     if (f2 == 0)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("division by zero"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("division by zero").atPos(pos).debugThrow();
 
     if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
         v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first operand of the division") / f2);
@@ -3569,10 +3548,7 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
         NixInt i2 = state.forceInt(*args[1], pos, "while evaluating the second operand of the division");
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("overflow in integer division"),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>("overflow in integer division").atPos(pos).debugThrow();
 
         v.mkInt(i1 / i2);
     }
@@ -3703,10 +3679,7 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
     int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
 
     if (start < 0)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("negative start position in 'substring'"),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("negative start position in 'substring'").atPos(pos).debugThrow();
 
 
     int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
@@ -3770,10 +3743,7 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args,
     auto type = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.hashString");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        state.debugThrowLastTrace(Error({
-            .msg = hintfmt("unknown hash type '%1%'", type),
-            .errPos = state.positions[pos]
-        }));
+        state.error<EvalError>("unknown hash algorithm '%1%'", type).atPos(pos).debugThrow();
 
     NixStringContext context; // discarded
     auto s = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString");
@@ -3843,15 +3813,13 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>("memory limit exceeded by regular expression '%s'", re)
+                .atPos(pos)
+                .debugThrow();
         } else
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("invalid regular expression '%s'", re),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>("invalid regular expression '%s'", re)
+                .atPos(pos)
+                .debugThrow();
     }
 }
 
@@ -3947,15 +3915,13 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>("memory limit exceeded by regular expression '%s'", re)
+                .atPos(pos)
+                .debugThrow();
         } else
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("invalid regular expression '%s'", re),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>("invalid regular expression '%s'", re)
+                .atPos(pos)
+                .debugThrow();
     }
 }
 
@@ -4031,7 +3997,9 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.replaceStrings");
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.replaceStrings");
     if (args[0]->listSize() != args[1]->listSize())
-        state.error("'from' and 'to' arguments passed to builtins.replaceStrings have different lengths").atPos(pos).debugThrow<EvalError>();
+        state.error<EvalError>(
+            "'from' and 'to' arguments passed to builtins.replaceStrings have different lengths"
+        ).atPos(pos).debugThrow();
 
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index db940f277..1eec8b316 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -98,30 +98,30 @@ static void prim_addDrvOutputDependencies(EvalState & state, const PosIdx pos, V
 
 	auto contextSize = context.size();
     if (contextSize != 1) {
-        throw EvalError({
-            .msg = hintfmt("context of string '%s' must have exactly one element, but has %d", *s, contextSize),
-            .errPos = state.positions[pos]
-        });
+        state.error<EvalError>(
+            "context of string '%s' must have exactly one element, but has %d",
+            *s,
+            contextSize
+        ).atPos(pos).debugThrow();
     }
     NixStringContext context2 {
         (NixStringContextElem { std::visit(overloaded {
             [&](const NixStringContextElem::Opaque & c) -> NixStringContextElem::DrvDeep {
                 if (!c.path.isDerivation()) {
-                    throw EvalError({
-                        .msg = hintfmt("path '%s' is not a derivation",
-                            state.store->printStorePath(c.path)),
-                        .errPos = state.positions[pos],
-                    });
+                    state.error<EvalError>(
+                        "path '%s' is not a derivation",
+                        state.store->printStorePath(c.path)
+                    ).atPos(pos).debugThrow();
                 }
                 return NixStringContextElem::DrvDeep {
                     .drvPath = c.path,
                 };
             },
             [&](const NixStringContextElem::Built & c) -> NixStringContextElem::DrvDeep {
-                throw EvalError({
-                    .msg = hintfmt("`addDrvOutputDependencies` can only act on derivations, not on a derivation output such as '%1%'", c.output),
-                    .errPos = state.positions[pos],
-                });
+                state.error<EvalError>(
+                    "`addDrvOutputDependencies` can only act on derivations, not on a derivation output such as '%1%'",
+                    c.output
+                ).atPos(pos).debugThrow();
             },
             [&](const NixStringContextElem::DrvDeep & c) -> NixStringContextElem::DrvDeep {
                 /* Reuse original item because we want this to be idempotent. */
@@ -261,10 +261,10 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
     for (auto & i : *args[1]->attrs) {
         const auto & name = state.symbols[i.name];
         if (!state.store->isStorePath(name))
-            throw EvalError({
-                .msg = hintfmt("context key '%s' is not a store path", name),
-                .errPos = state.positions[i.pos]
-            });
+            state.error<EvalError>(
+                "context key '%s' is not a store path",
+                name
+            ).atPos(i.pos).debugThrow();
         auto namePath = state.store->parseStorePath(name);
         if (!settings.readOnlyMode)
             state.store->ensurePath(namePath);
@@ -281,10 +281,10 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
         if (iter != i.value->attrs->end()) {
             if (state.forceBool(*iter->value, iter->pos, "while evaluating the `allOutputs` attribute of a string context")) {
                 if (!isDerivation(name)) {
-                    throw EvalError({
-                        .msg = hintfmt("tried to add all-outputs context of %s, which is not a derivation, to a string", name),
-                        .errPos = state.positions[i.pos]
-                    });
+                    state.error<EvalError>(
+                        "tried to add all-outputs context of %s, which is not a derivation, to a string",
+                        name
+                    ).atPos(i.pos).debugThrow();
                 }
                 context.emplace(NixStringContextElem::DrvDeep {
                     .drvPath = namePath,
@@ -296,10 +296,10 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
         if (iter != i.value->attrs->end()) {
             state.forceList(*iter->value, iter->pos, "while evaluating the `outputs` attribute of a string context");
             if (iter->value->listSize() && !isDerivation(name)) {
-                throw EvalError({
-                    .msg = hintfmt("tried to add derivation output context of %s, which is not a derivation, to a string", name),
-                    .errPos = state.positions[i.pos]
-                });
+                state.error<EvalError>(
+                    "tried to add derivation output context of %s, which is not a derivation, to a string",
+                    name
+                ).atPos(i.pos).debugThrow();
             }
             for (auto elem : iter->value->listItems()) {
                 auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
diff --git a/src/libexpr/primops/fetchClosure.cc b/src/libexpr/primops/fetchClosure.cc
index 7fe8203f4..ca2a3df9c 100644
--- a/src/libexpr/primops/fetchClosure.cc
+++ b/src/libexpr/primops/fetchClosure.cc
@@ -26,7 +26,7 @@ static void runFetchClosureWithRewrite(EvalState & state, const PosIdx pos, Stor
                     state.store->printStorePath(fromPath),
                     state.store->printStorePath(rewrittenPath),
                     state.store->printStorePath(*toPathMaybe)),
-                .errPos = state.positions[pos]
+                .pos = state.positions[pos]
             });
         if (!toPathMaybe)
             throw Error({
@@ -35,7 +35,7 @@ static void runFetchClosureWithRewrite(EvalState & state, const PosIdx pos, Stor
                     "Use this value for the 'toPath' attribute passed to 'fetchClosure'",
                     state.store->printStorePath(fromPath),
                     state.store->printStorePath(rewrittenPath)),
-                .errPos = state.positions[pos]
+                .pos = state.positions[pos]
             });
     }
 
@@ -53,7 +53,7 @@ static void runFetchClosureWithRewrite(EvalState & state, const PosIdx pos, Stor
                 "The 'toPath' value '%s' is input-addressed, so it can't possibly be the result of rewriting to a content-addressed path.\n\n"
                 "Set 'toPath' to an empty string to make Nix report the correct content-addressed path.",
                 state.store->printStorePath(toPath)),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
     }
 
@@ -79,7 +79,7 @@ static void runFetchClosureWithContentAddressedPath(EvalState & state, const Pos
                 "to the 'fetchClosure' arguments.\n\n"
                 "Note that to ensure authenticity input-addressed store paths, users must configure a trusted binary cache public key on their systems. This is not needed for content-addressed paths.",
                 state.store->printStorePath(fromPath)),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
     }
 
@@ -102,7 +102,7 @@ static void runFetchClosureWithInputAddressedPath(EvalState & state, const PosId
                 "The store object referred to by 'fromPath' at '%s' is not input-addressed, but 'inputAddressed' is set to 'true'.\n\n"
                 "Remove the 'inputAddressed' attribute (it defaults to 'false') to expect 'fromPath' to be content-addressed",
                 state.store->printStorePath(fromPath)),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
     }
 
@@ -153,14 +153,14 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
         else
             throw Error({
                 .msg = hintfmt("attribute '%s' isn't supported in call to 'fetchClosure'", attrName),
-                .errPos = state.positions[pos]
+                .pos = state.positions[pos]
             });
     }
 
     if (!fromPath)
         throw Error({
             .msg = hintfmt("attribute '%s' is missing in call to 'fetchClosure'", "fromPath"),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
 
     bool inputAddressed = inputAddressedMaybe.value_or(false);
@@ -171,14 +171,14 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
                 .msg = hintfmt("attribute '%s' is set to true, but '%s' is also set. Please remove one of them",
                     "inputAddressed",
                     "toPath"),
-                .errPos = state.positions[pos]
+                .pos = state.positions[pos]
             });
     }
 
     if (!fromStoreUrl)
         throw Error({
             .msg = hintfmt("attribute '%s' is missing in call to 'fetchClosure'", "fromStore"),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
 
     auto parsedURL = parseURL(*fromStoreUrl);
@@ -188,13 +188,13 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
         !(getEnv("_NIX_IN_TEST").has_value() && parsedURL.scheme == "file"))
         throw Error({
             .msg = hintfmt("'fetchClosure' only supports http:// and https:// stores"),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
 
     if (!parsedURL.query.empty())
         throw Error({
             .msg = hintfmt("'fetchClosure' does not support URL query parameters (in '%s')", *fromStoreUrl),
-            .errPos = state.positions[pos]
+            .pos = state.positions[pos]
         });
 
     auto fromStore = openStore(parsedURL.to_string());
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index b9ff01c16..261722d1b 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -38,17 +38,11 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.fetchMercurial");
             else
-                throw EvalError({
-                    .msg = hintfmt("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]),
-                    .errPos = state.positions[attr.pos]
-                });
+                state.error<EvalError>("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]).atPos(attr.pos).debugThrow();
         }
 
         if (url.empty())
-            throw EvalError({
-                .msg = hintfmt("'url' argument required"),
-                .errPos = state.positions[pos]
-            });
+            state.error<EvalError>("'url' argument required").atPos(pos).debugThrow();
 
     } else
         url = state.coerceToString(pos, *args[0], context,
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index d108d7d50..cc84e89b0 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -123,16 +123,14 @@ static void fetchTree(
 
         if (auto aType = args[0]->attrs->get(state.sType)) {
             if (type)
-                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("unexpected attribute 'type'"),
-                    .errPos = state.positions[pos]
-                }));
+                state.error<EvalError>(
+                    "unexpected attribute 'type'"
+                ).atPos(pos).debugThrow();
             type = state.forceStringNoCtx(*aType->value, aType->pos, "while evaluating the `type` attribute passed to builtins.fetchTree");
         } else if (!type)
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>(
+                "attribute 'type' is missing in call to 'fetchTree'"
+            ).atPos(pos).debugThrow();
 
         attrs.emplace("type", type.value());
 
@@ -153,16 +151,15 @@ static void fetchTree(
             else if (attr.value->type() == nInt)
                 attrs.emplace(state.symbols[attr.name], uint64_t(attr.value->integer));
             else
-                state.debugThrowLastTrace(TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
-                    state.symbols[attr.name], showType(*attr.value)));
+                state.error<TypeError>("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
+                    state.symbols[attr.name], showType(*attr.value)).debugThrow();
         }
 
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
-                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
-                    .errPos = state.positions[pos]
-                }));
+                state.error<EvalError>(
+                    "attribute 'name' isn’t supported in call to 'fetchTree'"
+                ).atPos(pos).debugThrow();
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
@@ -183,8 +180,9 @@ static void fetchTree(
     if (!evalSettings.pureEval && !input.isDirect())
         input = lookupInRegistries(state.store, input).first;
 
-    if (evalSettings.pureEval && !input.isLocked())
-        state.debugThrowLastTrace(EvalError("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]));
+    if (evalSettings.pureEval && !input.isLocked()) {
+        state.error<EvalError>("in pure evaluation mode, 'fetchTree' requires a locked input").atPos(pos).debugThrow();
+    }
 
     auto [tree, input2] = input.fetch(state.store);
 
@@ -225,17 +223,13 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the name of the content we should fetch");
             else
-                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("unsupported argument '%s' to '%s'", n, who),
-                    .errPos = state.positions[attr.pos]
-                }));
+                state.error<EvalError>("unsupported argument '%s' to '%s'", n, who)
+                .atPos(pos).debugThrow();
         }
 
         if (!url)
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("'url' argument required"),
-                .errPos = state.positions[pos]
-            }));
+            state.error<EvalError>(
+                "'url' argument required").atPos(pos).debugThrow();
     } else
         url = state.forceStringNoCtx(*args[0], pos, "while evaluating the url we should fetch");
 
@@ -248,7 +242,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
         name = baseNameOf(*url);
 
     if (evalSettings.pureEval && !expectedHash)
-        state.debugThrowLastTrace(EvalError("in pure evaluation mode, '%s' requires a 'sha256' argument", who));
+        state.error<EvalError>("in pure evaluation mode, '%s' requires a 'sha256' argument", who).atPos(pos).debugThrow();
 
     // early exit if pinned and already in the store
     if (expectedHash && expectedHash->type == htSHA256) {
@@ -277,9 +271,15 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
         auto hash = unpack
             ? state.store->queryPathInfo(storePath)->narHash
             : hashFile(htSHA256, state.store->toRealPath(storePath));
-        if (hash != *expectedHash)
-            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)));
+        if (hash != *expectedHash) {
+            state.error<EvalError>(
+                "hash mismatch in file downloaded from '%s':\n  specified: %s\n  got:       %s",
+                *url,
+                expectedHash->to_string(Base32, true),
+                hash.to_string(Base32, true)
+            ).withExitStatus(102)
+            .debugThrow();
+        }
     }
 
     state.allowAndSetStorePathString(storePath, v);
diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index 2f4d4022e..94be7960a 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -83,10 +83,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
     try {
         visit(val, toml::parse(tomlStream, "fromTOML" /* the "filename" */));
     } catch (std::exception & e) { // TODO: toml::syntax_error
-        throw EvalError({
-            .msg = hintfmt("while parsing a TOML string: %s", e.what()),
-            .errPos = state.positions[pos]
-        });
+        state.error<EvalError>("while parsing TOML: %s", e.what()).atPos(pos).debugThrow();
     }
 }
 
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index ac3986c87..ab479520b 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -80,7 +80,7 @@ json printValueAsJSON(EvalState & state, bool strict,
                 try {
                     out.push_back(printValueAsJSON(state, strict, *elem, pos, context, copyToStore));
                 } catch (Error & e) {
-                    e.addTrace({},
+                    e.addTrace(state.positions[pos],
                         hintfmt("while evaluating list element at index %1%", i));
                     throw;
                 }
@@ -99,13 +99,12 @@ json printValueAsJSON(EvalState & state, bool strict,
 
         case nThunk:
         case nFunction:
-            auto e = TypeError({
-                .msg = hintfmt("cannot convert %1% to JSON", showType(v)),
-                .errPos = state.positions[v.determinePos(pos)]
-            });
-            e.addTrace(state.positions[pos], hintfmt("message for the trace"));
-            state.debugThrowLastTrace(e);
-            throw e;
+            state.error<TypeError>(
+                "cannot convert %1% to JSON",
+                showType(v)
+            )
+            .atPos(v.determinePos(pos))
+            .debugThrow();
     }
     return out;
 }
@@ -119,7 +118,8 @@ void printValueAsJSON(EvalState & state, bool strict,
 json ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
     NixStringContext & context, bool copyToStore) const
 {
-    state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
+    state.error<TypeError>("cannot convert %1% to JSON", showType())
+    .debugThrow();
 }
 
 
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index b7b9ed94a..450216ec0 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -105,7 +105,7 @@ class ExternalValueBase
      * Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const;
+    virtual std::string coerceToString(EvalState & state, const PosIdx & pos, NixStringContext & context, bool copyMore, bool copyToStore) const;
 
     /**
      * Compare to another value of the same type. Defaults to uncomparable,
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 9c2ad039a..1b021d0b0 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -339,7 +339,7 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
         return 1;
     } catch (BaseError & e) {
         logError(e.info());
-        return e.status;
+        return e.info().status;
     } catch (std::bad_alloc & e) {
         printError(error + "out of memory");
         return 1;
diff --git a/src/libstore/build/entry-points.cc b/src/libstore/build/entry-points.cc
index 7f0a05d5d..d4bead28e 100644
--- a/src/libstore/build/entry-points.cc
+++ b/src/libstore/build/entry-points.cc
@@ -33,7 +33,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     }
 
     if (failed.size() == 1 && ex) {
-        ex->status = worker.failingExitStatus();
+        ex->withExitStatus(worker.failingExitStatus());
         throw std::move(*ex);
     } else if (!failed.empty()) {
         if (ex) logError(ex->info());
@@ -104,7 +104,7 @@ void Store::ensurePath(const StorePath & path)
 
     if (goal->exitCode != Goal::ecSuccess) {
         if (goal->ex) {
-            goal->ex->status = worker.failingExitStatus();
+            goal->ex->withExitStatus(worker.failingExitStatus());
             throw std::move(*goal->ex);
         } else
             throw Error(worker.failingExitStatus(), "path '%s' does not exist and cannot be created", printStorePath(path));
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 18b2f64c1..170d1fb35 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -119,7 +119,7 @@ struct TunnelLogger : public Logger
             if (GET_PROTOCOL_MINOR(clientVersion) >= 26) {
                 to << STDERR_ERROR << *ex;
             } else {
-                to << STDERR_ERROR << ex->what() << ex->status;
+                to << STDERR_ERROR << ex->what() << ex->info().status;
             }
         }
     }
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 9fac82a02..649caed86 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -334,7 +334,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
      *      try {
      *          e->eval(*this, env, v);
      *          if (v.type() != nAttrs)
-     *              throwTypeError("expected a set but found %1%", v);
+     *              error<TypeError>("expected a set but found %1%", v);
      *      } catch (Error & e) {
      *          e.addTrace(pos, errorCtx);
      *          throw;
@@ -348,7 +348,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
      *      e->eval(*this, env, v);
      *      try {
      *          if (v.type() != nAttrs)
-     *              throwTypeError("expected a set but found %1%", v);
+     *              error<TypeError>("expected a set but found %1%", v);
      *      } catch (Error & e) {
      *          e.addTrace(pos, errorCtx);
      *          throw;
@@ -410,7 +410,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     oss << einfo.msg << "\n";
 
-    printPosMaybe(oss, "", einfo.errPos);
+    printPosMaybe(oss, "", einfo.pos);
 
     auto suggestions = einfo.suggestions.trim();
     if (!suggestions.suggestions.empty()) {
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 234cbe1f6..00dbd7ae2 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -84,9 +84,14 @@ inline bool operator>=(const Trace& lhs, const Trace& rhs);
 struct ErrorInfo {
     Verbosity level;
     hintformat msg;
-    std::shared_ptr<Pos> errPos;
+    std::shared_ptr<Pos> pos;
     std::list<Trace> traces;
 
+    /**
+     * Exit status.
+     */
+    unsigned int status = 1;
+
     Suggestions suggestions;
 
     static std::optional<std::string> programName;
@@ -103,18 +108,21 @@ class BaseError : public std::exception
 protected:
     mutable ErrorInfo err;
 
+    /**
+     * Cached formatted contents of `err.msg`.
+     */
     mutable std::optional<std::string> what_;
+    /**
+     * Format `err.msg` and set `what_` to the resulting value.
+     */
     const std::string & calcWhat() const;
 
 public:
-    unsigned int status = 1; // exit status
-
     BaseError(const BaseError &) = default;
 
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
-        : err { .level = lvlError, .msg = hintfmt(args...) }
-        , status(status)
+        : err { .level = lvlError, .msg = hintfmt(args...), .status = status }
     { }
 
     template<typename... Args>
@@ -149,6 +157,15 @@ public:
     const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
 
+    void withExitStatus(unsigned int status)
+    {
+        err.status = status;
+    }
+
+    void atPos(std::shared_ptr<Pos> pos) {
+        err.pos = pos;
+    }
+
     void pushTrace(Trace trace)
     {
         err.traces.push_front(trace);
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index b4b180760..7d61c0611 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -196,7 +196,7 @@ struct JSONLogger : Logger {
         json["level"] = ei.level;
         json["msg"] = oss.str();
         json["raw_msg"] = ei.msg.str();
-        to_json(json, ei.errPos);
+        to_json(json, ei.pos);
 
         if (loggerSettings.showTrace.get() && !ei.traces.empty()) {
             nlohmann::json traces = nlohmann::json::array();
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index d53e93b7f..75d8b6c6d 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -932,8 +932,8 @@ static void opServe(Strings opFlags, Strings opArgs)
                     store->buildPaths(toDerivedPaths(paths));
                     out << 0;
                 } catch (Error & e) {
-                    assert(e.status);
-                    out << e.status << e.msg();
+                    assert(e.info().status);
+                    out << e.info().status << e.msg();
                 }
                 break;
             }
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 3dcc483fa..9ec861a89 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -104,7 +104,7 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
                     }
                 }
                 else
-                    throw TypeError("value at '%s' is not a string or an attribute set", state->positions[pos]);
+                    state->error<TypeError>("value at '%s' is not a string or an attribute set", state->positions[pos]).debugThrow();
             };
 
             recurse(*v, pos, *writeTo);
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 0da9b1ce9..a11e39534 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -808,10 +808,10 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
         auto templateDir = templateDirAttr->getString();
 
         if (!store->isInStore(templateDir))
-            throw TypeError(
+            evalState->error<TypeError>(
                 "'%s' was not found in the Nix store\n"
                 "If you've set '%s' to a string, try using a path instead.",
-                templateDir, templateDirAttr->getAttrPathStr());
+                templateDir, templateDirAttr->getAttrPathStr()).debugThrow();
 
         std::vector<Path> changedFiles;
         std::vector<Path> conflictedFiles;
@@ -1281,7 +1281,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                 {
                     auto aType = visitor.maybeGetAttr("type");
                     if (!aType || aType->getString() != "app")
-                        throw EvalError("not an app definition");
+                        state->error<EvalError>("not an app definition").debugThrow();
                     if (json) {
                         j.emplace("type", "app");
                     } else {
diff --git a/tests/functional/lang/eval-fail-attr-name-type.err.exp b/tests/functional/lang/eval-fail-attr-name-type.err.exp
index c8d56ba7d..6848a35ed 100644
--- a/tests/functional/lang/eval-fail-attr-name-type.err.exp
+++ b/tests/functional/lang/eval-fail-attr-name-type.err.exp
@@ -14,3 +14,8 @@ error:
             8|
 
        error: expected a string but found an integer: 1
+       at /pwd/lang/eval-fail-attr-name-type.nix:7:17:
+            6| in
+            7|   attrs.puppy.${key}
+             |                 ^
+            8|
diff --git a/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp b/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
index 73f9df8cc..9bbb251e1 100644
--- a/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
+++ b/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
@@ -5,4 +5,4 @@ error:
              | ^
             2|   key = "value"
 
-       error: while parsing a TOML string: Dates and times are not supported
+       error: while parsing TOML: Dates and times are not supported
diff --git a/tests/functional/lang/eval-fail-toJSON.err.exp b/tests/functional/lang/eval-fail-toJSON.err.exp
index 4f6003437..ad267711b 100644
--- a/tests/functional/lang/eval-fail-toJSON.err.exp
+++ b/tests/functional/lang/eval-fail-toJSON.err.exp
@@ -20,6 +20,11 @@ error:
             3|     true
 
        … while evaluating list element at index 3
+         at /pwd/lang/eval-fail-toJSON.nix:2:3:
+            1| builtins.toJSON {
+            2|   a.b = [
+             |   ^
+            3|     true
 
        … while evaluating attribute 'c'
          at /pwd/lang/eval-fail-toJSON.nix:7:7:
diff --git a/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp b/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
index 94784c651..4326c9650 100644
--- a/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
+++ b/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
@@ -7,3 +7,8 @@ error:
             6|
 
        error: expected a string but found a set: { }
+       at /pwd/lang/eval-fail-using-set-as-attr-name.nix:5:10:
+            4| in
+            5|   attr.${key}
+             |          ^
+            6|
diff --git a/tests/unit/libexpr/error_traces.cc b/tests/unit/libexpr/error_traces.cc
index c3e88d95e..7649fa587 100644
--- a/tests/unit/libexpr/error_traces.cc
+++ b/tests/unit/libexpr/error_traces.cc
@@ -12,33 +12,33 @@ namespace nix {
 
     TEST_F(ErrorTraceTest, TraceBuilder) {
         ASSERT_THROW(
-            state.error("Not much").debugThrow<EvalError>(),
+            state.error<EvalError>("puppy").debugThrow(),
             EvalError
         );
 
         ASSERT_THROW(
-            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>(),
+            state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow(),
             EvalError
         );
 
         ASSERT_THROW(
             try {
                 try {
-                    state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+                    state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow();
                 } catch (Error & e) {
-                    e.addTrace(state.positions[noPos], "Something", "");
+                    e.addTrace(state.positions[noPos], "beans", "");
                     throw;
                 }
             } catch (BaseError & e) {
                 ASSERT_EQ(PrintToString(e.info().msg),
-                          PrintToString(hintfmt("Not much")));
+                          PrintToString(hintfmt("puppy")));
                 auto trace = e.info().traces.rbegin();
                 ASSERT_EQ(e.info().traces.size(), 2);
                 ASSERT_EQ(PrintToString(trace->hint),
-                          PrintToString(hintfmt("No more")));
+                          PrintToString(hintfmt("doggy")));
                 trace++;
                 ASSERT_EQ(PrintToString(trace->hint),
-                          PrintToString(hintfmt("Something")));
+                          PrintToString(hintfmt("beans")));
                 throw;
             }
             , EvalError
@@ -47,12 +47,12 @@ namespace nix {
 
     TEST_F(ErrorTraceTest, NestedThrows) {
         try {
-            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+            state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow();
         } catch (BaseError & e) {
             try {
-                state.error("Not much more").debugThrow<EvalError>();
+                state.error<EvalError>("beans").debugThrow();
             } catch (Error & e2) {
-                e.addTrace(state.positions[noPos], "Something", "");
+                e.addTrace(state.positions[noPos], "beans2", "");
                 //e2.addTrace(state.positions[noPos], "Something", "");
                 ASSERT_TRUE(e.info().traces.size() == 2);
                 ASSERT_TRUE(e2.info().traces.size() == 0);