From 3a5855353e5dc2986c38bb66eaff5538dea70da1 Mon Sep 17 00:00:00 2001
From: Guillaume Maudoux <guillaume.maudoux@tweag.io>
Date: Fri, 4 Mar 2022 21:47:58 +0100
Subject: [PATCH] Add detailed error mesage for coerceTo{String,Path}

---
 src/libexpr/eval.cc                   | 34 +++++++-------
 src/libexpr/eval.hh                   |  7 +--
 src/libexpr/flake/flake.cc            |  2 +-
 src/libexpr/get-drvs.cc               |  6 +--
 src/libexpr/primops.cc                | 66 +++++++++++++++------------
 src/libexpr/primops/context.cc        |  4 +-
 src/libexpr/primops/fetchMercurial.cc |  4 +-
 src/libexpr/primops/fetchTree.cc      |  4 +-
 src/libexpr/value.hh                  |  2 +-
 src/nix-env/user-env.cc               |  4 +-
 src/nix/bundle.cc                     |  4 +-
 src/nix/eval.cc                       |  2 +-
 src/nix/flake.cc                      |  2 +-
 src/nix/repl.cc                       |  4 +-
 14 files changed, 79 insertions(+), 66 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 6a21701f3..b2a8651d9 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1775,7 +1775,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
             /* skip canonization of first path, which would only be not
             canonized in the first place if it's coming from a ./${foo} type
             path */
-            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first);
+            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first, "");
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -1958,14 +1958,15 @@ std::optional<std::string> EvalState::tryAttrsToString(const Pos & pos, Value &
     if (i != v.attrs->end()) {
         Value v1;
         callFunction(*i->value, v, v1, pos);
-        return coerceToString(pos, v1, context, coerceMore, copyToStore).toOwned();
+        return coerceToString(pos, v1, context, coerceMore, copyToStore,
+                "While evaluating the result of the `toString` attribute").toOwned();
     }
 
     return {};
 }
 
 BackedStringView EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath)
+    bool coerceMore, bool copyToStore, bool canonicalizePath, const std::string & errorCtx)
 {
     forceValue(v, pos);
 
@@ -1988,12 +1989,12 @@ BackedStringView EvalState::coerceToString(const Pos & pos, Value & v, PathSet &
         if (maybeString)
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end()) throwTypeError(pos, "cannot coerce a set to a string");
-        return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
+        if (i == v.attrs->end()) throwTypeError(pos, "%2%: cannot coerce %1% to a string", v, errorCtx);
+        return coerceToString(pos, *i->value, context, coerceMore, copyToStore, canonicalizePath, errorCtx);
     }
 
     if (v.type() == nExternal)
-        return v.external->coerceToString(pos, context, coerceMore, copyToStore);
+        return v.external->coerceToString(pos, context, coerceMore, copyToStore, errorCtx);
 
     if (coerceMore) {
 
@@ -2008,7 +2009,8 @@ BackedStringView EvalState::coerceToString(const Pos & pos, Value & v, PathSet &
         if (v.isList()) {
             std::string result;
             for (auto [n, v2] : enumerate(v.listItems())) {
-                result += *coerceToString(pos, *v2, context, coerceMore, copyToStore);
+                result += *coerceToString(pos, *v2, context, coerceMore, copyToStore, canonicalizePath,
+                        errorCtx + ": While evaluating one element of the list");
                 if (n < v.listSize() - 1
                     /* !!! not quite correct */
                     && (!v2->isList() || v2->listSize() != 0))
@@ -2018,7 +2020,7 @@ BackedStringView EvalState::coerceToString(const Pos & pos, Value & v, PathSet &
         }
     }
 
-    throwTypeError(pos, "cannot coerce %1% to a string", v);
+    throwTypeError(pos, "%2%: cannot coerce %1% to a string", v, errorCtx);
 }
 
 
@@ -2046,22 +2048,22 @@ std::string EvalState::copyPathToStore(PathSet & context, const Path & path)
 }
 
 
-Path EvalState::coerceToPath(const Pos & pos, Value & v, PathSet & context)
+Path EvalState::coerceToPath(const Pos & pos, Value & v, PathSet & context, const std::string & errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
     if (path == "" || path[0] != '/')
-        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", path);
+        throwEvalError(pos, "%2%: string '%1%' doesn't represent an absolute path", path, errorCtx);
     return path;
 }
 
 
-StorePath EvalState::coerceToStorePath(const Pos & pos, Value & v, PathSet & context)
+StorePath EvalState::coerceToStorePath(const Pos & pos, Value & v, PathSet & context, const std::string & errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, false, false, errorCtx).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
     throw EvalError({
-        .msg = hintfmt("path '%1%' is not in the Nix store", path),
+        .msg = hintfmt("%2%: path '%1%' is not in the Nix store", path, errorCtx),
         .errPos = pos
     });
 }
@@ -2263,10 +2265,10 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, const std::string & errorCtx) const
 {
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType()),
+        .msg = hintfmt("%2%: cannot coerce %1% to a string", showType(), errorCtx),
         .errPos = pos
     });
 }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index ae2a5f4f1..9e8ac4926 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -262,17 +262,18 @@ public:
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const Pos & pos, Value & v, PathSet & context,
         bool coerceMore = false, bool copyToStore = true,
-        bool canonicalizePath = true);
+        bool canonicalizePath = true,
+        const std::string & errorCtx = "");
 
     std::string copyPathToStore(PathSet & context, const Path & path);
 
     /* Path coercion.  Converts strings, paths and derivations to a
        path.  The result is guaranteed to be a canonicalised, absolute
        path.  Nothing is copied to the store. */
-    Path coerceToPath(const Pos & pos, Value & v, PathSet & context);
+    Path coerceToPath(const Pos & pos, Value & v, PathSet & context, const std::string & errorCtx);
 
     /* Like coerceToPath, but the result must be a store path. */
-    StorePath coerceToStorePath(const Pos & pos, Value & v, PathSet & context);
+    StorePath coerceToStorePath(const Pos & pos, Value & v, PathSet & context, const std::string & errorCtx);
 
 public:
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index b0f087b11..fe2daaf21 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -260,7 +260,7 @@ static Flake getFlake(
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
                     setting.name,
-                    state.coerceToString(*setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
+                    state.coerceToString(*setting.pos, *setting.value, emptyContext, false, true, true, "") .toOwned());
             }
             else if (setting.value->type() == nInt)
                 flake.config.settings.insert({setting.name, state.forceInt(*setting.value, *setting.pos, "")});
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 975490d36..9bb545430 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -74,7 +74,7 @@ std::optional<StorePath> DrvInfo::queryDrvPath() const
         if (i == attrs->end())
             drvPath = {std::nullopt};
         else
-            drvPath = {state->coerceToStorePath(*i->pos, *i->value, context)};
+            drvPath = {state->coerceToStorePath(*i->pos, *i->value, context, "Whyle evaluating the drv path of a DrvInfo")};
     }
     return drvPath.value_or(std::nullopt);
 }
@@ -94,7 +94,7 @@ StorePath DrvInfo::queryOutPath() const
         Bindings::iterator i = attrs->find(state->sOutPath);
         PathSet context;
         if (i != attrs->end())
-            outPath = state->coerceToStorePath(*i->pos, *i->value, context);
+            outPath = state->coerceToStorePath(*i->pos, *i->value, context, "While evaluating the output path of a DrvInfo");
     }
     if (!outPath)
         throw UnimplementedError("CA derivations are not yet supported");
@@ -122,7 +122,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
                 Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
                 if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
                 PathSet context;
-                outputs.emplace(name, state->coerceToStorePath(*outPath->pos, *outPath->value, context));
+                outputs.emplace(name, state->coerceToStorePath(*outPath->pos, *outPath->value, context, "While evaluating the outPath of an output path of a DrvInfo"));
             }
         } else
             outputs.emplace("out", queryOutPath());
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e05e6d45a..14db24f68 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -103,7 +103,7 @@ static Path realisePath(EvalState & state, const Pos & pos, Value & v, const Rea
     auto path = [&]()
     {
         try {
-            return state.coerceToPath(pos, v, context);
+            return state.coerceToPath(pos, v, context, "While realising the context of a path");
         } catch (Error & e) {
             e.addTrace(pos, "while realising the context of a path");
             throw;
@@ -350,10 +350,12 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
         });
     }
     PathSet context;
-    auto program = state.coerceToString(pos, *elems[0], context, false, false).toOwned();
+    auto program = state.coerceToString(pos, *elems[0], context, false, false,
+           "While evaluating the first element of the argument passed to builtins.exec").toOwned();
     Strings commandArgs;
     for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
-        commandArgs.push_back(state.coerceToString(pos, *elems[i], context, false, false).toOwned());
+        commandArgs.push_back(state.coerceToString(pos, *elems[i], context, false, false,
+                    "While evaluating an element of the argument passed to builtins.exec").toOwned());
     }
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
@@ -714,7 +716,8 @@ static RegisterPrimOp primop_abort({
     .fun = [](EvalState & state, const Pos & pos, Value * * args, Value & v)
     {
         PathSet context;
-        auto s = state.coerceToString(pos, *args[0], context).toOwned();
+        auto s = state.coerceToString(pos, *args[0], context,
+                "While evaluating the error message passed to builtins.abort").toOwned();
         throw Abort("evaluation aborted with the following error message: '%1%'", s);
     }
 });
@@ -732,7 +735,8 @@ static RegisterPrimOp primop_throw({
     .fun = [](EvalState & state, const Pos & pos, Value * * args, Value & v)
     {
       PathSet context;
-      auto s = state.coerceToString(pos, *args[0], context).toOwned();
+      auto s = state.coerceToString(pos, *args[0], context,
+              "While evaluating the error message passed to builtin.throw").toOwned();
       throw ThrownError(s);
     }
 });
@@ -744,7 +748,8 @@ static void prim_addErrorContext(EvalState & state, const Pos & pos, Value * * a
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(std::nullopt, state.coerceToString(pos, *args[0], context).toOwned());
+        e.addTrace(std::nullopt, state.coerceToString(pos, *args[0], context,
+                    "While evaluating the error message passed to builtins.addErrorContext").toOwned());
         throw;
     }
 }
@@ -757,7 +762,8 @@ static RegisterPrimOp primop_addErrorContext(RegisterPrimOp::Info {
 
 static void prim_ceil(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos), "While evaluating the first argument passed to builtins.ceil");
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos),
+            "While evaluating the first argument passed to builtins.ceil");
     v.mkInt(ceil(value));
 }
 
@@ -1028,7 +1034,8 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             }
 
             if (i->name == state.sContentAddressed) {
-                contentAddressed = state.forceBool(*i->value, pos, "While evaluating the `__contentAddressed` attribute passed to builtins.derivationStrict");
+                contentAddressed = state.forceBool(*i->value, pos,
+                        "While evaluating the `__contentAddressed` attribute passed to builtins.derivationStrict");
                 if (contentAddressed)
                     settings.requireExperimentalFeature(Xp::CaDerivations);
             }
@@ -1036,9 +1043,11 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
             else if (i->name == state.sArgs) {
-                state.forceList(*i->value, pos, "While evaluating the `args` attribute passed to builtins.derivationStrict");
+                state.forceList(*i->value, pos,
+                        "While evaluating the `args` attribute passed to builtins.derivationStrict");
                 for (auto elem : i->value->listItems()) {
-                    auto s = state.coerceToString(posDrvName, *elem, context, true).toOwned();
+                    auto s = state.coerceToString(posDrvName, *elem, context, true,
+                            "While evaluating an element of the `args` argument passed to builtins.derivationStrict").toOwned();
                     drv.args.push_back(s);
                 }
             }
@@ -1074,7 +1083,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                     }
 
                 } else {
-                    auto s = state.coerceToString(*i->pos, *i->value, context, true).toOwned();
+                    auto s = state.coerceToString(*i->pos, *i->value, context, true, "While evaluating an attribute passed to builtins.derivationStrict").toOwned();
                     drv.env.emplace(key, s);
                     if (i->name == state.sBuilder) drv.builder = std::move(s);
                     else if (i->name == state.sSystem) drv.platform = std::move(s);
@@ -1306,7 +1315,7 @@ static RegisterPrimOp primop_placeholder({
 static void prim_toPath(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context);
+    Path path = state.coerceToPath(pos, *args[0], context, "While evaluating the first argument passed to builtins.toPath");
     v.mkString(canonPath(path), context);
 }
 
@@ -1337,7 +1346,7 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
         });
 
     PathSet context;
-    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
+    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "While evaluating the first argument passed to builtins.storePath"));
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
@@ -1407,7 +1416,7 @@ static RegisterPrimOp primop_pathExists({
 static void prim_baseNameOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context, false, false)), context);
+    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context, false, false, "While evaluating the first argument passed to builtins.baseNameOf")), context);
 }
 
 static RegisterPrimOp primop_baseNameOf({
@@ -1427,7 +1436,7 @@ static RegisterPrimOp primop_baseNameOf({
 static void prim_dirOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto path = state.coerceToString(pos, *args[0], context, false, false);
+    auto path = state.coerceToString(pos, *args[0], context, false, false, "While evaluating the first argument passed to builtins.dirOf");
     auto dir = dirOf(*path);
     if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
 }
@@ -1484,7 +1493,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         std::string prefix;
         Bindings::iterator i = v2->attrs->find(state.sPrefix);
         if (i != v2->attrs->end())
-            prefix = state.forceStringNoCtx(*i->value, pos, "While evaluating the `prefix` attribute passed to an element of the list passed to builtins.findFile");
+            prefix = state.forceStringNoCtx(*i->value, pos, "While evaluating the `prefix` attribute of an element of the list passed to builtins.findFile");
 
         i = getAttr(
             state,
@@ -1495,7 +1504,8 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         );
 
         PathSet context;
-        auto path = state.coerceToString(pos, *i->value, context, false, false).toOwned();
+        auto path = state.coerceToString(pos, *i->value, context, false, false,
+                "While evaluating the `path` attribute of an element of the list passed to builtins.findFile").toOwned();
 
         try {
             auto rewrites = state.realiseContext(context);
@@ -1945,7 +1955,7 @@ static void addPath(
 static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context);
+    Path path = state.coerceToPath(pos, *args[1], context, "While evaluating the second argument (the path to filter) passed to builtins.filterSource");
 
     state.forceValue(*args[0], pos);
     if (args[0]->type() != nFunction)
@@ -2027,7 +2037,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
     for (auto & attr : *args[0]->attrs) {
         auto & n(attr.name);
         if (n == "path")
-            path = state.coerceToPath(*attr.pos, *attr.value, context);
+            path = state.coerceToPath(*attr.pos, *attr.value, context, "While evaluating the `path` attribute passed to builtins.path");
         else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, *attr.pos, "while evaluating the `name` attribute passed to builtins.path");
         else if (n == "filter") {
@@ -2045,7 +2055,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
     }
     if (path.empty())
         throw EvalError({
-            .msg = hintfmt("'path' required"),
+            .msg = hintfmt("Missing required 'path' attribute in the first argument to builtins.path"),
             .errPos = pos
         });
     if (name.empty())
@@ -3318,7 +3328,7 @@ static RegisterPrimOp primop_lessThan({
 static void prim_toString(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context, true, false);
+    auto s = state.coerceToString(pos, *args[0], context, true, false, "While evaluating the first argument passed to builtins.toString");
     v.mkString(*s, context);
 }
 
@@ -3352,10 +3362,10 @@ static RegisterPrimOp primop_toString({
    non-negative. */
 static void prim_substring(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    int start = state.forceInt(*args[0], pos, "While evaluating the first argument passed to builtins.substring");
-    int len = state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.substring");
+    int start = state.forceInt(*args[0], pos, "While evaluating the first argument (the start offset) passed to builtins.substring");
+    int len = state.forceInt(*args[1], pos, "While evaluating the second argument (the substring length) passed to builtins.substring");
     PathSet context;
-    auto s = state.coerceToString(pos, *args[2], context);
+    auto s = state.coerceToString(pos, *args[2], context, "While evaluating the third argument (the string) passed to builtins.substring");
 
     if (start < 0)
         throw EvalError({
@@ -3389,7 +3399,7 @@ static RegisterPrimOp primop_substring({
 static void prim_stringLength(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "While evaluating the argument passed to builtins.stringLength");
     v.mkInt(s->size());
 }
 
@@ -3641,8 +3651,8 @@ static void prim_concatStringsSep(EvalState & state, const Pos & pos, Value * *
 {
     PathSet context;
 
-    auto sep = state.forceString(*args[0], context, pos, "While evaluating the first argument passed to builtins.concatStringsSep");
-    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.concatStringsSep");
+    auto sep = state.forceString(*args[0], context, pos, "While evaluating the first argument (the separator string) passed to builtins.concatStringsSep");
+    state.forceList(*args[1], pos, "While evaluating the second argument (the list of strings to concat) passed to builtins.concatStringsSep");
 
     std::string res;
     res.reserve((args[1]->listSize() + 32) * sep.size());
@@ -3650,7 +3660,7 @@ static void prim_concatStringsSep(EvalState & state, const Pos & pos, Value * *
 
     for (auto elem : args[1]->listItems()) {
         if (first) first = false; else res += sep;
-        res += *state.coerceToString(pos, *elem, context);
+        res += *state.coerceToString(pos, *elem, context, "While evaluating one element of the list of strings to concat passed to builtins.concatStringsSep");
     }
 
     v.mkString(res, context);
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index f1feb4dc6..78320dc09 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -7,7 +7,7 @@ namespace nix {
 static void prim_unsafeDiscardStringContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "While evaluating the argument passed to builtins.unsafeDiscardStringContext");
     v.mkString(*s);
 }
 
@@ -33,7 +33,7 @@ static RegisterPrimOp primop_hasContext("__hasContext", 1, prim_hasContext);
 static void prim_unsafeDiscardOutputDependency(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "While evaluating the argument passed to builtins.unsafeDiscardOutputDependency");
 
     PathSet context2;
     for (auto & p : context)
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index 199f7508e..dcaf3d362 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -22,7 +22,7 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(attr.name);
             if (n == "url")
-                url = state.coerceToString(*attr.pos, *attr.value, context, false, false).toOwned();
+                url = state.coerceToString(*attr.pos, *attr.value, context, false, false, "While evaluating the `url` attribute passed to builtins.fetchMercurial").toOwned();
             else if (n == "rev") {
                 // Ugly: unlike fetchGit, here the "rev" attribute can
                 // be both a revision or a branch/tag name.
@@ -48,7 +48,7 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
             });
 
     } else
-        url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        url = state.coerceToString(pos, *args[0], context, false, false, "While evaluating the first argument passed to builtins.fetchMercurial").toOwned();
 
     // FIXME: git externals probably can be used to bypass the URI
     // whitelist. Ah well.
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index a40eddfca..2baa272cb 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -125,7 +125,7 @@ static void fetchTree(
             if (attr.name == state.sType) continue;
             state.forceValue(*attr.value, *attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
-                auto s = state.coerceToString(*attr.pos, *attr.value, context, false, false).toOwned();
+                auto s = state.coerceToString(*attr.pos, *attr.value, context, false, false, "").toOwned();
                 attrs.emplace(attr.name,
                     attr.name == "url"
                     ? type == "git"
@@ -151,7 +151,7 @@ static void fetchTree(
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
-        auto url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        auto url = state.coerceToString(pos, *args[0], context, false, false, "While evaluating the first argument passed to the fetcher").toOwned();
 
         if (type == "git") {
             fetchers::Attrs attrs;
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index d0fa93e92..858227018 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -85,7 +85,7 @@ class ExternalValueBase
     /* Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const;
+    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, const std::string & errorCtx) const;
 
     /* Compare to another value of the same type. Defaults to uncomparable,
      * i.e. always false.
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index af4f350ff..b30ccc7f6 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -132,9 +132,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     state.forceValue(topLevel, [&]() { return topLevel.determinePos(noPos); });
     PathSet context;
     Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
-    auto topLevelDrv = state.coerceToStorePath(*aDrvPath.pos, *aDrvPath.value, context);
+    auto topLevelDrv = state.coerceToStorePath(*aDrvPath.pos, *aDrvPath.value, context, "");
     Attr & aOutPath(*topLevel.attrs->find(state.sOutPath));
-    auto topLevelOut = state.coerceToStorePath(*aOutPath.pos, *aOutPath.value, context);
+    auto topLevelOut = state.coerceToStorePath(*aOutPath.pos, *aOutPath.value, context, "");
 
     /* Realise the resulting store expression. */
     debug("building user environment");
diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index aa074edc2..9ff66899e 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -97,13 +97,13 @@ struct CmdBundle : InstallableCommand
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
         PathSet context2;
-        auto drvPath = evalState->coerceToStorePath(*attr1->pos, *attr1->value, context2);
+        auto drvPath = evalState->coerceToStorePath(*attr1->pos, *attr1->value, context2, "");
 
         auto attr2 = vRes->attrs->get(evalState->sOutPath);
         if (!attr2)
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
-        auto outPath = evalState->coerceToStorePath(*attr2->pos, *attr2->value, context2);
+        auto outPath = evalState->coerceToStorePath(*attr2->pos, *attr2->value, context2, "");
 
         store->buildPaths({ DerivedPath::Built { drvPath } });
 
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 8cd04d5fe..8ee3ba45b 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -107,7 +107,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         else if (raw) {
             stopProgressBar();
-            std::cout << *state->coerceToString(noPos, *v, context);
+            std::cout << *state->coerceToString(noPos, *v, context, "While generating the eval command output");
         }
 
         else if (json) {
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 703c3beb2..356f89713 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -448,7 +448,7 @@ struct CmdFlakeCheck : FlakeCommand
                 if (auto attr = v.attrs->get(state->symbols.create("path"))) {
                     if (attr->name == state->symbols.create("path")) {
                         PathSet context;
-                        auto path = state->coerceToPath(*attr->pos, *attr->value, context);
+                        auto path = state->coerceToPath(*attr->pos, *attr->value, context, "");
                         if (!store->isInStore(path))
                             throw Error("template '%s' has a bad 'path' attribute");
                         // TODO: recursively check the flake in 'path'.
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 7011ff939..0cb68552c 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -461,7 +461,7 @@ bool NixRepl::processLine(std::string line)
 
         if (v.type() == nPath || v.type() == nString) {
             PathSet context;
-            auto filename = state->coerceToString(noPos, v, context);
+            auto filename = state->coerceToString(noPos, v, context, "While evaluating the filename to edit");
             pos.file = state->symbols.create(*filename);
         } else if (v.isLambda()) {
             pos = v.lambda.fun->pos;
@@ -780,7 +780,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
             Bindings::iterator i = v.attrs->find(state->sDrvPath);
             PathSet context;
             if (i != v.attrs->end())
-                str << state->store->printStorePath(state->coerceToStorePath(*i->pos, *i->value, context));
+                str << state->store->printStorePath(state->coerceToStorePath(*i->pos, *i->value, context, "While evaluating the drvPath of a derivation"));
             else
                 str << "???";
             str << "»";