Show both attrPos and select pos in trace

src/libexpr/eval.cc

@@ -1450,7 +1450,10 @@ static std::string showAttrSelectionPath(EvalState & state, Env & env, std::span
 void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 {
     Value vTmp;
-    PosIdx pos2;
+    // current or last *definition site* (the attr, not the select)
+    PosIdx attrPos;
+    const AttrName * unresolvedOrEnd = attrPathStart;
+    // cursor, result if successful
     Value * vAttrs = &vTmp;
 
     e->eval(state, env, vTmp);
@@ -1483,28 +1486,34 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                         allAttrNames.insert(std::string(state.symbols[attr.name]));
                     auto suggestions = Suggestions::bestMatches(allAttrNames, state.symbols[name]);
                     state.error<EvalError>("attribute '%1%' missing", state.symbols[name])
-                        .atPos(pos)
                         .withSuggestions(suggestions)
                         .withFrame(env, *this)
                         .debugThrow();
                 }
             }
             vAttrs = j->value;
-            pos2 = j->pos;
+            attrPos = j->pos;
+            unresolvedOrEnd = &i + 1;
             if (state.countCalls)
-                state.attrSelects->try_emplace_or_visit(pos2, 1, [](auto & i) { i.second++; });
+                state.attrSelects->try_emplace_or_visit(attrPos, 1, [](auto & i) { i.second++; });
         }
 
-        state.forceValue(*vAttrs, (pos2 ? pos2 : this->pos));
+        state.forceValue(*vAttrs, (attrPos ? attrPos : this->pos));
 
     } catch (Error & e) {
-        if (pos2) {
-            auto pos2r = state.positions[pos2];
-            auto origin = std::get_if<SourcePath>(&pos2r.origin);
-            if (!(origin && *origin == state.derivationInternal))
-                state.addErrorTrace(
-                    e, pos2, "while evaluating the attribute '%1%'", showAttrSelectionPath(state, env, getAttrPath()));
+        // Traces are printed in reverse, so we add context before the main item.
+        if (attrPos) {
+            auto attrPosR = state.positions[attrPos];
+            auto origin = std::get_if<SourcePath>(&attrPosR.origin);
+            if (!(origin && *origin == state.derivationInternal)) {
+                auto successPath =
+                    showAttrSelectionPath(state, env, std::span<const AttrName>(attrPathStart, unresolvedOrEnd));
+                state.addErrorTrace(e, attrPos, "from the definition of '%1%'", successPath);
+            }
         }
+        // Add main item: the selection site itself (`a.b`), ie the actual access
+        state.addErrorTrace(
+            e, getPos(), "while evaluating the attribute '%1%'", showAttrSelectionPath(state, env, getAttrPath()));
         throw;
     }
 

src/libexpr/include/nix/expr/nixexpr.hh

@@ -316,6 +316,9 @@ struct ExprSelect : Expr
         return pos;
     }
 
+    /**
+     * @return `std::span<const AttrName>` starting at `attrPathStart`
+     */
     std::span<const AttrName> getAttrPath() const
     {
         return {attrPathStart, nAttrPath};

src/libexpr/parser.y

@@ -384,7 +384,7 @@ expr_simple
   /* Let expressions `let {..., body = ...}' are just desugared
      into `(rec {..., body = ...}).body'. */
   | LET '{' binds '}'
-    { $3->recursive = true; $3->pos = CUR_POS; $$ = state->exprs.add<ExprSelect>(state->exprs.alloc, noPos, $3, state->s.body); }
+    { $3->recursive = true; $3->pos = CUR_POS; $$ = state->exprs.add<ExprSelect>(state->exprs.alloc, CUR_POS, $3, state->s.body); }
   | REC '{' binds '}'
     { $3->recursive = true; $3->pos = CUR_POS; $$ = $3; }
   | '{' binds1 '}'

tests/functional/lang/eval-fail-assert.err.exp

@@ -1,5 +1,11 @@
 error:
        … while evaluating the attribute 'body'
+         at /pwd/lang/eval-fail-assert.nix:1:1:
+            1| let {
+             | ^
+            2|   x =
+
+       … from the definition of 'body'
          at /pwd/lang/eval-fail-assert.nix:7:3:
             6|
             7|   body = x "x";

tests/functional/lang/eval-fail-attr-name-type.err.exp

@@ -1,5 +1,12 @@
 error:
        … while evaluating the attribute 'puppy."${key}"'
+         at /pwd/lang/eval-fail-attr-name-type.nix:7:1:
+            6| in
+            7| attrs.puppy.${key}
+             | ^
+            8|
+
+       … from the definition of 'puppy'
          at /pwd/lang/eval-fail-attr-name-type.nix:3:5:
             2|   attrs = {
             3|     puppy.doggy = { };

tests/functional/lang/eval-fail-blackhole.err.exp

@@ -1,5 +1,11 @@
 error:
        … while evaluating the attribute 'body'
+         at /pwd/lang/eval-fail-blackhole.nix:1:1:
+            1| let {
+             | ^
+            2|   body = x;
+
+       … from the definition of 'body'
          at /pwd/lang/eval-fail-blackhole.nix:2:3:
             1| let {
             2|   body = x;

tests/functional/lang/eval-fail-remove.err.exp

@@ -1,15 +1,23 @@
 error:
        … while evaluating the attribute 'body'
+         at /pwd/lang/eval-fail-remove.nix:1:1:
+            1| let {
+             | ^
+            2|   attrs = {
+
+       … from the definition of 'body'
          at /pwd/lang/eval-fail-remove.nix:7:3:
             6|
             7|   body = (removeAttrs attrs [ "x" ]).x;
              |   ^
             8| }
 
-       error: attribute 'x' missing
-       at /pwd/lang/eval-fail-remove.nix:7:10:
+       … while evaluating the attribute 'x'
+         at /pwd/lang/eval-fail-remove.nix:7:10:
             6|
             7|   body = (removeAttrs attrs [ "x" ]).x;
              |          ^
             8| }
+
+       error: attribute 'x' missing
        Did you mean y?

tests/functional/lang/eval-fail-scope-5.err.exp

@@ -1,5 +1,11 @@
 error:
        … while evaluating the attribute 'body'
+         at /pwd/lang/eval-fail-scope-5.nix:1:1:
+            1| let {
+             | ^
+            2|
+
+       … from the definition of 'body'
          at /pwd/lang/eval-fail-scope-5.nix:13:3:
            12|
            13|   body = f { };

tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp

@@ -1,4 +1,11 @@
 error:
+       … while evaluating the attribute '"${key}"'
+         at /pwd/lang/eval-fail-using-set-as-attr-name.nix:7:1:
+            6| in
+            7| attr.${key}
+             | ^
+            8|
+
        … while evaluating an attribute name
          at /pwd/lang/eval-fail-using-set-as-attr-name.nix:7:8:
             6| in

tests/functional/repl/doc-constant.expected

@@ -27,16 +27,18 @@ Unchangeably constant.
 nix-repl> :doc lib.attr.undocument
 error:
        … while evaluating the attribute 'attr.undocument'
+         at «string»:1:1:
+            1| lib.attr.undocument
+             | ^
+
+       … from the definition of 'attr'
          at /path/to/tests/functional/repl/doc-comments.nix:52:3:
            51|   */
            52|   lib.attr.empty = { };
              |   ^
            53|
 
        error: attribute 'undocument' missing
-       at «string»:1:1:
-            1| lib.attr.undocument
-             | ^
        Did you mean undocumented?
 
 nix-repl> :doc (import ./doc-comments.nix).constant
@@ -89,28 +91,36 @@ error: undefined variable 'missing'
              | ^
 
 nix-repl> :doc lib.missing
-error: attribute 'missing' missing
-       at «string»:1:1:
+error:
+       … while evaluating the attribute 'missing'
+         at «string»:1:1:
             1| lib.missing
              | ^
 
+       error: attribute 'missing' missing
+
 nix-repl> :doc lib.missing.attr
-error: attribute 'missing' missing
-       at «string»:1:1:
+error:
+       … while evaluating the attribute 'missing'
+         at «string»:1:1:
             1| lib.missing.attr
              | ^
 
+       error: attribute 'missing' missing
+
 nix-repl> :doc lib.attr.undocumental
 error:
        … while evaluating the attribute 'attr.undocumental'
+         at «string»:1:1:
+            1| lib.attr.undocumental
+             | ^
+
+       … from the definition of 'attr'
          at /path/to/tests/functional/repl/doc-comments.nix:52:3:
            51|   */
            52|   lib.attr.empty = { };
              |   ^
            53|
 
        error: attribute 'undocumental' missing
-       at «string»:1:1:
-            1| lib.attr.undocumental
-             | ^
        Did you mean undocumented?

tests/functional/repl/inherit-missing-shows-pos.expected

@@ -6,8 +6,11 @@ nix-repl> a = { x = 1; }
 nix-repl> inherit (a) y
 
 nix-repl> y
-error: attribute 'y' missing
-       at «string»:1:13:
+error:
+       … while evaluating the attribute 'y'
+         at «string»:1:13:
             1| inherit (a) y
              |             ^
+
+       error: attribute 'y' missing
        Did you mean x?

tests/functional/repl/missing-attr-shows-def.expected

@@ -0,0 +1,18 @@
+Nix <nix version>
+Type :? for help.
+
+nix-repl> s = { a = { }; }
+
+nix-repl> s.a.b
+error:
+       … while evaluating the attribute 'a.b'
+         at «string»:1:1:
+            1| s.a.b
+             | ^
+
+       … from the definition of 'a'
+         at «string»:1:7:
+            1| s = { a = { }; }
+             |       ^
+
+       error: attribute 'b' missing

tests/functional/repl/missing-attr-shows-def.in

@@ -0,0 +1,3 @@
+# COM: a missing nested attribute reports where the containing set is defined
+s = { a = { }; }
+s.a.b