From 0c6e46e34965ba0db4e0b755ee473868a4fef21f Mon Sep 17 00:00:00 2001
From: regnat <rg@regnat.ovh>
Date: Tue, 8 Mar 2022 06:16:51 +0100
Subject: [PATCH] Add some suggestions to the evaluator
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Make the evaluator show some suggestions when trying to access an
invalid field from an attrset.

```console
$ nix eval --expr '{ foo = 1; }.foa'
error: attribute 'foa' missing

       at «string»:1:1:

            1| { foo = 1; }.foa
             | ^
       Did you mean foo?
```
---
 src/libexpr/eval.cc  | 20 ++++++++++++++++++--
 tests/suggestions.sh |  4 ++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5bf161cc0..1d88e8709 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -727,6 +727,15 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const std::string & s2
     throw EvalError(s, s2);
 }
 
+LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const Suggestions & suggestions, const char * s, const std::string & s2))
+{
+    throw EvalError({
+        .msg = hintfmt(s, s2),
+        .errPos = pos,
+        .suggestions = suggestions,
+    });
+}
+
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const std::string & s2))
 {
     throw EvalError({
@@ -1281,8 +1290,15 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                 }
             } else {
                 state.forceAttrs(*vAttrs, pos);
-                if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
-                    throwEvalError(pos, "attribute '%1%' missing", name);
+                if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end()) {
+                    std::set<std::string> allAttrNames;
+                    for (auto & attr : *vAttrs->attrs)
+                        allAttrNames.insert(attr.name);
+                    throwEvalError(
+                        pos,
+                        Suggestions::bestMatches(allAttrNames, name),
+                        "attribute '%1%' missing", name);
+                }
             }
             vAttrs = j->value;
             pos2 = j->pos;
diff --git a/tests/suggestions.sh b/tests/suggestions.sh
index 16a5a7004..29d5b364b 100644
--- a/tests/suggestions.sh
+++ b/tests/suggestions.sh
@@ -34,3 +34,7 @@ NIX_BUILD_STDERR_WITH_SUGGESTIONS=$(! nix build .\#fob 2>&1 1>/dev/null)
 NIX_BUILD_STDERR_WITH_NO_CLOSE_SUGGESTION=$(! nix build .\#bar 2>&1 1>/dev/null)
 [[ ! "$NIX_BUILD_STDERR_WITH_NO_CLOSE_SUGGESTION" =~ "Did you mean" ]] || \
     fail "The nix build stderr shouldn’t suggest anything if there’s nothing relevant to suggest"
+
+NIX_EVAL_STDERR_WITH_SUGGESTIONS=$(! nix build --impure --expr '(builtins.getFlake (builtins.toPath ./.)).packages.'$system'.fob' 2>&1 1>/dev/null)
+[[ "$NIX_EVAL_STDERR_WITH_SUGGESTIONS" =~ "Did you mean one of fo1, fo2, foo or fooo?" ]] || \
+    fail "The evaluator should suggest the three closest possiblities"