Resolve to a concrete impl instead of using fuzzy search

Author: oli-obk

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -831,18 +831,21 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         );
         let mut diag = struct_span_code_err!(self.dcx(), span, E0277, "{}", err_msg);
         *diag.long_ty_path() = file;
-        if !self.predicate_may_hold(&Obligation::new(
+        let obligation = Obligation::new(
             self.tcx,
             ObligationCause::dummy(),
             param_env,
             trait_ref,
-        )) {
+        );
+        if !self.predicate_may_hold(&obligation) {
             diag.downgrade_to_delayed_bug();
         }
-        for candidate in self.find_similar_impl_candidates(trait_ref) {
-            let CandidateSimilarity::Exact { .. } = candidate.similarity else { continue };
-            let impl_did = candidate.impl_def_id;
-            let trait_did = candidate.trait_ref.def_id;
+
+        if let Ok(Some(ImplSource::UserDefined(impl_data))) =
+            SelectionContext::new(self).select(&obligation.with(self.tcx, trait_ref.skip_binder()))
+        {
+            let impl_did = impl_data.impl_def_id;
+            let trait_did = trait_ref.def_id();
             let impl_span = self.tcx.def_span(impl_did);
             let trait_name = self.tcx.item_name(trait_did);
 

tests/ui/consts/issue-94675.stderr

@@ -3,19 +3,27 @@ error[E0277]: the trait bound `Vec<usize>: [const] Index<_>` is not satisfied
    |
 LL |         self.bar[0] = baz.len();
    |         ^^^^^^^^^^^
+   |
+note: trait `Index` is implemented but not `const`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 
 error[E0277]: the trait bound `Vec<usize>: [const] IndexMut<usize>` is not satisfied
   --> $DIR/issue-94675.rs:11:9
    |
 LL |         self.bar[0] = baz.len();
    |         ^^^^^^^^^^^
+   |
+note: trait `IndexMut` is implemented but not `const`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 
 error[E0277]: the trait bound `Vec<usize>: [const] Index<usize>` is not satisfied
   --> $DIR/issue-94675.rs:11:9
    |
 LL |         self.bar[0] = baz.len();
    |         ^^^^^^^^^^^
    |
+note: trait `Index` is implemented but not `const`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 note: required by a bound in `std::ops::IndexMut::index_mut`
   --> $SRC_DIR/core/src/ops/index.rs:LL:COL
 

tests/ui/traits/const-traits/call-generic-method-fail.stderr

@@ -3,11 +3,6 @@ error[E0277]: the trait bound `T: [const] PartialEq` is not satisfied
    |
 LL |     *t == *t
    |     ^^^^^^^^
-   |
-note: trait `PartialEq` is implemented but not `const`
-  --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
-note: trait `PartialEq` is implemented but not `const`
-  --> $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
 
 error: aborting due to 1 previous error