Make derive(Trait) suggestion more accurate

Only suggest `derive(PartialEq)` when both LHS and RHS types
are the same, otherwise the suggestion is not useful.

Author: estebank

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -2252,6 +2252,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         err: &mut Diagnostic,
         errors: Vec<FulfillmentError<'tcx>>,
+        suggest_derive: bool,
     ) {
         let all_local_types_needing_impls =
             errors.iter().all(|e| match e.obligation.predicate.kind().skip_binder() {
@@ -2322,18 +2323,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .iter()
             .map(|e| (e.obligation.predicate, None, Some(e.obligation.cause.clone())))
             .collect();
-        self.suggest_derive(err, &preds);
+        if suggest_derive {
+            self.suggest_derive(err, &preds);
+        } else {
+            // The predicate comes from a binop where the lhs and rhs have different types.
+            let _ = self.note_predicate_source_and_get_derives(err, &preds);
+        }
     }
 
-    pub fn suggest_derive(
+    fn note_predicate_source_and_get_derives(
         &self,
         err: &mut Diagnostic,
         unsatisfied_predicates: &[(
             ty::Predicate<'tcx>,
             Option<ty::Predicate<'tcx>>,
             Option<ObligationCause<'tcx>>,
         )],
-    ) {
+    ) -> Vec<(String, Span, String)> {
         let mut derives = Vec::<(String, Span, Symbol)>::new();
         let mut traits = Vec::new();
         for (pred, _, _) in unsatisfied_predicates {
@@ -2419,6 +2425,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             );
         }
 
+        derives_grouped
+    }
+
+    pub fn suggest_derive(
+        &self,
+        err: &mut Diagnostic,
+        unsatisfied_predicates: &[(
+            ty::Predicate<'tcx>,
+            Option<ty::Predicate<'tcx>>,
+            Option<ObligationCause<'tcx>>,
+        )],
+    ) {
+        let derives_grouped =
+            self.note_predicate_source_and_get_derives(err, unsatisfied_predicates);
         for (self_name, self_span, traits) in &derives_grouped {
             err.span_suggestion_verbose(
                 self_span.shrink_to_lo(),

compiler/rustc_hir_typeck/src/op.rs

@@ -318,7 +318,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             lhs_expr.span,
                             format!("cannot use `{}=` on type `{}`", op.node.as_str(), lhs_ty),
                         );
-                        self.note_unmet_impls_on_type(&mut err, errors);
+                        self.note_unmet_impls_on_type(&mut err, errors, false);
                         (err, None)
                     }
                     IsAssign::No => {
@@ -375,7 +375,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             err.span_label(lhs_expr.span, lhs_ty.to_string());
                             err.span_label(rhs_expr.span, rhs_ty.to_string());
                         }
-                        self.note_unmet_impls_on_type(&mut err, errors);
+                        let suggest_derive = self.can_eq(self.param_env, lhs_ty, rhs_ty);
+                        self.note_unmet_impls_on_type(&mut err, errors, suggest_derive);
                         (err, output_def_id)
                     }
                 };
@@ -852,7 +853,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             Str | Never | Char | Tuple(_) | Array(_, _) => {}
                             Ref(_, lty, _) if *lty.kind() == Str => {}
                             _ => {
-                                self.note_unmet_impls_on_type(&mut err, errors);
+                                self.note_unmet_impls_on_type(&mut err, errors, true);
                             }
                         }
                     }

tests/ui/binop/binary-op-suggest-deref.stderr

@@ -270,11 +270,6 @@ note: an implementation of `PartialEq<&&{integer}>` might be missing for `Foo`
    |
 LL |     struct Foo;
    |     ^^^^^^^^^^ must implement `PartialEq<&&{integer}>`
-help: consider annotating `Foo` with `#[derive(PartialEq)]`
-   |
-LL +     #[derive(PartialEq)]
-LL |     struct Foo;
-   |
 
 error[E0277]: can't compare `&String` with `str`
   --> $DIR/binary-op-suggest-deref.rs:69:20

tests/ui/issues/issue-62375.stderr

@@ -11,11 +11,6 @@ note: an implementation of `PartialEq<fn(()) -> A {A::Value}>` might be missing
    |
 LL | enum A {
    | ^^^^^^ must implement `PartialEq<fn(()) -> A {A::Value}>`
-help: consider annotating `A` with `#[derive(PartialEq)]`
-   |
-LL + #[derive(PartialEq)]
-LL | enum A {
-   |
 help: use parentheses to construct this tuple variant
    |
 LL |     a == A::Value(/* () */);