Add a diagnostic for similarly named traits

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

@@ -487,6 +487,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             );
                         }
 
+                        self.suggest_impl_similarly_named_trait(&mut err, &obligation, leaf_trait_predicate);
+
                         self.try_to_add_help_message(
                             &root_obligation,
                             &obligation,

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

@@ -5108,6 +5108,44 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             _ => {}
         }
     }
+    pub(crate) fn suggest_impl_similarly_named_trait(
+        &self,
+        err: &mut Diag<'_>,
+        obligation: &PredicateObligation<'tcx>,
+        trait_predicate: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        let trait_def_id = trait_predicate.def_id();
+        let trait_name = self.tcx.item_name(trait_def_id);
+        if let Some(other_trait_def_id) = self.tcx.all_traits_including_private().find(|def_id| {
+            trait_def_id != *def_id
+                && trait_name == self.tcx.item_name(def_id)
+                && self
+                    .tcx
+                    .all_impls(*def_id)
+                    .find(|impl_def_id| {
+                        let Some(trait_ref) = self.tcx.impl_trait_ref(impl_def_id) else {
+                            return false;
+                        };
+                        self.predicate_must_hold_modulo_regions(&Obligation::new(
+                            self.tcx,
+                            obligation.cause.clone(),
+                            obligation.param_env,
+                            trait_ref
+                                .skip_binder()
+                                .with_self_ty(self.tcx, trait_predicate.skip_binder().self_ty()),
+                        ))
+                    })
+                    .is_some()
+        }) {
+            err.note(format!(
+                "`{}` implements similarly named `{}`, but not `{}`",
+                trait_predicate.self_ty(),
+                self.tcx.def_path_str(other_trait_def_id),
+                trait_predicate.print_modifiers_and_trait_path()
+            ));
+        }
+        ()
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.

tests/run-make/crate-loading-crate-depends-on-itself/foo.stderr

@@ -4,6 +4,7 @@ error[E0277]: the trait bound `foo::Struct: Trait` is not satisfied
 13 |     check_trait::<foo::Struct>();
    |                   ^^^^^^^^^^^ the trait `Trait` is not implemented for `foo::Struct`
    |
+   = note: `foo::Struct` implements similarly named `foo::Trait`, but not `Trait`
 note: there are multiple different versions of crate `foo` in the dependency graph
   --> foo-current.rs:7:1
    |

tests/run-make/crate-loading/multiple-dep-versions.stderr

@@ -6,6 +6,7 @@ LL |     do_something(Type);
    |     |
    |     required by a bound introduced by this call
    |
+   = note: `dep_2_reexport::Type` implements similarly named `dependency::Trait`, but not `Trait`
 note: there are multiple different versions of crate `dependency` in the dependency graph
   --> replaced
    |
@@ -92,6 +93,7 @@ LL |     do_something(OtherType);
    |     |
    |     required by a bound introduced by this call
    |
+   = note: `OtherType` implements similarly named `dependency::Trait`, but not `Trait`
 note: there are multiple different versions of crate `dependency` in the dependency graph
   --> replaced
    |

tests/ui/traits/bound/same-crate-name.stderr

@@ -12,6 +12,7 @@ help: trait impl with same name found
 LL | impl Bar for Foo {}
    | ^^^^^^^^^^^^^^^^
    = note: perhaps two different versions of crate `crate_a2` are being used?
+   = note: `Foo` implements similarly named `main::a::Bar`, but not `main::a::Bar`
    = help: the trait `main::a::Bar` is implemented for `ImplementsTraitForUsize<usize>`
 note: required by a bound in `try_foo`
   --> $DIR/auxiliary/crate_a1.rs:3:24
@@ -48,6 +49,7 @@ help: trait impl with same name found
 LL | impl Bar for ImplementsWrongTraitConditionally<isize> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: perhaps two different versions of crate `crate_a2` are being used?
+   = note: `ImplementsWrongTraitConditionally<isize>` implements similarly named `main::a::Bar`, but not `main::a::Bar`
    = help: the trait `main::a::Bar` is implemented for `ImplementsTraitForUsize<usize>`
 note: required by a bound in `try_foo`
   --> $DIR/auxiliary/crate_a1.rs:3:24

tests/ui/traits/similarly_named_trait.rs

@@ -0,0 +1,17 @@
+trait Trait {} //~ HELP this trait has no implementations, consider adding one
+
+mod m {
+    pub trait Trait {}
+    pub struct St;
+    impl Trait for St {}
+}
+
+fn func<T: Trait>(_: T) {} //~ NOTE required by a bound in `func`
+//~^ NOTE required by this bound in `func`
+
+fn main() {
+    func(m::St); //~ ERROR the trait bound `St: Trait` is not satisfied
+    //~^ NOTE the trait `Trait` is not implemented for `St`
+    //~| NOTE required by a bound introduced by this call
+    //~| NOTE `St` implements similarly named `m::Trait`, but not `Trait`
+}

tests/ui/traits/similarly_named_trait.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the trait bound `St: Trait` is not satisfied
+  --> $DIR/similarly_named_trait.rs:13:10
+   |
+LL |     func(m::St);
+   |     ---- ^^^^^ the trait `Trait` is not implemented for `St`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: `St` implements similarly named `m::Trait`, but not `Trait`
+help: this trait has no implementations, consider adding one
+  --> $DIR/similarly_named_trait.rs:1:1
+   |
+LL | trait Trait {}
+   | ^^^^^^^^^^^
+note: required by a bound in `func`
+  --> $DIR/similarly_named_trait.rs:9:12
+   |
+LL | fn func<T: Trait>(_: T) {}
+   |            ^^^^^ required by this bound in `func`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.