Add a note when a type implements a trait with the same name as the required one

This is useful when you have two dependencies that use different trait for
the same thing and with the same name. The user can accidentally implement
the bad one which might be confusing.

Author: rperier

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, leaf_trait_predicate);
+
                         self.try_to_add_help_message(
                             &root_obligation,
                             &obligation,

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

@@ -5108,6 +5108,38 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             _ => {}
         }
     }
+    pub(crate) fn suggest_impl_similarly_named_trait(
+        &self,
+        err: &mut Diag<'_>,
+        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 imp = self.tcx.impl_trait_header(impl_def_id).unwrap();
+                        if imp.polarity != ty::ImplPolarity::Positive {
+                            return false;
+                        }
+                        let imp = imp.trait_ref.skip_binder();
+                        trait_predicate.skip_binder().self_ty() == imp.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/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`.