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_middle/src/ty/generics.rs

@@ -102,7 +102,7 @@ impl GenericParamDef {
     }
 }
 
-#[derive(Default)]
+#[derive(Default, PartialEq)]
 pub struct GenericParamCount {
     pub lifetimes: usize,
     pub types: usize,

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

@@ -498,6 +498,8 @@ 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

@@ -5127,6 +5127,45 @@ 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);
+
+        // Ensure that the current trait and the other_trait have the same number of args and each arg
+        // has the same kind
+        let trait_has_same_params = |other_trait_def_id: DefId| -> bool {
+            self.tcx.generics_of(trait_def_id).own_counts()
+                == self.tcx.generics_of(other_trait_def_id).own_counts()
+        };
+
+        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)
+                && trait_has_same_params(*def_id)
+                && self.predicate_must_hold_modulo_regions(&Obligation::new(
+                    self.tcx,
+                    obligation.cause.clone(),
+                    obligation.param_env,
+                    trait_predicate.map_bound(|tr| ty::TraitPredicate {
+                        trait_ref: ty::TraitRef::new(self.tcx, *def_id, tr.trait_ref.args),
+                        ..tr
+                    }),
+                ))
+        }) {
+            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 +1,27 @@
+trait Trait {} //~ HELP this trait has no implementations, consider adding one
+trait TraitWithParam<T> {} //~ HELP this trait has no implementations, consider adding one
+
+mod m {
+    pub trait Trait {}
+    pub trait TraitWithParam<T> {}
+    pub struct St;
+    impl Trait for St {}
+    impl<T> TraitWithParam<T> for St {}
+}
+
+fn func<T: Trait>(_: T) {} //~ NOTE required by a bound in `func`
+//~^ NOTE required by this bound in `func`
+
+fn func2<T: TraitWithParam<T>> (_: T) {} //~ NOTE required by a bound in `func2`
+//~^ NOTE required by this bound in `func2`
+
+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`
+    func2(m::St); //~ ERROR the trait bound `St: TraitWithParam<St>` is not satisfied
+    //~^ NOTE the trait `TraitWithParam<St>` is not implemented for `St`
+    //~| NOTE required by a bound introduced by this call
+    //~| NOTE `St` implements similarly named `m::TraitWithParam`, but not `TraitWithParam<St>`
+}

tests/ui/traits/similarly_named_trait.stderr

@@ -0,0 +1,43 @@
+error[E0277]: the trait bound `St: Trait` is not satisfied
+  --> $DIR/similarly_named_trait.rs:19:10
+   |
+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:12:12
+   |
+LL | fn func<T: Trait>(_: T) {}
+   |            ^^^^^ required by this bound in `func`
+
+error[E0277]: the trait bound `St: TraitWithParam<St>` is not satisfied
+  --> $DIR/similarly_named_trait.rs:23:11
+   |
+LL |     func2(m::St);
+   |     ----- ^^^^^ the trait `TraitWithParam<St>` is not implemented for `St`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: `St` implements similarly named `m::TraitWithParam`, but not `TraitWithParam<St>`
+help: this trait has no implementations, consider adding one
+  --> $DIR/similarly_named_trait.rs:2:1
+   |
+LL | trait TraitWithParam<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `func2`
+  --> $DIR/similarly_named_trait.rs:15:13
+   |
+LL | fn func2<T: TraitWithParam<T>> (_: T) {}
+   |             ^^^^^^^^^^^^^^^^^ required by this bound in `func2`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/union/issue-81199.stderr

@@ -16,6 +16,7 @@ error[E0277]: the trait bound `T: Pointee` is not satisfied
 LL |     components: PtrComponents<T>,
    |                 ^^^^^^^^^^^^^^^^ the trait `Pointee` is not implemented for `T`
    |
+   = note: `T` implements similarly named `std::ptr::Pointee`, but not `Pointee`
 note: required by a bound in `PtrComponents`
   --> $DIR/issue-81199.rs:11:25
    |