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

@@ -460,7 +460,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             span,
                             leaf_trait_predicate,
                         );
-                        self.note_version_mismatch(&mut err, leaf_trait_predicate);
+                        self.note_different_trait_with_same_name(&mut err, &obligation, leaf_trait_predicate);
                         self.suggest_remove_await(&obligation, &mut err);
                         self.suggest_derive(&obligation, &mut err, leaf_trait_predicate);
 
@@ -2354,10 +2354,13 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
     /// with the same path as `trait_ref`, a help message about
-    /// a probable version mismatch is added to `err`
-    fn note_version_mismatch(
+    /// a probable version mismatch is added to `err`. Otherwise if it implements
+    /// another trait with the same name, a note message about a similarly named
+    /// trait is added to `err`.
+    fn note_different_trait_with_same_name(
         &self,
         err: &mut Diag<'_>,
+        obligation: &PredicateObligation<'tcx>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         let get_trait_impls = |trait_def_id| {
@@ -2384,6 +2387,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let traits_with_same_path =
             traits_with_same_path.into_items().into_sorted_stable_ord_by_key(|(p, _)| p);
         let mut suggested = false;
+
         for (_, trait_with_same_path) in traits_with_same_path {
             let trait_impls = get_trait_impls(trait_with_same_path);
             if trait_impls.is_empty() {
@@ -2401,6 +2405,72 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             err.note(crate_msg);
             suggested = true;
         }
+
+        if suggested {
+            return true;
+        }
+
+        let trait_def_id = trait_pred.def_id();
+        let trait_name = self.tcx.item_name(trait_def_id);
+        let trait_krate_name = self.tcx.crate_name(trait_def_id.krate);
+
+        // FIXME: If there are multiple different versions of a crate in the dependency graph, there is already
+        // a suggestion designed for this purpose, see FnCtxt::detect_and_explain_multiple_crate_versions_of_trait_item.
+        // We should merge them all into this one. We should not have 2 subtly different impls of this note.
+        if self
+            .tcx
+            .all_traits_including_private()
+            .find(|def_id| {
+                def_id.krate != trait_def_id.krate
+                    && self.tcx.crate_name(def_id.krate) == trait_krate_name
+                    && self.tcx.item_name(def_id) == trait_name
+            })
+            .is_some()
+        {
+            return false;
+        }
+
+        let trait_has_same_params = |other_trait_def_id: DefId| -> bool {
+            let trait_generics = self.tcx.generics_of(trait_def_id);
+            let other_trait_generics = self.tcx.generics_of(other_trait_def_id);
+
+            if trait_generics.count() != other_trait_generics.count() {
+                return false;
+            }
+            trait_generics.own_params.iter().zip(other_trait_generics.own_params.iter()).all(
+                |(a, b)| {
+                    (matches!(a.kind, ty::GenericParamDefKind::Type { .. })
+                        && matches!(b.kind, ty::GenericParamDefKind::Type { .. }))
+                        || (matches!(a.kind, ty::GenericParamDefKind::Lifetime,)
+                            && matches!(b.kind, ty::GenericParamDefKind::Lifetime))
+                        || (matches!(a.kind, ty::GenericParamDefKind::Const { .. })
+                            && matches!(b.kind, ty::GenericParamDefKind::Const { .. }))
+                },
+            )
+        };
+        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)
+                && trait_has_same_params(*def_id)
+                && self.predicate_must_hold_modulo_regions(&Obligation::new(
+                    self.tcx,
+                    obligation.cause.clone(),
+                    obligation.param_env,
+                    trait_pred.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_pred.self_ty(),
+                self.tcx.def_path_str(other_trait_def_id),
+                trait_pred.print_modifiers_and_trait_path()
+            ));
+            suggested = true;
+        }
         suggested
     }
 

tests/ui/traits/similarly_named_trait.rs

@@ -0,0 +1,28 @@
+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;  //~ HELP the trait `Trait` is not implemented for `St`
+    //~| HELP the trait `TraitWithParam<St>` is not implemented for `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 unsatisfied trait bound
+    //~| 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 unsatisfied trait bound
+    //~| 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 +1,53 @@
+error[E0277]: the trait bound `St: Trait` is not satisfied
+  --> $DIR/similarly_named_trait.rs:20:10
+   |
+LL |     func(m::St);
+   |     ---- ^^^^^ unsatisfied trait bound
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: the trait `Trait` is not implemented for `St`
+  --> $DIR/similarly_named_trait.rs:7:5
+   |
+LL |     pub struct St;
+   |     ^^^^^^^^^^^^^
+   = 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:13:12
+   |
+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:24:11
+   |
+LL |     func2(m::St);
+   |     ----- ^^^^^ unsatisfied trait bound
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: the trait `TraitWithParam<St>` is not implemented for `St`
+  --> $DIR/similarly_named_trait.rs:7:5
+   |
+LL |     pub struct St;
+   |     ^^^^^^^^^^^^^
+   = 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:16:13
+   |
+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
    |