Do not suggest 'Trait<Assoc=arg>' when in trait impl

because that would be illegal syntax

compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs

@@ -129,6 +129,44 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         if self.missing_lifetimes() { "lifetime" } else { "generic" }
     }
 
+    /// Returns true if the generic type is a trait
+    /// and is being referred to from one of its trait impls
+    fn is_in_trait_impl(&self) -> bool {
+        if self.tcx.is_trait(self.def_id) {
+            // Here we check if the reference to the generic type
+            // is from the 'of_trait' field of the enclosing impl
+
+            let parent = self.tcx.hir().get_parent(self.path_segment.hir_id);
+            let parent_item = self
+                .tcx
+                .hir()
+                .get_by_def_id(self.tcx.hir().get_parent_item(self.path_segment.hir_id).def_id);
+
+            // Get the HIR id of the trait ref
+            let hir::Node::TraitRef(hir::TraitRef { hir_ref_id: trait_ref_id, .. }) = parent else {
+                return false;
+            };
+
+            // Get the HIR id of the 'of_trait' field of the impl
+            let hir::Node::Item(hir::Item {
+                kind:
+                    hir::ItemKind::Impl(hir::Impl {
+                        of_trait: Some(hir::TraitRef { hir_ref_id: id_in_of_trait, .. }),
+                        ..
+                    }),
+                ..
+            }) = parent_item
+            else {
+                return false;
+            };
+
+            // Check that trait is referred to from the of_trait field of impl
+            trait_ref_id == id_in_of_trait
+        } else {
+            false
+        }
+    }
+
     fn num_provided_args(&self) -> usize {
         if self.missing_lifetimes() {
             self.num_provided_lifetime_args()
@@ -948,20 +986,26 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         // If there is a single unbound associated type and a single excess generic param
         // suggest replacing the generic param with the associated type bound
         if provided_args_matches_unbound_traits && !unbound_types.is_empty() {
-            let unused_generics = &self.gen_args.args[self.num_expected_type_or_const_args()..];
+            // Don't suggest if we're in a trait impl as
-            let suggestions = iter::zip(unused_generics, &unbound_types)
+            // that would result in invalid syntax (fixes #116464)
-                .map(|(potential, name)| (potential.span().shrink_to_lo(), format!("{name} = ")))
+            if !self.is_in_trait_impl() {
-                .collect::<Vec<_>>();
+                let unused_generics = &self.gen_args.args[self.num_expected_type_or_const_args()..];
+                let suggestions = iter::zip(unused_generics, &unbound_types)
+                    .map(|(potential, name)| {
+                        (potential.span().shrink_to_lo(), format!("{name} = "))
+                    })
+                    .collect::<Vec<_>>();
 
-            if !suggestions.is_empty() {
+                if !suggestions.is_empty() {
-                err.multipart_suggestion_verbose(
+                    err.multipart_suggestion_verbose(
-                    format!(
+                        format!(
-                        "replace the generic bound{s} with the associated type{s}",
+                            "replace the generic bound{s} with the associated type{s}",
-                        s = pluralize!(unbound_types.len())
+                            s = pluralize!(unbound_types.len())
-                    ),
+                        ),
-                    suggestions,
+                        suggestions,
-                    Applicability::MaybeIncorrect,
+                        Applicability::MaybeIncorrect,
-                );
+                    );
+                }
             }
         } else if remove_entire_generics {
             let span = self

tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.rs

@@ -0,0 +1,43 @@
+// Regression test for #116464
+// Checks that we do not suggest Trait<..., Assoc=arg> when the trait
+// is referred to from one of its impls but do so at all other places
+
+pub trait Trait<T> {
+    type Assoc;
+}
+
+impl<T, S> Trait<T> for i32 {
+    type Assoc = String;
+}
+
+// Should not not trigger suggestion here...
+impl<T, S> Trait<T, S> for () {}
+//~^ ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+
+//... but should do so in all of the below cases except the last one
+fn func<T: Trait<u32, String>>(t: T) -> impl Trait<(), i32> {
+//~^ ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+//~| ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+    3
+}
+
+struct Struct<T: Trait<u32, String>> {
+//~^ ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+    a: T
+}
+
+trait AnotherTrait<T: Trait<T, i32>> {}
+//~^ ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+
+impl<T: Trait<u32, String>> Struct<T> {}
+//~^ ERROR trait takes 1 generic argument but 2 generic arguments were supplied
+
+// Test for self type. Should not trigger suggestion as it doesn't have an
+// associated type
+trait YetAnotherTrait {}
+impl<T: Trait<u32, Assoc=String>, U> YetAnotherTrait for Struct<T, U> {}
+//~^ ERROR struct takes 1 generic argument but 2 generic arguments were supplied
+
+
+fn main() {
+}

tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.stderr

@@ -0,0 +1,109 @@
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:14:12
+   |
+LL | impl<T, S> Trait<T, S> for () {}
+   |            ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:18:12
+   |
+LL | fn func<T: Trait<u32, String>>(t: T) -> impl Trait<(), i32> {
+   |            ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+help: replace the generic bound with the associated type
+   |
+LL | fn func<T: Trait<u32, Assoc = String>>(t: T) -> impl Trait<(), i32> {
+   |                       +++++++
+
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:18:46
+   |
+LL | fn func<T: Trait<u32, String>>(t: T) -> impl Trait<(), i32> {
+   |                                              ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+help: replace the generic bound with the associated type
+   |
+LL | fn func<T: Trait<u32, String>>(t: T) -> impl Trait<(), Assoc = i32> {
+   |                                                        +++++++
+
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:24:18
+   |
+LL | struct Struct<T: Trait<u32, String>> {
+   |                  ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+help: replace the generic bound with the associated type
+   |
+LL | struct Struct<T: Trait<u32, Assoc = String>> {
+   |                             +++++++
+
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:29:23
+   |
+LL | trait AnotherTrait<T: Trait<T, i32>> {}
+   |                       ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+help: replace the generic bound with the associated type
+   |
+LL | trait AnotherTrait<T: Trait<T, Assoc = i32>> {}
+   |                                +++++++
+
+error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:32:9
+   |
+LL | impl<T: Trait<u32, String>> Struct<T> {}
+   |         ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:5:11
+   |
+LL | pub trait Trait<T> {
+   |           ^^^^^ -
+help: replace the generic bound with the associated type
+   |
+LL | impl<T: Trait<u32, Assoc = String>> Struct<T> {}
+   |                    +++++++
+
+error[E0107]: struct takes 1 generic argument but 2 generic arguments were supplied
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:38:58
+   |
+LL | impl<T: Trait<u32, Assoc=String>, U> YetAnotherTrait for Struct<T, U> {}
+   |                                                          ^^^^^^    - help: remove this generic argument
+   |                                                          |
+   |                                                          expected 1 generic argument
+   |
+note: struct defined here, with 1 generic parameter: `T`
+  --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:24:8
+   |
+LL | struct Struct<T: Trait<u32, String>> {
+   |        ^^^^^^ -
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0107`.