Don't compute name of associated item if it's an RPITIT

compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs

@@ -487,14 +487,14 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
             items
                 .in_definition_order()
                 .filter(|item| item.is_type())
+                .filter(|item| !item.is_impl_trait_in_trait())
                 .filter(|item| {
                     !self
                         .gen_args
                         .constraints
                         .iter()
                         .any(|constraint| constraint.ident.name == item.name())
                 })
-                .filter(|item| !item.is_impl_trait_in_trait())
                 .map(|item| self.tcx.item_ident(item.def_id).to_string())
                 .collect()
         } else {

tests/ui/impl-trait/in-trait/dont-probe-missing-item-name-2.rs

@@ -0,0 +1,12 @@
+trait Foo {
+    fn rpitit() -> impl Sized;
+}
+
+// Ensure that we don't try to probe the name of the RPITIT when looking for
+// fixes to suggest for the redundant generic below.
+
+fn test<T: Foo<i32, Assoc = i32>>() {}
+//~^ ERROR trait takes 0 generic arguments but 1 generic argument was supplied
+//~| ERROR associated type `Assoc` not found for `Foo`
+
+fn main() {}

tests/ui/impl-trait/in-trait/dont-probe-missing-item-name-2.stderr

@@ -0,0 +1,24 @@
+error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/dont-probe-missing-item-name-2.rs:8:12
+   |
+LL | fn test<T: Foo<i32, Assoc = i32>>() {}
+   |            ^^^------------------ help: remove the unnecessary generics
+   |            |
+   |            expected 0 generic arguments
+   |
+note: trait defined here, with 0 generic parameters
+  --> $DIR/dont-probe-missing-item-name-2.rs:1:7
+   |
+LL | trait Foo {
+   |       ^^^
+
+error[E0220]: associated type `Assoc` not found for `Foo`
+  --> $DIR/dont-probe-missing-item-name-2.rs:8:21
+   |
+LL | fn test<T: Foo<i32, Assoc = i32>>() {}
+   |                     ^^^^^ associated type `Assoc` not found
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0107, E0220.
+For more information about an error, try `rustc --explain E0107`.