Filter out RPITITs when suggesting unconstrained assoc type on too many generics

compiler/rustc_hir_analysis/src/errors/wrong_number_of_generic_args.rs

@@ -495,6 +495,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
                         .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-consider-unconstrained-rpitits.rs

@@ -0,0 +1,14 @@
+// There's a suggestion that turns `Iterator<u32>` into `Iterator<Item = u32>`
+// if we have more generics than the trait wants. Let's not consider RPITITs
+// for this, since that makes no sense right now.
+
+trait Foo {
+    fn bar(self) -> impl Sized;
+}
+
+impl Foo<u8> for () {
+    //~^ ERROR trait takes 0 generic arguments but 1 generic argument was supplied
+    fn bar(self) -> impl Sized {}
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/dont-consider-unconstrained-rpitits.stderr

@@ -0,0 +1,17 @@
+error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/dont-consider-unconstrained-rpitits.rs:9:6
+   |
+LL | impl Foo<u8> for () {
+   |      ^^^---- help: remove the unnecessary generics
+   |      |
+   |      expected 0 generic arguments
+   |
+note: trait defined here, with 0 generic parameters
+  --> $DIR/dont-consider-unconstrained-rpitits.rs:5:7
+   |
+LL | trait Foo {
+   |       ^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0107`.