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

Author: compiler-errors

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 +1,12 @@
+// 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 main() {}

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

@@ -0,0 +1,24 @@
+error[E0412]: cannot find type `Bar` in this scope
+  --> $DIR/dont-consider-unconstrained-rpitits.rs:9:18
+   |
+LL | impl Foo<u8> for Bar {}
+   |                  ^^^ not found in this scope
+
+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 Bar {}
+   |      ^^^---- 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 2 previous errors
+
+Some errors have detailed explanations: E0107, E0412.
+For more information about an error, try `rustc --explain E0107`.