Rollup merge of #104295 - compiler-errors:rpitit-generics-parity, r=eholk

Check generics parity before collecting return-position `impl Trait`s in trait

The only thing is that this duplicates the error message for number of generics mismatch, but we already deduplicate that error message in Cargo. I could add a flag to delay the error if the reviewer cares.

Fixes #104281

Also drive-by adds a few comments to the `collect_trait_impl_trait_tys` method, and removes an unused argument from `compare_number_of_generics`.

src/test/ui/impl-trait/in-trait/trait-more-generics-than-impl.rs

@@ -0,0 +1,17 @@
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+struct S;
+
+trait Foo {
+    fn bar<T>() -> impl Sized;
+}
+
+impl Foo for S {
+    fn bar() -> impl Sized {}
+    //~^ ERROR method `bar` has 0 type parameters but its trait declaration has 1 type parameter
+}
+
+fn main() {
+    S::bar();
+}

src/test/ui/impl-trait/in-trait/trait-more-generics-than-impl.stderr

@@ -0,0 +1,12 @@
+error[E0049]: method `bar` has 0 type parameters but its trait declaration has 1 type parameter
+  --> $DIR/trait-more-generics-than-impl.rs:11:11
+   |
+LL |     fn bar<T>() -> impl Sized;
+   |            - expected 1 type parameter
+...
+LL |     fn bar() -> impl Sized {}
+   |           ^ found 0 type parameters
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0049`.