Add new ui test for returning an Fn trait that returns impl Trait

Change description from compiletest to regression test

Co-authored-by: 许杰友 Jieyou Xu (Joe) <39484203+jieyouxu@users.noreply.github.com>

Improve test name, location, and description

Update tests/ui/impl-trait/impl-fn-rpit-opaque-107883.rs

Co-authored-by: waffle <waffle.lapkin@gmail.com>

tests/ui/impl-trait/impl-fn-rpit-opaque-107883.rs

@@ -0,0 +1,37 @@
+//@ check-pass
+// Regression test for <https;//github.com/rust-lang/rust/issues/107883>
+#![feature(impl_trait_in_fn_trait_return)]
+#![feature(unboxed_closures)] // only for `h`
+
+use std::fmt::Debug;
+
+fn f<T>() -> impl Fn(T) -> impl Debug {
+    |_x| 15
+}
+
+fn g<T>() -> impl MyFn<(T,), Out = impl Debug> {
+    |_x| 15
+}
+
+trait MyFn<T> {
+    type Out;
+}
+
+impl<T, U, F: Fn(T) -> U> MyFn<(T,)> for F {
+    type Out = U;
+}
+
+fn h<T>() -> impl Fn<(T,), Output = impl Debug> {
+    |_x| 15
+}
+
+fn f_<T>() -> impl Fn(T) -> impl Debug {
+    std::convert::identity(|_x| 15)
+}
+
+fn f__<T>() -> impl Fn(T) -> impl Debug {
+    let r = |_x| 15;
+    r
+}
+
+fn main() {}