Move some tests to more reasonable directories - 2

Address comments

Update limits

src/test/ui/impl-trait/nested_impl_trait.rs

@@ -0,0 +1,30 @@
+use std::fmt::Debug;
+
+fn fine(x: impl Into<u32>) -> impl Into<u32> { x }
+
+fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
+//~^ ERROR nested `impl Trait` is not allowed
+
+fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
+//~^ ERROR nested `impl Trait` is not allowed
+//~^^ `impl Trait` not allowed
+
+fn bad_in_arg_position(_: impl Into<impl Debug>) { }
+//~^ ERROR nested `impl Trait` is not allowed
+
+struct X;
+impl X {
+    fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
+    //~^ ERROR nested `impl Trait` is not allowed
+}
+
+fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
+    vec![|| println!("woot")].into_iter()
+}
+
+fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
+//~^ `impl Trait` not allowed
+    || 5
+}
+
+fn main() {}