Disallow projections from impl Trait types

src/test/ui/impl_trait_projections.rs

@@ -0,0 +1,43 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![feature(conservative_impl_trait, universal_impl_trait)]
+
+use std::fmt::Debug;
+use std::option;
+
+fn parametrized_type_is_allowed() -> Option<impl Debug> {
+    Some(5i32)
+}
+
+fn path_parametrized_type_is_allowed() -> option::Option<impl Debug> {
+    Some(5i32)
+}
+
+fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
+//~^ ERROR `impl Trait` is not allowed in path parameters
+//~^^ ERROR ambiguous associated type
+    x.next().unwrap()
+}
+
+fn projection_with_named_trait_is_disallowed(x: impl Iterator)
+    -> <impl Iterator as Iterator>::Item
+//~^ ERROR `impl Trait` is not allowed in path parameters
+{
+    x.next().unwrap()
+}
+
+fn projection_with_named_trait_inside_path_is_disallowed()
+    -> <::std::ops::Range<impl Debug> as Iterator>::Item
+//~^ ERROR `impl Trait` is not allowed in path parameters
+{
+    (1i32..100).next().unwrap()
+}
+
+fn main() {}

src/test/ui/impl_trait_projections.stderr

@@ -0,0 +1,28 @@
+error[E0667]: `impl Trait` is not allowed in path parameters
+  --> $DIR/impl_trait_projections.rs:23:51
+   |
+23 | fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
+   |                                                   ^^^^^^^^^^^^^
+
+error[E0667]: `impl Trait` is not allowed in path parameters
+  --> $DIR/impl_trait_projections.rs:30:9
+   |
+30 |     -> <impl Iterator as Iterator>::Item
+   |         ^^^^^^^^^^^^^
+
+error[E0667]: `impl Trait` is not allowed in path parameters
+  --> $DIR/impl_trait_projections.rs:37:27
+   |
+37 |     -> <::std::ops::Range<impl Debug> as Iterator>::Item
+   |                           ^^^^^^^^^^
+
+error[E0223]: ambiguous associated type
+  --> $DIR/impl_trait_projections.rs:23:50
+   |
+23 | fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
+   |                                                  ^^^^^^^^^^^^^^^^^^^^^ ambiguous associated type
+   |
+   = note: specify the type using the syntax `<impl std::iter::Iterator as Trait>::Item`
+
+error: aborting due to 4 previous errors
+