syntax: add uniform_paths feature-gate.

Author: eddyb

src/libsyntax/feature_gate.rs

@@ -507,6 +507,9 @@ declare_features! (
 
     // Support for arbitrary delimited token streams in non-macro attributes.
     (active, unrestricted_attribute_tokens, "1.30.0", Some(44690), None),
+
+    // Allows `use x::y;` to resolve through `self::x`, not just `::x`.
+    (active, uniform_paths, "1.30.0", Some(53130), None),
 );
 
 declare_features! (

src/test/ui/feature-gate-uniform-paths.rs

@@ -0,0 +1,22 @@
+// 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.
+
+pub mod foo {
+    pub use bar::Bar;
+    //~^ ERROR unresolved import `bar`
+
+    pub mod bar {
+        pub struct Bar;
+    }
+}
+
+fn main() {
+    let _ = foo::Bar;
+}

src/test/ui/feature-gate-uniform-paths.stderr

@@ -0,0 +1,9 @@
+error[E0432]: unresolved import `bar`
+  --> $DIR/feature-gate-uniform-paths.rs:12:13
+   |
+LL |     pub use bar::Bar;
+   |             ^^^ Did you mean `self::bar`?
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0432`.