Feature gate associated type defaults

There are multiple issues with them as designed and implemented.

cc #27364

src/libcore/lib.rs

@@ -65,6 +65,7 @@
 #![allow(raw_pointer_derive)]
 #![deny(missing_docs)]
 
+#![feature(associated_type_defaults)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
 #![feature(on_unimplemented)]

src/libsyntax/feature_gate.rs

@@ -165,7 +165,10 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
     ("static_recursion", "1.3.0", Active),
 
     // Allows default type parameters to influence type inference.
-    ("default_type_parameter_fallback", "1.3.0", Active)
+    ("default_type_parameter_fallback", "1.3.0", Active),
+
+    // Allows associated type defaults
+    ("associated_type_defaults", "1.2.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -763,6 +766,10 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                     self.gate_feature("const_fn", ti.span, "const fn is unstable");
                 }
             }
+            ast::TypeTraitItem(_, Some(_)) => {
+                self.gate_feature("associated_type_defaults", ti.span,
+                                  "associated type defaults are unstable");
+            }
             _ => {}
         }
         visit::walk_trait_item(self, ti);

src/test/auxiliary/xcrate_associated_type_defaults.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 pub trait Foo {
     type Input = usize;
     fn bar(&self, _: Self::Input) {}

src/test/compile-fail/feature-gate-assoc-type-defaults.rs

@@ -0,0 +1,15 @@
+// Copyright 2015 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.
+
+trait Foo {
+    type Bar = u8; //~ ERROR associated type defaults are unstable
+}
+
+fn main() {}

src/test/run-pass/default-associated-types.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 trait Foo<T> {
     type Out = T;
     fn foo(&self) -> Self::Out;

src/test/run-pass/issue-25339.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 use std::marker::PhantomData;
 
 pub trait Routing<I> {