Fix orphan checking (cc #19470). (This is not a complete fix of #19470 because of the backwards compatibility feature gate.)

This is a [breaking-change]. The new rules require that, for an impl of a trait defined
in some other crate, two conditions must hold:

1. Some type must be local.
2. Every type parameter must appear "under" some local type.

Here are some examples that are legal:

```rust
struct MyStruct<T> { ... }

// Here `T` appears "under' `MyStruct`.
impl<T> Clone for MyStruct<T> { }

// Here `T` appears "under' `MyStruct` as well. Note that it also appears
// elsewhere.
impl<T> Iterator<T> for MyStruct<T> { }
```

Here is an illegal example:

```rust
// Here `U` does not appear "under" `MyStruct` or any other local type.
// We call `U` "uncovered".
impl<T,U> Iterator<U> for MyStruct<T> { }
```

There are a couple of ways to rewrite this last example so that it is
legal:

1. In some cases, the uncovered type parameter (here, `U`) should be converted
   into an associated type. This is however a non-local change that requires access
   to the original trait. Also, associated types are not fully baked.
2. Add `U` as a type parameter of `MyStruct`:
   ```rust
   struct MyStruct<T,U> { ... }
   impl<T,U> Iterator<U> for MyStruct<T,U> { }
   ```
3. Create a newtype wrapper for `U`
   ```rust
   impl<T,U> Iterator<Wrapper<U>> for MyStruct<T,U> { }
   ```

Because associated types are not fully baked, which in the case of the
`Hash` trait makes adhering to this rule impossible, you can
temporarily disable this rule in your crate by using
`#![feature(old_orphan_check)]`. Note that the `old_orphan_check`
feature will be removed before 1.0 is released.

2 files changed, 23 insertions, 3 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index b2c2d7eb626..23c4ee8f64b 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -77,8 +77,11 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     // to bootstrap fix for #5723.
     ("issue_5723_bootstrap", Accepted),
 
-    // A way to temporary opt out of opt in copy. This will *never* be accepted.
-    ("opt_out_copy", Active),
+    // A way to temporarily opt out of opt in copy. This will *never* be accepted.
+    ("opt_out_copy", Deprecated),
+
+    // A way to temporarily opt out of the new orphan rules. This will *never* be accepted.
+    ("old_orphan_check", Deprecated),
 
     // These are used to test this portion of the compiler, they don't actually
     // mean anything
@@ -91,6 +94,10 @@ enum Status {
     /// currently being considered for addition/removal.
     Active,
 
+    /// Represents a feature gate that is temporarily enabling deprecated behavior.
+    /// This gate will never be accepted.
+    Deprecated,
+
     /// Represents a feature which has since been removed (it was once Active)
     Removed,
 
@@ -108,6 +115,7 @@ pub struct Features {
     pub visible_private_types: bool,
     pub quote: bool,
     pub opt_out_copy: bool,
+    pub old_orphan_check: bool,
 }
 
 impl Features {
@@ -120,6 +128,7 @@ impl Features {
             visible_private_types: false,
             quote: false,
             opt_out_copy: false,
+            old_orphan_check: false,
         }
     }
 }
@@ -442,7 +451,16 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::C
                     };
                     match KNOWN_FEATURES.iter()
                                         .find(|& &(n, _)| name == n) {
-                        Some(&(name, Active)) => { cx.features.push(name); }
+                        Some(&(name, Active)) => {
+                            cx.features.push(name);
+                        }
+                        Some(&(name, Deprecated)) => {
+                            cx.features.push(name);
+                            span_handler.span_warn(
+                                mi.span,
+                                "feature is deprecated and will only be available \
+                                 for a limited time, please rewrite code that relies on it");
+                        }
                         Some(&(_, Removed)) => {
                             span_handler.span_err(mi.span, "feature has been removed");
                         }
@@ -469,6 +487,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::C
         visible_private_types: cx.has_feature("visible_private_types"),
         quote: cx.has_feature("quote"),
         opt_out_copy: cx.has_feature("opt_out_copy"),
+        old_orphan_check: cx.has_feature("old_orphan_check"),
     },
     unknown_features)
 }
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index d5093c5055c..7a6824ac27c 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -26,6 +26,7 @@
 #![feature(macro_rules, globs, default_type_params, phase, slicing_syntax)]
 #![feature(quote, unsafe_destructor)]
 #![feature(unboxed_closures)]
+#![feature(old_orphan_check)]
 
 extern crate arena;
 extern crate fmt_macros;