librustc: Match trait self types exactly.

This can break code that looked like:

    impl Foo for Box<Any> {
        fn f(&self) { ... }
    }

    let x: Box<Any + Send> = ...;
    x.f();

Change such code to:

    impl Foo for Box<Any> {
        fn f(&self) { ... }
    }

    let x: Box<Any> = ...;
    x.f();

That is, upcast before calling methods.

This is a conservative solution to #5781. A more proper treatment (see
the xfail'd `trait-contravariant-self.rs`) would take variance into
account. This change fixes the soundness hole.

Some library changes had to be made to make this work. In particular,
`Box<Any>` is no longer showable, and only `Box<Any+Send>` is showable.
Eventually, this restriction can be lifted; for now, it does not prove
too onerous, because `Any` is only used for propagating the result of
task failure.

This patch also adds a test for the variance inference work in #12828,
which accidentally landed as part of DST.

Closes #5781.

[breaking-change]

src/liballoc/owned.rs

@@ -16,6 +16,7 @@ use core::cmp::{PartialEq, PartialOrd, Eq, Ord, Ordering};
 use core::default::Default;
 use core::fmt;
 use core::intrinsics;
+use core::kinds::Send;
 use core::mem;
 use core::raw::TraitObject;
 use core::result::{Ok, Err, Result};
@@ -106,6 +107,34 @@ impl AnyOwnExt for Box<Any> {
     }
 }
 
+/// Extension methods for an owning `Any+Send` trait object
+pub trait AnySendOwnExt {
+    /// Returns the boxed value if it is of type `T`, or
+    /// `Err(Self)` if it isn't.
+    fn move_send<T: 'static>(self) -> Result<Box<T>, Self>;
+}
+
+impl AnySendOwnExt for Box<Any+Send> {
+    #[inline]
+    fn move_send<T: 'static>(self) -> Result<Box<T>, Box<Any+Send>> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let to: TraitObject =
+                    *mem::transmute::<&Box<Any+Send>, &TraitObject>(&self);
+
+                // Prevent destructor on self being run
+                intrinsics::forget(self);
+
+                // Extract the data pointer
+                Ok(mem::transmute(to.data))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
 impl<T: fmt::Show> fmt::Show for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         (**self).fmt(f)