std: Stabilize TypeId and tweak BoxAny

This commit aims to stabilize the `TypeId` abstraction by moving it out of the
`intrinsics` module into the `any` module of the standard library. Specifically,

* `TypeId` is now defined at `std::any::TypeId`
* `TypeId::hash` has been removed in favor of an implementation of `Hash`.

This commit also performs a final pass over the `any` module, confirming the
following:

* `Any::get_type_id` remains unstable as *usage* of the `Any` trait will likely
  never require this, and the `Any` trait does not need to be implemented for
  any other types. As a result, this implementation detail can remain unstable
  until associated statics are implemented.
* `Any::downcast_ref` is now stable
* `Any::downcast_mut` is now stable
* `BoxAny` remains unstable. While a direct impl on `Box<Any>` is allowed today
  it does not allow downcasting of trait objects like `Box<Any + Send>` (those
  returned from `Thread::join`). This is covered by #18737.
* `BoxAny::downcast` is now stable.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 8ad0c152dc8..a2cc98c7d01 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -125,8 +125,11 @@ impl<S: hash::Hasher, T: ?Sized + Hash<S>> Hash<S> for Box<T> {
 }
 
 /// Extension methods for an owning `Any` trait object.
-#[unstable = "post-DST and coherence changes, this will not be a trait but \
-              rather a direct `impl` on `Box<Any>`"]
+#[unstable = "this trait will likely disappear once compiler bugs blocking \
+              a direct impl on `Box<Any>` have been fixed "]
+// FIXME(#18737): this should be a direct impl on `Box<Any>`. If you're
+//                removing this please make sure that you can downcase on
+//                `Box<Any + Send>` as well as `Box<Any>`
 pub trait BoxAny {
     /// Returns the boxed value if it is of type `T`, or
     /// `Err(Self)` if it isn't.
@@ -134,10 +137,9 @@ pub trait BoxAny {
     fn downcast<T: 'static>(self) -> Result<Box<T>, Self>;
 }
 
+#[stable]
 impl BoxAny for Box<Any> {
     #[inline]
-    #[unstable = "method may be renamed with respect to other downcasting \
-                  methods"]
     fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
         if self.is::<T>() {
             unsafe {