Remove the implicit 'static bound on Send.

Previously Send was defined as `trait Send: 'static {}`. As detailed in
https://github.com/rust-lang/rfcs/pull/458, the `'static` bound is not
actually necessary for safety, we can use lifetimes to enforce that more
flexibly.

`unsafe` code that was previously relying on `Send` to insert a
`'static` bound now may allow incorrect patterns, and so should be
audited (a quick way to ensure safety immediately and postpone the audit
is to add an explicit `'static` bound to any uses of the `Send` type).

cc #22251.

src/libcore/marker.rs

@@ -32,9 +32,19 @@ use clone::Clone;
            reason = "will be overhauled with new lifetime rules; see RFC 458")]
 #[lang="send"]
 #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
+#[cfg(stage0)] // SNAP ac134f7 remove after stage0
 pub unsafe trait Send: 'static {
     // empty.
 }
+/// Types able to be transferred across thread boundaries.
+#[unstable(feature = "core",
+           reason = "will be overhauled with new lifetime rules; see RFC 458")]
+#[lang="send"]
+#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
+#[cfg(not(stage0))]
+pub unsafe trait Send {
+    // empty.
+}
 
 /// Types with a constant size known at compile-time.
 #[stable(feature = "rust1", since = "1.0.0")]