std: Add a new top-level thread_local module

This commit removes the `std::local_data` module in favor of a new
`std::thread_local` module providing thread local storage. The module provides
two variants of TLS: one which owns its contents and one which is based on
scoped references. Each implementation has pros and cons listed in the
documentation.

Both flavors have accessors through a function called `with` which yield a
reference to a closure provided. Both flavors also panic if a reference cannot
be yielded and provide a function to test whether an access would panic or not.
This is an implementation of [RFC 461][rfc] and full details can be found in
that RFC.

This is a breaking change due to the removal of the `std::local_data` module.
All users can migrate to the new thread local system like so:

    thread_local!(static FOO: Rc<RefCell<Option<T>>> = Rc::new(RefCell::new(None)))

The old `local_data` module inherently contained the `Rc<RefCell<Option<T>>>` as
an implementation detail which must now be explicitly stated by users.

[rfc]: https://github.com/rust-lang/rfcs/pull/461
[breaking-change]

1 files changed, 4 insertions, 9 deletions

diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 4272b57a4dc..3a3407aedba 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -560,15 +560,10 @@ pub type IdentInterner = StrInterner;
 // fresh one.
 // FIXME(eddyb) #8726 This should probably use a task-local reference.
 pub fn get_ident_interner() -> Rc<IdentInterner> {
-    local_data_key!(key: Rc<::parse::token::IdentInterner>)
-    match key.get() {
-        Some(interner) => interner.clone(),
-        None => {
-            let interner = Rc::new(mk_fresh_ident_interner());
-            key.replace(Some(interner.clone()));
-            interner
-        }
-    }
+    thread_local!(static KEY: Rc<::parse::token::IdentInterner> = {
+        Rc::new(mk_fresh_ident_interner())
+    })
+    KEY.with(|k| k.clone())
 }
 
 /// Represents a string stored in the task-local interner. Because the