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]

src/libsyntax/attr.rs

@@ -25,21 +25,20 @@ use parse::token::InternedString;
 use parse::token;
 use ptr::P;
 
-use std::collections::HashSet;
+use std::cell::{RefCell, Cell};
 use std::collections::BitvSet;
+use std::collections::HashSet;
 
-local_data_key!(used_attrs: BitvSet)
+thread_local!(static USED_ATTRS: RefCell<BitvSet> = RefCell::new(BitvSet::new()))
 
 pub fn mark_used(attr: &Attribute) {
-    let mut used = used_attrs.replace(None).unwrap_or_else(|| BitvSet::new());
     let AttrId(id) = attr.node.id;
-    used.insert(id);
-    used_attrs.replace(Some(used));
+    USED_ATTRS.with(|slot| slot.borrow_mut().insert(id));
 }
 
 pub fn is_used(attr: &Attribute) -> bool {
     let AttrId(id) = attr.node.id;
-    used_attrs.get().map_or(false, |used| used.contains(&id))
+    USED_ATTRS.with(|slot| slot.borrow().contains(&id))
 }
 
 pub trait AttrMetaMethods {
@@ -167,11 +166,14 @@ pub fn mk_word_item(name: InternedString) -> P<MetaItem> {
     P(dummy_spanned(MetaWord(name)))
 }
 
-local_data_key!(next_attr_id: uint)
+thread_local!(static NEXT_ATTR_ID: Cell<uint> = Cell::new(0))
 
 pub fn mk_attr_id() -> AttrId {
-    let id = next_attr_id.replace(None).unwrap_or(0);
-    next_attr_id.replace(Some(id + 1));
+    let id = NEXT_ATTR_ID.with(|slot| {
+        let r = slot.get();
+        slot.set(r + 1);
+        r
+    });
     AttrId(id)
 }