add `#[thread_local]` attribute

This provides a building block for fast thread-local storage. It does
not change the safety semantics of `static mut`.

Closes #10310

doc/rust.md

@@ -1754,6 +1754,8 @@ names are effectively reserved. Some significant attributes include:
 * The `deriving` attribute, for automatically generating
   implementations of certain traits.
 * The `static_assert` attribute, for asserting that a static bool is true at compiletime
+* The `thread_local` attribute, for defining a `static mut` as a thread-local. Note that this is
+  only a low-level building block, and is not local to a *task*, nor does it provide safety.
 
 Other attributes may be added or removed during development of the language.
 

src/librustc/lib/llvm.rs

@@ -1749,6 +1749,12 @@ pub fn SetUnnamedAddr(Global: ValueRef, Unnamed: bool) {
     }
 }
 
+pub fn set_thread_local(global: ValueRef, is_thread_local: bool) {
+    unsafe {
+        llvm::LLVMSetThreadLocal(global, is_thread_local as Bool);
+    }
+}
+
 pub fn ConstICmp(Pred: IntPredicate, V1: ValueRef, V2: ValueRef) -> ValueRef {
     unsafe {
         llvm::LLVMConstICmp(Pred as c_ushort, V1, V2)

src/librustc/middle/lint.rs

@@ -816,6 +816,7 @@ static obsolete_attrs: &'static [(&'static str, &'static str)] = &[
 static other_attrs: &'static [&'static str] = &[
     // item-level
     "address_insignificant", // can be crate-level too
+    "thread_local", // for statics
     "allow", "deny", "forbid", "warn", // lint options
     "deprecated", "experimental", "unstable", "stable", "locked", "frozen", //item stability
     "crate_map", "cfg", "doc", "export_name", "link_section", "no_freeze",

src/librustc/middle/trans/base.rs

@@ -2543,6 +2543,10 @@ pub fn get_item_val(ccx: @mut CrateContext, id: ast::NodeId) -> ValueRef {
                                     inlineable = true;
                                 }
 
+                                if attr::contains_name(i.attrs, "thread_local") {
+                                    lib::llvm::set_thread_local(g, true);
+                                }
+
                                 if !inlineable {
                                     debug!("{} not inlined", sym);
                                     ccx.non_inlineable_statics.insert(id);