Auto merge of #43931 - eddyb:const-local-key, r=alexcrichton

Make the LocalKey facade of thread_local! inlineable cross-crate.

Fixes (almost*) #25088 by changing the `LocalKey` `static` `thread_local!` generates to a `const`.
This can be done because a `LocalKey` value holds no actual TLS data, only function pointers to get at said data, and it could even be made `Copy` without any negative consequences.
The recent stabilization of rvalue promotion to `'static` allows doing this without changing the API.
r? @alexcrichton

*almost because we can't yet inline `__getit` because it breaks on MSVC, see https://github.com/rust-lang/rust/pull/43931#issuecomment-323534214

src/libstd/thread/local.rs

@@ -159,8 +159,9 @@ macro_rules! thread_local {
 #[allow_internal_unstable]
 #[allow_internal_unsafe]
 macro_rules! __thread_local_inner {
-    ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => {
-        $(#[$attr])* $vis static $name: $crate::thread::LocalKey<$t> = {
+    (@key $(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => {
+        {
+            #[inline]
             fn __init() -> $t { $init }
 
             unsafe fn __getit() -> $crate::option::Option<
@@ -182,7 +183,16 @@ macro_rules! __thread_local_inner {
             unsafe {
                 $crate::thread::LocalKey::new(__getit, __init)
             }
-        };
+        }
+    };
+    ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => {
+        #[cfg(stage0)]
+        $(#[$attr])* $vis static $name: $crate::thread::LocalKey<$t> =
+            __thread_local_inner!(@key $(#[$attr])* $vis $name, $t, $init);
+
+        #[cfg(not(stage0))]
+        $(#[$attr])* $vis const $name: $crate::thread::LocalKey<$t> =
+            __thread_local_inner!(@key $(#[$attr])* $vis $name, $t, $init);
     }
 }
 

src/test/compile-fail/macro-local-data-key-priv.rs

@@ -16,5 +16,5 @@ mod bar {
 
 fn main() {
     bar::baz.with(|_| ());
-    //~^ ERROR static `baz` is private
+    //~^ ERROR `baz` is private
 }