Use UnsafeCell for fast constant thread locals

Author: Zoxc

library/std/src/sys/thread_local/fast_local.rs

@@ -13,22 +13,21 @@ pub macro thread_local_inner {
  (@key $t:ty, const $init:expr) => {{
  #[inline]
  #[deny(unsafe_op_in_unsafe_fn)]
- // FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_refs` lint
- #[cfg_attr(bootstrap, allow(static_mut_ref))]
- #[cfg_attr(not(bootstrap), allow(static_mut_refs))]
  unsafe fn __getit(
  _init: $crate::option::Option<&mut $crate::option::Option<$t>>,
  ) -> $crate::option::Option<&'static $t> {
  const INIT_EXPR: $t = $init;
  // If the platform has support for `#[thread_local]`, use it.
  #[thread_local]
- static mut VAL: $t = INIT_EXPR;
+ // We use `UnsafeCell` here instead of `static mut` to ensure any generated TLS shims
+ // have a nonnull attribute on their return value.
+ static VAL: $crate::cell::UnsafeCell<$t> = $crate::cell::UnsafeCell::new(INIT_EXPR);
 
  // If a dtor isn't needed we can do something "very raw" and
  // just get going.
  if !$crate::mem::needs_drop::<$t>() {
  unsafe {
- return $crate::option::Option::Some(&VAL)
+ return $crate::option::Option::Some(&*VAL.get())
  }
  }
 
@@ -55,15 +54,15 @@ pub macro thread_local_inner {
  // so now.
  0 => {
  $crate::thread::local_impl::Key::<$t>::register_dtor(
- $crate::ptr::addr_of_mut!(VAL) as *mut $crate::primitive::u8,
+ VAL.get() as *mut $crate::primitive::u8,
  destroy,
  );
  STATE.set(1);
- $crate::option::Option::Some(&VAL)
+ $crate::option::Option::Some(&*VAL.get())
  }
  // 1 == the destructor is registered and the value
  // is valid, so return the pointer.
- 1 => $crate::option::Option::Some(&VAL),
+ 1 => $crate::option::Option::Some(&*VAL.get()),
  // otherwise the destructor has already run, so we
  // can't give access.
  _ => $crate::option::Option::None,