Code change:
<ins> // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Scoped thread-local storage //! //! This module provides the ability to generate *scoped* thread-local //! variables. In this sense, scoped indicates that thread local storage //! actually stores a reference to a value, and this reference is only placed //! in storage for a scoped amount of time. //! //! There are no restrictions on what types can be placed into a scoped //! variable, but all scoped variables are initialized to the equivalent of //! null. Scoped thread local storage is useful when a value is present for a known //! period of time and it is not required to relinquish ownership of the //! contents. //! //! # Examples //! //! ``` //! # #![feature(scoped_tls)] //! scoped_thread_local!(static FOO: u32); //! //! // Initially each scoped slot is empty. //! assert!(!FOO.is_set()); //! //! // When inserting a value, the value is only in place for the duration //! // of the closure specified. //! FOO.set(&1, || { //!     FOO.with(|slot| { //!         assert_eq!(*slot, 1); //!     }); //! }); //! ``` #![unstable(feature = "thread_local_internals")] use prelude::v1::*; // macro hygiene sure would be nice, wouldn't it? #[doc(hidden)] pub mod __impl { pub use super::imp::KeyInner; pub use sys_common::thread_local::INIT as OS_INIT; } /// Type representing a thread local storage key corresponding to a reference /// to the type parameter `T`. /// /// Keys are statically allocated and can contain a reference to an instance of /// type `T` scoped to a particular lifetime. Keys provides two methods, `set` /// and `with`, both of which currently use closures to control the scope of /// their contents. #[unstable(feature = "scoped_tls", reason = "scoped TLS has yet to have wide enough use to fully consider  stabilizing its interface")] pub struct ScopedKey<T> { #[doc(hidden)] pub inner: __impl::KeyInner<T> } /// Declare a new scoped thread local storage key. /// /// This macro declares a `static` item on which methods are used to get and /// set the value stored within. #[macro_export] #[allow_internal_unstable] macro_rules! scoped_thread_local { (static $name:ident: $t:ty) => ( __scoped_thread_local_inner!(static $name: $t); ); (pub static $name:ident: $t:ty) => ( __scoped_thread_local_inner!(pub static $name: $t); ); } #[macro_export] #[doc(hidden)] #[allow_internal_unstable] macro_rules! __scoped_thread_local_inner { (static $name:ident: $t:ty) => ( #[cfg_attr(not(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64")), thread_local)] static $name: ::std::thread::ScopedKey<$t> = __scoped_thread_local_inner!($t); ); (pub static $name:ident: $t:ty) => ( #[cfg_attr(not(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64")), thread_local)] pub static $name: ::std::thread::ScopedKey<$t> = __scoped_thread_local_inner!($t); ); ($t:ty) => ({ use std::thread::ScopedKey as __Key; #[cfg(not(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64")))] const _INIT: __Key<$t> = __Key { inner: ::std::thread::__scoped::KeyInner { inner: ::std::cell::UnsafeCell { value: 0 as *mut _ }, } }; #[cfg(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64"))] const _INIT: __Key<$t> = __Key { inner: ::std::thread::__scoped::KeyInner { inner: ::std::thread::__scoped::OS_INIT, marker: ::std::marker::PhantomData::<::std::cell::Cell<$t>>, } }; _INIT }) } #[unstable(feature = "scoped_tls", reason = "scoped TLS has yet to have wide enough use to fully consider  stabilizing its interface")] impl<T> ScopedKey<T> { /// Insert a value into this scoped thread local storage slot for a /// duration of a closure. /// /// While `cb` is running, the value `t` will be returned by `get` unless /// this function is called recursively inside of `cb`. /// /// Upon return, this function will restore the previous value, if any /// was available. /// /// # Examples /// /// ``` /// # #![feature(scoped_tls)] /// scoped_thread_local!(static FOO: u32); /// /// FOO.set(&100, || { ///     let val = FOO.with(|v| *v); ///     assert_eq!(val, 100); /// ///     // set can be called recursively ///     FOO.set(&101, || { ///         // ... ///     }); /// ///     // Recursive calls restore the previous value. ///     let val = FOO.with(|v| *v); ///     assert_eq!(val, 100); /// }); /// ``` pub fn set<R, F>(&'static self, t: &T, cb: F) -> R where F: FnOnce() -> R, { struct Reset<'a, T: 'a> { key: &'a __impl::KeyInner<T>, val: *mut T, } #[unsafe_destructor] impl<'a, T> Drop for Reset<'a, T> { fn drop(&mut self) { unsafe { self.key.set(self.val) } } } let prev = unsafe { let prev = self.inner.get(); self.inner.set(t as *const T as *mut T); prev }; let _reset = Reset { key: &self.inner, val: prev }; cb() } /// Get a value out of this scoped variable. /// /// This function takes a closure which receives the value of this /// variable. /// /// # Panics /// /// This function will panic if `set` has not previously been called. /// /// # Examples /// /// ```no_run /// # #![feature(scoped_tls)] /// scoped_thread_local!(static FOO: u32); /// /// FOO.with(|slot| { ///     // work with `slot` /// }); /// ``` pub fn with<R, F>(&'static self, cb: F) -> R where F: FnOnce(&T) -> R { unsafe { let ptr = self.inner.get(); assert!(!ptr.is_null(), "cannot access a scoped thread local  variable without calling `set` first"); cb(&*ptr) } } /// Test whether this TLS key has been `set` for the current thread. pub fn is_set(&'static self) -> bool { unsafe { !self.inner.get().is_null() } } } #[cfg(not(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64")))] mod imp { use std::cell::UnsafeCell; #[doc(hidden)] pub struct KeyInner<T> { pub inner: UnsafeCell<*mut T> } unsafe impl<T> ::marker::Sync for KeyInner<T> { } #[doc(hidden)] impl<T> KeyInner<T> { #[doc(hidden)] pub unsafe fn set(&self, ptr: *mut T) { *self.inner.get() = ptr; } #[doc(hidden)] pub unsafe fn get(&self) -> *mut T { *self.inner.get() } } } #[cfg(any(windows, target_os = "android", target_os = "ios", target_os = "openbsd", target_arch = "aarch64"))] mod imp { use marker; use std::cell::Cell; use sys_common::thread_local::StaticKey as OsStaticKey; #[doc(hidden)] pub struct KeyInner<T> { pub inner: OsStaticKey, pub marker: marker::PhantomData<Cell<T>>, } unsafe impl<T> ::marker::Sync for KeyInner<T> { } #[doc(hidden)] impl<T> KeyInner<T> { #[doc(hidden)] pub unsafe fn set(&self, ptr: *mut T) { self.inner.set(ptr as *mut _) } #[doc(hidden)] pub unsafe fn get(&self) -> *mut T { self.inner.get() as *mut _ } } } #[cfg(test)] mod tests { use cell::Cell; use prelude::v1::*; scoped_thread_local!(static FOO: u32); #[test] fn smoke() { scoped_thread_local!(static BAR: u32); assert!(!BAR.is_set()); BAR.set(&1, || { assert!(BAR.is_set()); BAR.with(|slot| { assert_eq!(*slot, 1); }); }); assert!(!BAR.is_set()); } #[test] fn cell_allowed() { scoped_thread_local!(static BAR: Cell<u32>); BAR.set(&Cell::new(1), || { BAR.with(|slot| { assert_eq!(slot.get(), 1); }); }); } #[test] fn scope_item_allowed() { assert!(!FOO.is_set()); FOO.set(&1, || { assert!(FOO.is_set()); FOO.with(|slot| { assert_eq!(*slot, 1); }); }); assert!(!FOO.is_set()); } } </ins>

Related discussion:
text: I have no idea what's causing this, but on the current nightly, this fails: with "use of unstable library feature 'threadlocalinternals'." Enabling the feature works, but this should be a stable function. Interestingly, tweaking the works: I wonder if it's because there is a (private) sub-module called which has a module-wide marker. Version info:
This change was introduced just recently: I reproduced the problem without using anything thread-related: Commands: Output: If I remove the attribute from the module, the code gives me the error. I mark as stable, then the code compiles. If I instead remove the function, I get the bar error. cc
With merged, is this fixed?
The exact issue at hand has been resolved, but the underlying issue remains. Specifically a module and free-function of the same name but different stability level will cause problems.
Also hit by this; still affecting the nightly for osx right now, but hopefully the next build will include the fix.
Stability is an internal detail. This is unlikely to impact out-of-tree code.