Tracking issue for `thread_local` stabilization

[aturon]

The #[thread_local] attribute is currently feature-gated. This issue tracks its stabilization.

Known problems:

• #[thread_local] translates directly to the thread_local attribute in LLVM. This isn't supported on all platforms, and it's not even supported on all distributions within the same platform (e.g. macOS 10.6 didn't support it but 10.7 does). I don't think this is necessarily a blocker, but I also don't think we have many attributes and such which are so platform specific like this.
• Statics that are thread local shouldn't require Sync - #[thread_local] statics shouldn't require Sync #18001
• Statics that are thread local should either not borrow for the 'static lifetime or should be unsafe to access - Safe access to a #[thread_local] should be disallowed #17954
• Statics can currently reference other thread local statics, but this is a bug - Statics should not be able to take the address of a thread_local static #18712
• Unsound with generators Keeping references to #[thread_local] statics is allowed across yields. #49682
• static mut can be given 'static lifetime with NLL (#[thread_local] static mut is allowed to have 'static lifetime #54366)

[aturon]

@alexcrichton Can you elaborate on the blockers?

[alexcrichton]

Certainly! Known issues to me:

• #[thread_local] translates directly to the thread_local attribute in LLVM. This isn't supported on all platforms, and it's not even supported on all distributions within the same platform (e.g. 10.6 doesn't support it but 10.7 does). I don't think this is necessarily a blocker, but I also don't think we have many attributes and such which are so platform specific like this.
• Statics that are thread local shouldn't require Sync - #[thread_local] statics shouldn't require Sync #18001
• Statics that are thread local should either not borrow for the 'static lifetime or should be unsafe to access - Safe access to a #[thread_local] should be disallowed #17954
• Statics can currently reference other thread local statics, but this is a bug - Statics should not be able to take the address of a thread_local static #18712

That's at least what I can think of at this time!

[alexcrichton]

A note, we've since implemented cfg(target_thread_local) which is in turn itself feature gated, but this may ease the "this isn't implemented on all platforms" worry.

[alexandermerritt]

Hi! Is there any update on the status? Nightly still requires statics to be Sync. I tried with:

rustc 1.13.0-nightly (acd3f796d 2016-08-28)
binary: rustc
commit-hash: acd3f796d26e9295db1eba1ef16e0d4cc3b96dd5
commit-date: 2016-08-28
host: x86_64-unknown-linux-gnu
release: 1.13.0-nightly

[mneumann]

@alexcrichton Any news on #[thread_local] becoming stabilized? AFAIK, at the moment it is impossible on DragonFly to access errno variable from stable code, other than directly from libstd. This blocks crates like nix on DragonFly, which want to access errno as well, but libstd is not exposing it, and stable code is not allowed to use feature(thread_local).

[alexcrichton]

@mneumann no, no progress. I'd recommend a C shim for now.

[mneumann]

@alexcrichton thanks. I am doing a shim now https://github.com/mneumann/errno-dragonfly-rs.

[real-felix]

The optimizations are too aggressive ;)

See this code:

#![feature(thread_local)]

#[thread_local]
pub static FOO: [&str; 1] = [ "Hello" ];

fn change_foo(s: &'static str) {
    FOO[0] = s;
}

fn main() {
    println!("{}", FOO[0]);
    change_foo("Test");
    println!("{}", FOO[0]);
}

The compiler does not detect the side effect in change_foo and removes the call in release. The output is:

Hello
Hello

[alexcrichton]

cc @eddyb, @Boiethios your example shouldn't actually compile because it should require static mut, not just static