rustc/library/std/src/sys_common/thread_info.rs

#![allow(dead_code)] // stack_guard isn't used right now on all platforms
#![allow(unused_unsafe)] // thread_local with `const {}` triggers this liny

use crate::cell::RefCell;
use crate::sys::thread::guard::Guard;
use crate::thread::Thread;

struct ThreadInfo {
    stack_guard: Option<Guard>,
    thread: Thread,
}

thread_local! { static THREAD_INFO: RefCell<Option<ThreadInfo>> = const { RefCell::new(None) } }

impl ThreadInfo {
    fn with<R, F>(f: F) -> Option<R>
    where
        F: FnOnce(&mut ThreadInfo) -> R,
    {
        THREAD_INFO
            .try_with(move |thread_info| {
                let mut thread_info = thread_info.borrow_mut();
                let thread_info = thread_info.get_or_insert_with(|| ThreadInfo {
                    stack_guard: None,
                    thread: Thread::new(None),
                });
                f(thread_info)
            })
            .ok()
    }
}

pub fn current_thread() -> Option<Thread> {
    ThreadInfo::with(|info| info.thread.clone())
}

pub fn stack_guard() -> Option<Guard> {
    ThreadInfo::with(|info| info.stack_guard.clone()).and_then(|o| o)
}

pub fn set(stack_guard: Option<Guard>, thread: Thread) {
    THREAD_INFO.with(move |thread_info| {
        let mut thread_info = thread_info.borrow_mut();
        rtassert!(thread_info.is_none());
        *thread_info = Some(ThreadInfo { stack_guard, thread });
    });
}