Declare a new thread local storage key of type std::thread::LocalKey.

Syntax

The macro wraps any number of static declarations and makes them thread local. Publicity and attributes for each static are allowed. Example:

use std::cell::{Cell, RefCell};

thread_local! {
    pub static FOO: Cell<u32> = const { Cell::new(1) };

    static BAR: RefCell<Vec<f32>> = RefCell::new(vec![1.0, 2.0]);
}

assert_eq!(FOO.get(), 1);
BAR.with_borrow(|v| assert_eq!(v[1], 2.0));

Note that only shared references (&T) to the inner data may be obtained, so a type such as Cell or RefCell is typically used to allow mutating access.

This macro supports a special const {} syntax that can be used when the initialization expression can be evaluated as a constant. This can enable a more efficient thread local implementation that can avoid lazy initialization. For types that do not [need to be dropped][crate::mem::needs_drop], this can enable an even more efficient implementation that does not need to track any additional state.

use std::cell::RefCell;

thread_local! {
    pub static FOO: RefCell<Vec<u32>> = const { RefCell::new(Vec::new()) };
}

FOO.with_borrow(|v| assert_eq!(v.len(), 0));

See LocalKey documentation for more information.