Macro tracing_core::stdlib::thread_local

macro_rules! thread_local {
    () => { ... };
    (
        $(#[$attr:meta])* $vis:vis static $name:ident : $t:ty = const
        $init:block; $($rest:tt)*
    ) => { ... };
    ($(#[$attr:meta])* $vis:vis static $name:ident : $t:ty = const $init:block) => { ... };
    (
        $(#[$attr:meta])* $vis:vis static $name:ident : $t:ty = $init:expr;
        $($rest:tt)*
    ) => { ... };
    ($(#[$attr:meta])* $vis:vis static $name:ident : $t:ty = $init:expr) => { ... };
}

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> = 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, 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.