std::thread_local!

macro_rules! thread_local {
    (static $name:ident: $t:ty = $init:expr) => (
        static $name: ::std::thread::LocalKey<$t> = {
            #[cfg_attr(all(any(target_os = "macos", target_os = "linux"),
                           not(target_arch = "aarch64")),
                       thread_local)]
            static __KEY: ::std::thread::__LocalKeyInner<$t> =
                ::std::thread::__LocalKeyInner::new();
            fn __init() -> $t { $init }
            fn __getit() -> &'static ::std::thread::__LocalKeyInner<$t> { &__KEY }
            ::std::thread::LocalKey::new(__getit, __init)
        };
    );
    (pub static $name:ident: $t:ty = $init:expr) => (
        pub static $name: ::std::thread::LocalKey<$t> = {
            #[cfg_attr(all(any(target_os = "macos", target_os = "linux"),
                           not(target_arch = "aarch64")),
                       thread_local)]
            static __KEY: ::std::thread::__LocalKeyInner<$t> =
                ::std::thread::__LocalKeyInner::new();
            fn __init() -> $t { $init }
            fn __getit() -> &'static ::std::thread::__LocalKeyInner<$t> { &__KEY }
            ::std::thread::LocalKey::new(__getit, __init)
        };
    );
}

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

See LocalKey documentation for more information.