// SPDX-License-Identifier: Apache-2.0 OR MIT use core::sync::atomic::{AtomicUsize, Ordering};use std::sync::Once; static WORKS: AtomicUsize = AtomicUsize::new(0);static INIT: Once = Once::new(); pub(crate) fn inside_proc_macro() -> bool {    match WORKS.load(Ordering::Relaxed) {        1 => return false,        2 => return true,        _ => {}    }     INIT.call_once(initialize);    inside_proc_macro()} pub(crate) fn force_fallback() {    WORKS.store(1, Ordering::Relaxed);} pub(crate) fn unforce_fallback() {    initialize();} #[cfg(not(no_is_available))]fn initialize() {    let available = proc_macro::is_available();    WORKS.store(available as usize + 1, Ordering::Relaxed);} // Swap in a null panic hook to avoid printing "thread panicked" to stderr,// then use catch_unwind to determine whether the compiler's proc_macro is// working. When proc-macro2 is used from outside of a procedural macro all// of the proc_macro crate's APIs currently panic.//// The Once is to prevent the possibility of this ordering:////     thread 1 calls take_hook, gets the user's original hook//     thread 1 calls set_hook with the null hook//     thread 2 calls take_hook, thinks null hook is the original hook//     thread 2 calls set_hook with the null hook//     thread 1 calls set_hook with the actual original hook//     thread 2 calls set_hook with what it thinks is the original hook//// in which the user's hook has been lost.//// There is still a race condition where a panic in a different thread can// happen during the interval that the user's original panic hook is// unregistered such that their hook is incorrectly not called. This is// sufficiently unlikely and less bad than printing panic messages to stderr// on correct use of this crate. Maybe there is a libstd feature request// here. For now, if a user needs to guarantee that this failure mode does// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from// the main thread before launching any other threads.#[cfg(no_is_available)]fn initialize() {    use std::panic::{self, PanicInfo};     type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static;     let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ });    let sanity_check = &*null_hook as *const PanicHook;    let original_hook = panic::take_hook();    panic::set_hook(null_hook);     let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok();    WORKS.store(works as usize + 1, Ordering::Relaxed);     let hopefully_null_hook = panic::take_hook();    panic::set_hook(original_hook);    if sanity_check != &*hopefully_null_hook {        panic!("observed race condition in proc_macro2::inside_proc_macro");    }}