1use core::sync::atomic::{AtomicUsize, Ordering};
2use std::sync::Once;
3
4static WORKS: AtomicUsize = AtomicUsize::new(0);
5static INIT: Once = Once::new();
6
7pub(crate) fn inside_proc_macro() -> bool {
8    match WORKS.load(Ordering::Relaxed) {
9        1 => return false,
10        2 => return true,
11        _ => {}
12    }
13
14    INIT.call_once(initialize);
15    inside_proc_macro()
16}
17
18pub(crate) fn force_fallback() {
19    WORKS.store(1, Ordering::Relaxed);
20}
21
22pub(crate) fn unforce_fallback() {
23    initialize();
24}
25
26#[cfg(not(no_is_available))]
27fn initialize() {
28    let available = proc_macro::is_available();
29    WORKS.store(available as usize + 1, Ordering::Relaxed);
30}
31
32// Swap in a null panic hook to avoid printing "thread panicked" to stderr,
33// then use catch_unwind to determine whether the compiler's proc_macro is
34// working. When proc-macro2 is used from outside of a procedural macro all
35// of the proc_macro crate's APIs currently panic.
36//
37// The Once is to prevent the possibility of this ordering:
38//
39//     thread 1 calls take_hook, gets the user's original hook
40//     thread 1 calls set_hook with the null hook
41//     thread 2 calls take_hook, thinks null hook is the original hook
42//     thread 2 calls set_hook with the null hook
43//     thread 1 calls set_hook with the actual original hook
44//     thread 2 calls set_hook with what it thinks is the original hook
45//
46// in which the user's hook has been lost.
47//
48// There is still a race condition where a panic in a different thread can
49// happen during the interval that the user's original panic hook is
50// unregistered such that their hook is incorrectly not called. This is
51// sufficiently unlikely and less bad than printing panic messages to stderr
52// on correct use of this crate. Maybe there is a libstd feature request
53// here. For now, if a user needs to guarantee that this failure mode does
54// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from
55// the main thread before launching any other threads.
56#[cfg(no_is_available)]
57fn initialize() {
58    use std::panic::{self, PanicInfo};
59
60    type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static;
61
62    let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ });
63    let sanity_check = &*null_hook as *const PanicHook;
64    let original_hook = panic::take_hook();
65    panic::set_hook(null_hook);
66
67    let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok();
68    WORKS.store(works as usize + 1, Ordering::Relaxed);
69
70    let hopefully_null_hook = panic::take_hook();
71    panic::set_hook(original_hook);
72    if sanity_check != &*hopefully_null_hook {
73        panic!("observed race condition in proc_macro2::inside_proc_macro");
74    }
75}
76