1 use core::sync::atomic::{AtomicUsize, Ordering};
2 use std::sync::Once;
3
4 static WORKS: AtomicUsize = AtomicUsize::new(0);
5 static INIT: Once = Once::new();
6
inside_proc_macro() -> bool7 pub(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
force_fallback()18 pub(crate) fn force_fallback() {
19 WORKS.store(1, Ordering::Relaxed);
20 }
21
unforce_fallback()22 pub(crate) fn unforce_fallback() {
23 initialize();
24 }
25
26 #[cfg(not(no_is_available))]
initialize()27 fn 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)]
initialize()57 fn 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