Hide panic output in nightly_works
diff --git a/src/unstable.rs b/src/unstable.rs
index e62c86a..4706ca7 100644
--- a/src/unstable.rs
+++ b/src/unstable.rs
@@ -23,16 +23,49 @@
fn nightly_works() -> bool {
use std::sync::atomic::*;
+ use std::sync::Once;
+
static WORKS: AtomicUsize = ATOMIC_USIZE_INIT;
+ static INIT: Once = Once::new();
match WORKS.load(Ordering::SeqCst) {
1 => return false,
2 => return true,
_ => {}
}
- let works = panic::catch_unwind(|| proc_macro::Span::call_site()).is_ok();
- WORKS.store(works as usize + 1, Ordering::SeqCst);
- works
+
+ // 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.
+ INIT.call_once(|| {
+ let original_hook = panic::take_hook();
+ panic::set_hook(Box::new(|_panic_info| { /* ignore */ }));
+ let works = panic::catch_unwind(|| proc_macro::Span::call_site()).is_ok();
+ WORKS.store(works as usize + 1, Ordering::SeqCst);
+ panic::set_hook(original_hook);
+ });
+ nightly_works()
}
fn mismatch() -> ! {