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use std::panic::{self, PanicInfo}; use std::sync::atomic::*; 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::SeqCst) { 1 => return false, 2 => return true, _ => {} } INIT.call_once(initialize); inside_proc_macro() } pub(crate) fn force_fallback() { WORKS.store(1, Ordering::SeqCst); } pub(crate) fn unforce_fallback() { initialize(); } // 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. fn initialize() { 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::SeqCst); 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"); } }