1 // SPDX-License-Identifier: Apache-2.0 OR MIT 2 3 use core::sync::atomic::{AtomicUsize, Ordering}; 4 use std::sync::Once; 5 6 static WORKS: AtomicUsize = AtomicUsize::new(0); 7 static INIT: Once = Once::new(); 8 9 pub(crate) fn inside_proc_macro() -> bool { 10 match WORKS.load(Ordering::Relaxed) { 11 1 => return false, 12 2 => return true, 13 _ => {} 14 } 15 16 INIT.call_once(initialize); 17 inside_proc_macro() 18 } 19 20 pub(crate) fn force_fallback() { 21 WORKS.store(1, Ordering::Relaxed); 22 } 23 24 pub(crate) fn unforce_fallback() { 25 initialize(); 26 } 27 28 #[cfg(not(no_is_available))] 29 fn initialize() { 30 let available = proc_macro::is_available(); 31 WORKS.store(available as usize + 1, Ordering::Relaxed); 32 } 33 34 // Swap in a null panic hook to avoid printing "thread panicked" to stderr, 35 // then use catch_unwind to determine whether the compiler's proc_macro is 36 // working. When proc-macro2 is used from outside of a procedural macro all 37 // of the proc_macro crate's APIs currently panic. 38 // 39 // The Once is to prevent the possibility of this ordering: 40 // 41 // thread 1 calls take_hook, gets the user's original hook 42 // thread 1 calls set_hook with the null hook 43 // thread 2 calls take_hook, thinks null hook is the original hook 44 // thread 2 calls set_hook with the null hook 45 // thread 1 calls set_hook with the actual original hook 46 // thread 2 calls set_hook with what it thinks is the original hook 47 // 48 // in which the user's hook has been lost. 49 // 50 // There is still a race condition where a panic in a different thread can 51 // happen during the interval that the user's original panic hook is 52 // unregistered such that their hook is incorrectly not called. This is 53 // sufficiently unlikely and less bad than printing panic messages to stderr 54 // on correct use of this crate. Maybe there is a libstd feature request 55 // here. For now, if a user needs to guarantee that this failure mode does 56 // not occur, they need to call e.g. `proc_macro2::Span::call_site()` from 57 // the main thread before launching any other threads. 58 #[cfg(no_is_available)] 59 fn initialize() { 60 use std::panic::{self, PanicInfo}; 61 62 type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static; 63 64 let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ }); 65 let sanity_check = &*null_hook as *const PanicHook; 66 let original_hook = panic::take_hook(); 67 panic::set_hook(null_hook); 68 69 let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok(); 70 WORKS.store(works as usize + 1, Ordering::Relaxed); 71 72 let hopefully_null_hook = panic::take_hook(); 73 panic::set_hook(original_hook); 74 if sanity_check != &*hopefully_null_hook { 75 panic!("observed race condition in proc_macro2::inside_proc_macro"); 76 } 77 } 78