76 lines
2.6 KiB
Rust
76 lines
2.6 KiB
Rust
use core::sync::atomic::{AtomicUsize, Ordering};
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use std::sync::Once;
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static WORKS: AtomicUsize = AtomicUsize::new(0);
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static INIT: Once = Once::new();
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pub(crate) fn inside_proc_macro() -> bool {
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match WORKS.load(Ordering::Relaxed) {
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1 => return false,
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2 => return true,
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_ => {}
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}
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INIT.call_once(initialize);
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inside_proc_macro()
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}
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pub(crate) fn force_fallback() {
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WORKS.store(1, Ordering::Relaxed);
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}
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pub(crate) fn unforce_fallback() {
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initialize();
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}
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#[cfg(not(no_is_available))]
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fn initialize() {
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let available = proc_macro::is_available();
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WORKS.store(available as usize + 1, Ordering::Relaxed);
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}
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// Swap in a null panic hook to avoid printing "thread panicked" to stderr,
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// then use catch_unwind to determine whether the compiler's proc_macro is
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// working. When proc-macro2 is used from outside of a procedural macro all
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// of the proc_macro crate's APIs currently panic.
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//
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// The Once is to prevent the possibility of this ordering:
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//
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// thread 1 calls take_hook, gets the user's original hook
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// thread 1 calls set_hook with the null hook
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// thread 2 calls take_hook, thinks null hook is the original hook
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// thread 2 calls set_hook with the null hook
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// thread 1 calls set_hook with the actual original hook
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// thread 2 calls set_hook with what it thinks is the original hook
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//
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// in which the user's hook has been lost.
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//
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// There is still a race condition where a panic in a different thread can
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// happen during the interval that the user's original panic hook is
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// unregistered such that their hook is incorrectly not called. This is
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// sufficiently unlikely and less bad than printing panic messages to stderr
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// on correct use of this crate. Maybe there is a libstd feature request
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// here. For now, if a user needs to guarantee that this failure mode does
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// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from
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// the main thread before launching any other threads.
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#[cfg(no_is_available)]
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fn initialize() {
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use std::panic::{self, PanicInfo};
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type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static;
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let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ });
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let sanity_check = &*null_hook as *const PanicHook;
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let original_hook = panic::take_hook();
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panic::set_hook(null_hook);
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let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok();
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WORKS.store(works as usize + 1, Ordering::Relaxed);
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let hopefully_null_hook = panic::take_hook();
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panic::set_hook(original_hook);
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if sanity_check != &*hopefully_null_hook {
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panic!("observed race condition in proc_macro2::inside_proc_macro");
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}
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}
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