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Commit | Line | Data |
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3c0e092e | 1 | use std::sync::atomic::{AtomicUsize, Ordering}; |
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2 | use std::sync::Once; |
3 | ||
4 | static WORKS: AtomicUsize = AtomicUsize::new(0); | |
5 | static INIT: Once = Once::new(); | |
6 | ||
7 | pub(crate) fn inside_proc_macro() -> bool { | |
5099ac24 | 8 | match WORKS.load(Ordering::Relaxed) { |
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9 | 1 => return false, |
10 | 2 => return true, | |
11 | _ => {} | |
12 | } | |
13 | ||
14 | INIT.call_once(initialize); | |
15 | inside_proc_macro() | |
16 | } | |
17 | ||
18 | pub(crate) fn force_fallback() { | |
5099ac24 | 19 | WORKS.store(1, Ordering::Relaxed); |
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20 | } |
21 | ||
22 | pub(crate) fn unforce_fallback() { | |
23 | initialize(); | |
24 | } | |
25 | ||
a2a8927a | 26 | #[cfg(not(no_is_available))] |
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27 | fn initialize() { |
28 | let available = proc_macro::is_available(); | |
5099ac24 | 29 | WORKS.store(available as usize + 1, Ordering::Relaxed); |
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30 | } |
31 | ||
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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. | |
a2a8927a | 56 | #[cfg(no_is_available)] |
f035d41b | 57 | fn initialize() { |
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58 | use std::panic::{self, PanicInfo}; |
59 | ||
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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(); | |
5099ac24 | 68 | WORKS.store(works as usize + 1, Ordering::Relaxed); |
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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 | } |