Move proc-macro detection to a module

Author: dtolnay

src/detection.rs

@@ -0,0 +1,58 @@
+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,
+        _ => {}
+    }
+
+    // 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(|| {
+        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");
+        }
+    });
+
+    inside_proc_macro()
+}

src/lib.rs

@@ -96,6 +96,7 @@ use std::path::PathBuf;
 use std::rc::Rc;
 use std::str::FromStr;
 
+mod detection;
 #[macro_use]
 mod strnom;
 mod fallback;

src/wrapper.rs

@@ -1,11 +1,12 @@
 use std::fmt;
 use std::iter;
 use std::ops::RangeBounds;
-use std::panic::{self, PanicInfo};
+use std::panic;
 #[cfg(super_unstable)]
 use std::path::PathBuf;
 use std::str::FromStr;
 
+use crate::detection::inside_proc_macro;
 use crate::{fallback, Delimiter, Punct, Spacing, TokenTree};
 
 #[derive(Clone)]
@@ -29,63 +30,6 @@ pub(crate) enum LexError {
     Fallback(fallback::LexError),
 }
 
-fn inside_proc_macro() -> bool {
-    use std::sync::atomic::*;
-    use std::sync::Once;
-
-    static WORKS: AtomicUsize = AtomicUsize::new(0);
-    static INIT: Once = Once::new();
-
-    match WORKS.load(Ordering::SeqCst) {
-        1 => return false,
-        2 => return true,
-        _ => {}
-    }
-
-    // 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(|| {
-        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");
-        }
-    });
-    inside_proc_macro()
-}
-
 fn mismatch() -> ! {
     panic!("stable/nightly mismatch")
 }