Use proc_macro::is_available() on rust 1.57+

build.rs

@@ -33,6 +33,12 @@
 //     location of a token. Enabled by procmacro2_semver_exempt or the
 //     "span-locations" Cargo cfg. This is behind a cfg because tracking
 //     location inside spans is a performance hit.
+//
+// "is_available"
+//     Use `proc_macro::is_available()` to detect if the proc macro API is
+//     available or needs to be polyfilled instead of trying to use the proc
+//     macro API and catching a panic if it isn't available.
+//     Enabled on Rust 1.57+
 
 use std::env;
 use std::iter;
@@ -82,6 +88,10 @@ fn main() {
         println!("cargo:rustc-cfg=literal_from_str");
     }
 
+    if version.minor >= 57 {
+        println!("cargo:rustc-cfg=is_available");
+    }
+
     let target = env::var("TARGET").unwrap();
     if !enable_use_proc_macro(&target) {
         return;

src/detection.rs

@@ -49,19 +49,27 @@ pub(crate) fn unforce_fallback() {
 // 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;
+    #[cfg(feature = "is_available")]
+    {
+        WORKS.store(proc_macro::is_available() as usize + 1, Ordering::SeqCst);
+    }
+
+    #[cfg(not(feature = "is_available"))]
+    {
+        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 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 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");
+        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");
+        }
     }
 }