Fix review comment

src/detection.rs

@@ -12,24 +12,16 @@ pub(crate) fn inside_proc_macro() -> bool {
         _ => {}
     }
 
-    #[cfg(feature = "is_available")]
-    {
-        proc_macro::is_available()
-    }
-
-    #[cfg(not(feature = "is_available"))]
-    {
     INIT.call_once(initialize);
     inside_proc_macro()
 }
-}
 
 pub(crate) fn force_fallback() {
     WORKS.store(1, Ordering::SeqCst);
 }
 
 pub(crate) fn unforce_fallback() {
-    WORKS.store(0, Ordering::SeqCst);
+    initialize();
 }
 
 // Swap in a null panic hook to avoid printing "thread panicked" to stderr,
@@ -57,6 +49,13 @@ 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() {
+    #[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 */ });
@@ -73,3 +72,4 @@ fn initialize() {
             panic!("observed race condition in proc_macro2::inside_proc_macro");
         }
     }
+}