Add a way to get mismatch backtrace using cfg

Author: dtolnay

.github/workflows/ci.yml

@@ -76,7 +76,7 @@ jobs:
       - name: RUSTFLAGS='-Z allow-features=' cargo test
         run: cargo test
         env:
-          RUSTFLAGS: -Z allow-features= ${{env.RUSTFLAGS}}
+          RUSTFLAGS: -Z allow-features= --cfg procmacro2_backtrace ${{env.RUSTFLAGS}}
 
   minimal:
     name: Minimal versions

src/wrapper.rs

@@ -40,7 +40,15 @@ impl LexError {
 
 #[cold]
 fn mismatch(line: u32) -> ! {
-    panic!("compiler/fallback mismatch #{}", line)
+    #[cfg(procmacro2_backtrace)]
+    {
+        let backtrace = std::backtrace::Backtrace::force_capture();
+        panic!("compiler/fallback mismatch #{}\n\n{}", line, backtrace)
+    }
+    #[cfg(not(procmacro2_backtrace))]
+    {
+        panic!("compiler/fallback mismatch #{}", line)
+    }
 }
 
 impl DeferredTokenStream {