mbe: Mark expansion errors cold_path

Author: tgross35

compiler/rustc_expand/src/lib.rs

@@ -4,6 +4,7 @@
 #![doc(rust_logo)]
 #![feature(array_windows)]
 #![feature(associated_type_defaults)]
+#![feature(cold_path)]
 #![feature(if_let_guard)]
 #![feature(macro_metavar_expr)]
 #![feature(proc_macro_diagnostic)]

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -229,6 +229,8 @@ fn expand_macro<'cx>(
             let tts = match transcribe(psess, &named_matches, rhs, rhs_span, transparency, id) {
                 Ok(tts) => tts,
                 Err(err) => {
+                    // Pessimize error handling for perf
+                    std::hint::cold_path();
                     let guar = err.emit();
                     return DummyResult::any(arm_span, guar);
                 }