Auto merge of #96150 - est31:unused_macro_rules, r=petrochenkov

Implement a lint to warn about unused macro rules

This implements a new lint to warn about unused macro rules (arms/matchers), similar to the `unused_macros` lint added by #41907 that warns about entire macros.

```rust
macro_rules! unused_empty {
    (hello) => { println!("Hello, world!") };
    () => { println!("empty") }; //~ ERROR: 1st rule of macro `unused_empty` is never used
}

fn main() {
    unused_empty!(hello);
}
```

Builds upon #96149 and #96156.

Fixes #73576

1 files changed, 2 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index cf9cf1b70aa..4407297c943 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -816,6 +816,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
     span: Span,
 ) -> Result<&'ll Value, ()> {
     // macros for error handling:
+    #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
     macro_rules! emit_error {
         ($msg: tt) => {
             emit_error!($msg, )
@@ -1144,6 +1145,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
         span: Span,
         args: &[OperandRef<'tcx, &'ll Value>],
     ) -> Result<&'ll Value, ()> {
+        #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
         macro_rules! emit_error {
             ($msg: tt) => {
                 emit_error!($msg, )