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, 1 insertions, 0 deletions

diff --git a/library/alloc/src/macros.rs b/library/alloc/src/macros.rs
index 22c19243e7f..093b02113c3 100644
--- a/library/alloc/src/macros.rs
+++ b/library/alloc/src/macros.rs
@@ -56,6 +56,7 @@ macro_rules! vec {
 // `slice::into_vec`  function which is only available with cfg(test)
 // NB see the slice::hack module in slice.rs for more information
 #[cfg(all(not(no_global_oom_handling), test))]
+#[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
 macro_rules! vec {
     () => (
         $crate::vec::Vec::new()