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

Author: bors

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()

stdarch

@@ -1 +1 @@
-Subproject commit d215afe9d1cf79c5edb0dfd3cdf4c50aca1f1760
+Subproject commit 28335054b1f417175ab5005cf1d9cf7937737930