manual clippy fixes

Author: pietroalbini

library/proc_macro/src/bridge/arena.rs

@@ -68,6 +68,7 @@ impl Arena {
     /// Allocates a byte slice with specified size from the current memory
     /// chunk. Returns `None` if there is no free space left to satisfy the
     /// request.
+    #[allow(clippy::mut_from_ref)]
     fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> {
         let start = self.start.get().addr();
         let old_end = self.end.get();