Rust already supports the kind of behaviour you are describing for borrows, because of non-lexical lifetimes. Code like the following now compiles:

    fn main() {
      let mut x = 42;
      let y = &x;
      println!("{y}");
      let z = &mut x;
    }

Even though y's scope overlaps with z's, and they introduce conflicting borrows, this code compiles because the compiler treats y's borrow as dead after its last use (this has been true since Rust Edition 2018, so for quite some time now). If you move the println after the mutable borrow then it fails to compile.

However values whose types have Drop are another matter. They are treated as if there's an explicit call to their drop function at the end of their lexical scope which pins their lifetime. This is intentional and desirable precisely because of the guard pattern (like for mutexes).

If you didn't have that guarantee, at worst your mutex's guard object would be immediately dropped because it's never referenced after it's created, or at best it would be very tricky to understand what the protected critical region is.