A read only no alias reference

chrefr:

It is already possible to do that:

This doesn't quite work:

    let a = 42u8;
    let r = ExclusiveReadOnly::new(&a);



error[E0308]: mismatched types
  --> src/main.rs:19:36
   |
19 |     let r = ExclusiveReadOnly::new(&a);
   |             ---------------------- ^^ types differ in mutability
   |             |
   |             arguments to this function are incorrect



    let a = 42u8;
    let r = ExclusiveReadOnly::new(&mut a);



error[E0596]: cannot borrow `a` as mutable, as it is not declared as mutable
  --> src/main.rs:19:36
   |
19 |     let r = ExclusiveReadOnly::new(&mut a);
   |                                    ^^^^^^ cannot borrow as mutable
   |