Restore compatibility with rustc pre 1.53
Restore compatibility with rustc pre 1.53

The signature of NonNull::as_ref is more restrictive on lifetimes in
older standard libraries.

    error[E0515]: cannot return value referencing local variable `repr`
      --> src/rust_str.rs:19:9
       |
    19 |         repr.as_ref()
       |         ----^^^^^^^^^
       |         |
       |         returns a value referencing data owned by the current function
       |         `repr` is borrowed here

dtolnay/cxx,dtolnay/cxx,dtolnay/cxx,dtolnay/cxx