error[E0515]: cannot return value referencing local variable `s`
  --> $DIR/issue-67765-async-diagnostic.rs:13:5
   |
LL |     let b = &s[..];
   |              - `s` is borrowed here
LL |
LL |     Err(b)?;
   |     ^^^^^^^ returns a value referencing data owned by the current function

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0515`.