error[E0308]: mismatched types
  --> $DIR/issue-89275.rs:28:29
   |
LL |     let other: &mut Other = downcast();
   |                ----------   ^^^^^^^^^^ types differ in mutability
   |                |
   |                expected due to this
   |
   = note: expected mutable reference `&mut Other`
                      found reference `&_`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.