error[E0308]: mismatched types
  --> $DIR/fn-trait-formatting.rs:6:17
   |
LL |     let _: () = (box |_: isize| {}) as Box<dyn FnOnce(isize)>;
   |            --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `Box`
   |            |
   |            expected due to this
   |
   = note: expected unit type `()`
                 found struct `Box<dyn FnOnce(isize)>`

error[E0308]: mismatched types
  --> $DIR/fn-trait-formatting.rs:10:17
   |
LL |     let _: () = (box |_: isize, isize| {}) as Box<dyn Fn(isize, isize)>;
   |            --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `Box`
   |            |
   |            expected due to this
   |
   = note: expected unit type `()`
                 found struct `Box<dyn Fn(isize, isize)>`

error[E0308]: mismatched types
  --> $DIR/fn-trait-formatting.rs:14:17
   |
LL |     let _: () = (box || -> isize { unimplemented!() }) as Box<dyn FnMut() -> isize>;
   |            --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `Box`
   |            |
   |            expected due to this
   |
   = note: expected unit type `()`
                 found struct `Box<dyn FnMut() -> isize>`

error[E0277]: expected a `Fn<(isize,)>` closure, found `{integer}`
  --> $DIR/fn-trait-formatting.rs:19:14
   |
LL | fn needs_fn<F>(x: F) where F: Fn(isize) -> isize {}
   |                               ------------------ required by this bound in `needs_fn`
...
LL |     needs_fn(1);
   |              ^ expected an `Fn<(isize,)>` closure, found `{integer}`
   |
   = help: the trait `Fn<(isize,)>` is not implemented for `{integer}`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.