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

error[E0308]: mismatched types
  --> $DIR/fn-trait-formatting.rs:10:17
   |
LL |     let _: () = Box::new(|_: 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)>`
help: use parentheses to call this trait object
   |
LL |     let _: () = (Box::new(|_: isize, isize| {}) as Box<dyn Fn(isize, isize)>)(/* isize */, /* isize */);
   |                 +                                                           +++++++++++++++++++++++++++

error[E0308]: mismatched types
  --> $DIR/fn-trait-formatting.rs:14:17
   |
LL |     let _: () = Box::new(|| -> 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 |     needs_fn(1);
   |     -------- ^ expected an `Fn<(isize,)>` closure, found `{integer}`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Fn<(isize,)>` is not implemented for `{integer}`
note: required by a bound in `needs_fn`
  --> $DIR/fn-trait-formatting.rs:1:31
   |
LL | fn needs_fn<F>(x: F) where F: Fn(isize) -> isize {}
   |                               ^^^^^^^^^^^^^^^^^^ required by this bound in `needs_fn`

error: aborting due to 4 previous errors

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