error[E0277]: the size for values of type `(dyn Fn() + 'static)` cannot be known at compilation time
  --> $DIR/dont-suggest-impl-as-closure-arg.rs:3:17
   |
LL |     let c = |f: dyn Fn()| f();
   |                 ^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Fn() + 'static)`
   = help: unsized fn params are gated as an unstable feature
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL |     let c = |f: &dyn Fn()| f();
   |                 +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.