function pointer vs Fn trait object

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fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 { // definition
    f(arg) + f(arg)
}

do_twice(|x| x + 1, 5) // call

This function accepts both, closures and function pointers. It takes a function pointer as parameter type.

When should I prefer this over using a trait object, like &dyn Fn(i32) -> i32 or Box<dyn Fn(i32)-> i32> instead of fn

fn do_twice(f: &dyn Fn(i32) -> i32, arg: i32) -> i32 { // definition
    f(arg) + f(arg)
}

do_twice(&|x| x + 1, 5) // call

or 

fn do_twice(f: Box<dyn Fn(i32) -> i32>, arg: i32) -> i32 { // definition
    f(arg) + f(arg)
}
trait-objects rust traits closures function
2 Answers

When should I prefer this over using a trait object

Trait objects are not the only other option. As @DarthKotik pointed out, accepting a fn pointer will not permit closures that capture their environment, but you can just use a normal type parameter, bounded by Fn to accept both functions and closures, without needing to box anything:

fn do_twice<F>(f: F, arg: i32) -> i32 
where
    F: Fn(i32) -> i32
{
    f(arg) + f(arg)
}

Or, equivalently, but avoiding an extra type variable:

fn do_twice(f: impl Fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}
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