Return obj that defines a closure parameter

1

How do I make this work

struct A<T, F: FnMut(T) -> T> {
    f: F,
    t: T,
}

fn foo<T, F: FnMut(T) -> T>() -> A<T, F> {
    A {
        t: 1,
        f: |x|{x + 1},
    }
}
2

Generics let the caller control the type. You can't use generics like that. You need to specify the return type of the function explicitly:

struct A<T, F: FnMut(T) -> T> {
    f: F,
    t: T,
}

fn foo() -> A<i32, impl FnMut(i32) -> i32> {
    A {
        t: 1,
        f: |x| x + 1,
    }
}

fn main() {
    let mut a = foo();
    println!("{:?}", (a.f)(3));
}