Rust. "the size for values of type cannot be known at compilation time" when try to return dynamic vector from function

I have a function Processor::process which can return dynamic vector of functions. When I try to use it I got an error:

error[E0277]: the size for values of type (dyn FnMut(String, Option<Vec<u8>>) -> Option<u8> + 'static) cannot be known at compilation time

This is my code:

fn handler1(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

fn handler2(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

fn handler3(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

struct Processor {}
impl Processor {
    pub fn process(data: u8) -> Vec<dyn FnMut(String, Option<Vec<u8>>) -> Option<u8>> {
        return match data {
            1 => vec![handler1],
            2 => vec![handler1, handler2],
            3 => vec![handler1, handler2, handler3],
            _ => {}
        }
    }
}

This is minimal sandbox implementation.

Could you please help to set correct typing for function return ?

>Solution :

Either you box them, or you return a reference with an specific lifetime. In this case ‘static:

fn handler1(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

fn handler2(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

fn handler3(a: String, b: Option<Vec<u8>>) -> Option<u8> {
    None
}

struct Processor {}
impl Processor {
    pub fn process(data: u8) -> Vec<&'static dyn FnMut(String, Option<Vec<u8>>) -> Option<u8>> {
        return match data {
            1 => vec![&handler1],
            2 => vec![&handler1, &handler2],
            3 => vec![&handler1, &handler2, &handler3],
            _ => vec![]
        }
    }
}

Playground

You can also just use function pointers instead of the trait dynamic dispatch:

impl Processor {
    pub fn process(data: u8) -> Vec<fn(String, Option<Vec<u8>>) -> Option<u8>> {
        return match data {
            1 => vec![handler1],
            2 => vec![handler1, handler2],
            3 => vec![handler1, handler2, handler3],
            _ => vec![]
        }
    }
}

Playground