Can not call boxed FnOnce

[chrysn]

Compared to alloc::boxed::Box, this can not be used to run a FnOnce from an owned pointer; it'd be great if that worked.

The test below currently fails to build ("the size of dyn FnOnce() cannot be statically determined"), whereas replacing BoxS with Box (and boxed_s! with Box::new) just works.

We could probably impl<Args> FnOnce(Args) for BoxS<T> where T: FnOnce(Args), but AIU that's blocked by rust-lang/rust#29625. Having a nightly feature would be an option, but given that this crate has worked for years without updates (and nightly features would be a constant whack-a-mole), probably not.

#!/usr/bin/env -S cargo +nightly -Zscript
//! ```cargo
//! [dependencies]
//! no_alloc = "*"
//! ```

use no_alloc::{BoxS, boxed_s};

fn run(f: BoxS<dyn FnOnce() + '_, [usize; 4]>) {
    f()
}

fn main() {
    let v = vec![42];
    let f = || drop(v);
    let b: BoxS<dyn FnOnce() + '_, [usize; 4]> = boxed_s!(f);
    run(b);
}