Just use Vec<Box<Trait>>;)

variadic_trait.rs

#![feature(unsize)]

use std::marker::Unsize;

pub trait Variadic<Trait: ?Sized> {
    type OneLess: Variadic<Trait>;
    
    fn pop(self) -> (Option<Box<Trait>>, Self::OneLess);
}

impl<Trait: ?Sized, A, B, C> Variadic<Trait> for (A, B, C) where A: Unsize<Trait>, B: Unsize<Trait>, C: Unsize<Trait> {
    type OneLess = (B, C);

    fn pop(self) -> (Option<Box<Trait>>, (B, C)) {
        let (a, b, c) = self;
        let a = Box::new(a) as Box<Trait>;
        (Some(a), (b, c))
    }
}

impl<Trait: ?Sized, A, B> Variadic<Trait> for (A, B) where A: Unsize<Trait>, B: Unsize<Trait> {
    type OneLess = (B,);
    
    fn pop(self) -> (Option<Box<Trait>>, (B,)) {
        let (a, b) = self;
        let a = Box::new(a) as Box<Trait>;
        (Some(a), (b,))
    }
}

impl<Trait: ?Sized, A> Variadic<Trait> for (A,) where A: Unsize<Trait> {
    type OneLess = ();
    
    fn pop(self) -> (Option<Box<Trait>>, ()) {
        let (a,) = self;
        let a = Box::new(a) as Box<Trait>;
        (Some(a), ())
    }
}

impl<Trait: ?Sized> Variadic<Trait> for () {
    type OneLess = ();
    
    fn pop(self) -> (Option<Box<Trait>>, ()) {
        (None, ())
    }
}

fn print_debug<A: Variadic<std::fmt::Debug + 'static>>(args: A) {
    if let (Some(a), rest) = args.pop() {
        println!("{:?}", a);
        print_debug(rest);
    }
}

fn main() {
    print_debug((1, "hello", 1.3));
}