以 Sized 为界的特征向量

Question

I am trying to see if there is any way to implement a vector of sized traits. I know about trait objects, and using Vec<Box<dyn traitName>> if traitName is ?Sized . But what if I have ( https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=43d9ee07cfe643d32a00963aa066c929 ):

trait A: Sized {}

struct B {
    c: u64,
}
impl A for B {}

fn lol() -> Vec<A> {
    let mut a: Vec<A> = Vec::new();
    
    let b = B { c: 2} ;
    
    a.push(b);
    
    a
}

The error I get is

8 | fn lol() -> Vec<A> {
  |                 ^
  |
help: add `dyn` keyword before this trait

but to fix that I have to use trait objects, which is not possible since A: Sized . Any way to fix this or is it impossible?

Answer 1

A where A is a trait is just an old syntax (forbidden in the 2021 edition) to dyn A . You can't create A or dyn A if A: Sized , ever.

if you'll run this with the 2018 edition, you'll get errors "the size for values of type (dyn A + 'static) cannot be known at compilation time" .