Allow dyn trait + Sized

[CreepySkeleton]

I would expect this code to work:

let a: Box<dyn Any + Sized> = Box::new(5);

error[E0225]: only auto traits can be used as additional traits in a trait object
  --> src/main.rs:22:26
   |
22 |     let a: Box<dyn Any + Sized> = Box::new(5);
   |                    ---   ^^^^^ additional non-auto trait
   |                    |
   |                    first non-auto trait
   |
   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Any + Sized {}`
   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>

I realize that trait objects don't work with multiple traits because of complications with multiple vtables. Sized is also technically not an auto trait, but for all intends and purposes here (no methods in Sized - no vtables) it should be equivalent to Send or Sync or whatever.

A somewhat more realistic example:

struct Arg {
    typed_validator: Box<dyn Fn(&str) -> Result<dyn Any + Sized, Error>>
}

[jonas-schievink]

dyn Trait can never be Sized, so this can't be made to work

[CreepySkeleton]

Oh right.

[petrochenkov]

@jonas-schievink

dyn Trait can never be Sized, so this can't be made to work

With rust-lang/rfcs#2027 accepted the requirement dyn Trait: Trait is no longer mandatory.

It makes very much sense to support dyn Sized because the traits in dyn Trait + ... are traits implemented by its underlying type, not by dyn Trait + ... itself, and the underlying type of dyn Trait + ... is always Sized currently.

[jonas-schievink]

I see, that makes sense, yeah.