Allow dyn trait + Sized #79701
Description
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>>
}