Attribute like `from` for `Box`ing large error variants

[clbarnes]

Some recent clippy update warns for large enum error variants, and recommends boxing them. This is probably reasonable. It would be convenient if there was an annotation like from which would create the box itself rather than having to manually implement an increasingly common use case.

Looks like thiserror_ext has a related utility already: https://docs.rs/thiserror-ext/latest/thiserror_ext/derive.Box.html but could probably be cleaner. Something like

use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
    Small(#[from] small::Error),
    Big(#[from(autobox)] Box<big::Error>)
}

which would generate something like

use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
    Small(small::Error),
    Big(Box<big::Error>)
}

impl From<small::Error> for MyError {
    fn from(value: small::Error) -> Self {
        Self::Small(value)
    }
}

impl From<big::Error> for MyError {
    fn from(value: big::Error) -> Self {
        Self::Big(Box::new(value))
    }
}

Maybe with an impl From<Box<big::Error>> for MyError too, for completeness.

[ollbx]

Would also love to see this. If someone finds this useful: this is my current workaround...

#[macro_export]
macro_rules! boxed_from {
	($dst_ty:ident :: $variant:ident, $src_ty:ty) => {
		impl From<$src_ty> for $dst_ty {
			fn from(value: $src_ty) -> Self {
				Self::$variant(Box::new(value))
			}
		}
	};
}

// ...

#[derive(Error, Debug)]
pub enum MyError {
    Small(#[from] small::Error),
    Big(Box<big::Error>)
}

boxed_from!(MyError::Big, big::Error);

[ilyagr]

See also #431