Feature request: allow adding inline to the generated from impls

[V0ldek]

I have the missing_inline_in_public_items lint enabled in my crate and I'm currently upping thiserror to 2.0. For some reason, the From impls from thiserror start tripping up this lint after the upgrade (see e.g. this action). I don't know why the upgrade causes it, since as far as I can see the From impls were never inline in 1.0 either, but that's a secondary issue.

It'd be nice if it was possible to add an inline attribute to the #[from] specifier. I could see two ways for this:

1. Just allow #[from(inline)] which slaps #[inline] onto the generated impl.
2. Allow a more generic mechanism where this:

#[derive(Error)]
enum MyError {
  IO(#[from] #[inline] #[deprecated("Reason")] #[any_other_attr("with", "args")] std::io::Error)
}

results in this impl:

impl From<std::io::Error> for MyError {
    #[inline]
    #[deprecated("Reason")]
    #[any_other_attr("with", "args")]
    fn from(value: std::io::Error) -> Self {
        Self::IO(value)
    }
}

I'm not familiar enough with macros to figure out how hard the second approach would be, though.

[dtolnay]

Please try 2.0.9. It should behave equivalently to 1.x in regard to missing_inline_in_public_items.

[V0ldek]

should behave equivalently to 1.x in regard to missing_inline_in_public_items.

Indeed it does, thank you.

What about the ability to mark the generated functions as #[inline] somehow?

[dtolnay]

I think that is almost certainly misguided. Those functions are only called on the error codepath and not on the usually performance sensitive happy path.