trait js::conversions::ToJSValConvertible: core::marker::Sized not satisfied

[malisas]

The compiler produces the following error:

error[E0277]: the trait bound `js::conversions::ToJSValConvertible: core::marker::Sized` is not satisfied
  --> /home/malisa/git_repos/malisa-servo/servo/components/script/body.rs:71:21
   |
71 |             promise.resolve_native(promise.global().r().get_cx(), val);
   |                     ^^^^^^^^^^^^^^ trait `js::conversions::ToJSValConvertible: core::marker::Sized` not satisfied
   |
   = note: `js::conversions::ToJSValConvertible` does not have a constant size known at compile-time

when I try to compile this code:

let val: &ToJSValConvertible = match r {
    FetchedData::Text(s) => &USVString(s),
    FetchedData::Json(ref j) => j,
    FetchedData::BlobData(ref b) => b,
    FetchedData::FormData(ref f) => f,
};
promise.resolve_native(promise.global().r().get_cx(), val);

[jdm]

The problem is that resolve_native takes a generic value bounded by ToJSValConvertible, and instead of passing a concrete type to it we're trying to pass a trait object. I'm not sure if there's a way to get the ergonomic code we want (eg. the snippet) in the previous comment without creating a new method on Promise that accepts an &ToJSValConvertible argument instead.

[nox]

Can't we just implement ToJSValConvertible for &ToJSValConvertible? https://play.rust-lang.org/?gist=aabe46af9b3ba6b7138b3acc5c66d6f5&version=stable&backtrace=1

[Ms2ger]

It's certainly possible, but would probably involve a virtual call... Might still be worth it.

[nox]

We can just make ToJSValConvertible be ?Sized if we really want to be able to support trait objects, no?

[nox]

Should this issue be closed? Not sure how it's actionable.

[jdm]

Yeah, no pressing need to fix this.