change `Error::Loadfiles::error` type from `Rc` to `Box`

[m4rch3n1ng]

change the type for Error::Loadfiles { error } from an Rc to a Box

context

i'm writing a music player using this library to play music via mpv

/// wrapper struct for mpv
pub struct Player(Mpv);

and i want to create a wrapper error enum and pull the inner error out of libmpv::Error::Loadfiles.

#[derive(Debug, thiserror::Error)]
pub enum PlayerError {
    #[error("invalid utf8")]
    InvalidUtf8,
    #[error("null error")]
    Null,
    // ...
}

but when want to impl From<libmpv::Error> for the enum, i have to do some annoying Rc fuckery to get at the inner value, even though the Rc is only created once and never cloned

impl From<libmpv::Error> for PlayerError {
    fn from(value: libmpv::Error) -> Self {
        match value {
            libmpv::Error::Loadfiles { index: _, error } => match Rc::into_inner(error) {
                Some(error) => PlayerError::from(error),
                // why?
                None => unreachable!()
            },
            // ...
        }
    }
}

if you were to change the type into a Box everything works the same, but actually using the error type is a lot less annyoing to work with

impl From<libmpv::Error> for PlayerError {
    fn from(value: libmpv::Error) -> Self {
        match value {
            libmpv::Error::Loadfiles { index: _, error } => PlayerError::from(*error),
            // ...
        }
    }
}

i couldn't find a reason for why the error type uses an Rc instead of a Box, but the Rc was added in commit 53b430d after a comment, that was added in commit 9043418

as an aside, the Rc prevents the error to be used with the anyhow or color_eyre crate, as the anyhow::Error result has type bounds for Send and Sync

impl Player {
    pub fn new() -> anyhow::Result<Self> { // same with color_eyre::Result
        let mpv = Mpv::new()?; // `std::rc::Rc<libmpv::Error>` cannot be sent between threads safely
        mpv.set_property("vo", "null")?; // `std::rc::Rc<libmpv::Error>` cannot be sent between threads safely

        let player = Player(mpv);
        Ok(player)
    }
}