Improve add_variable serialization error type

[michaelbeaumont]

At the moment errors returned by this library Box<dyn Error> are difficult to incorporate, for example with anyhow, because they have quite weak bounds. Another option would be to switch to structured errors, possibly with thiserror.

[clarkmcc]

Hey do you have a specific example of what you mean? We are using thiserror, I'm not aware of cases where we just using Box

https://github.com/clarkmcc/cel-rust/blob/master/interpreter/src/lib.rs

[michaelbeaumont]

OK, maybe this is just something left over then? Specifically this return type

[clarkmcc]

Ah, yeah looks leftover. The function always returns Ok(()) so that is unnecessary. I can fix.

Edit: I see now, it's because of the value conversion which depends on the type of value you're providing. A T: Serialize value will return a serialization error. A Value will return an Infallible error. I think I'll need to do some more involved type gymnastics to make this more clean.

pub trait TryIntoValue {
    type Error: std::error::Error + 'static;
    fn try_into_value(self) -> Result<Value, Self::Error>;
}

impl<T: serde::Serialize> TryIntoValue for T {
    type Error = crate::ser::SerializationError;
    fn try_into_value(self) -> Result<Value, Self::Error> {
        crate::ser::to_value(self)
    }
}
impl TryIntoValue for Value {
    type Error = Infallible;
    fn try_into_value(self) -> Result<Value, Self::Error> {
        Ok(self)
    }
}