impl Serializer for &mut fmt::Formatter

[jethrogb]

Allow using &mut fmt::Formatter as a serializer for primitive types.

One use-case I have in mind in particular is implementing Display for enums that also implement Serialize.

If you believe this impl should exist, due to coherence, this crate is the only place it could exist.

[dtolnay]

Thanks! Could you give an example of how this could be used?

[jethrogb]

#[derive(Serialize)]
#[serde(rename_all="kebab-case")]
enum MessageType {
    StartRequest,
    EndRequest,
}

impl fmt::Display for MessageType {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.serialize(f)
    }
}

[peterjoel]

This is actually a nice idea. I frequently end up with a dev dependency on serde_json, just to do a string comparison with a serialised value. This could be very convenient.

It doesn't actually need to implement Serializer for Formatter though; a new struct could delegate for the purpose, and live in a separate crate. It could also be implemented as a macro that generates a Display or Debug impl, without the need for an extra type.

[dtolnay]

Looks good; I am on board with providing this impl.

It looks like this doesn't yet work in the no-std build (https://travis-ci.org/github/serde-rs/serde/jobs/632277218) but I will follow up separately with a fix.

I will also add your example usage in a doc comment.

[dtolnay]

Published in 1.0.108.