The history of TryFrom/TryInto has spanned 3 years, from when it was originally proposed as an RFC in 2016. For a seemingly simple API, it's gone through a lot. Especially unusual was that it was stabilized a few releases ago and then had to be destabilized when a last-minute issue was discovered with the never type (`!`). The never type had been the primary blocker for stabilizing these APIs for the last year or so, but it was finally decided to simply use this temporary `Infallible` type, which would be mostly forwards compatible with the never type itself.
I've followed the issue closely because it's one of the features used in Ruma, my Matrix homeserver and libraries. In fact, for the library components of the project, it was the last unstable feature. With the stabilization of these APIs, I'll finally be able to release versions of the libraries that work on stable Rust. This will happen later today!
>> Can you eli5 why TryFrom and TryInto matters, and why it’s been stuck for so long ? (the RFC seems to be 3 years old)
> If you stabilise Try{From,Into}, you also want implementations of the types in std. So you want things like impl TryFrom for u16. But that requires an error type, and that was (I believe) the problem.
> u8 to u16 cannot fail, so you want the error type to be !. Except using ! as a type isn’t stable yet. So use a placeholder enum! But that means that once ! is stabilised, we’ve got this Infallible type kicking around that is redundant. So change it? But that would be breaking. So make the two isomorphic? Woah, woah, hold on there, this is starting to get crazy…
> new person bursts into the room “Hey, should ! automatically implement all traits, or not?”
> “Yes!” “No!” “Yes, and so should all variant-less enums!”
> Everyone in the room is shouting, and the curtains spontaneously catching fire. In the corner, the person who proposed Try{From,Into} sits, sobbing. It was supposed to all be so simple… but this damn ! thing is just ruining everything.
> … That’s not what happened, but it’s more entertaining than just saying “many people were unsure exactly what to do about the ! situation, which turned out to be more complicated than expected”.
I've followed the issue closely because it's one of the features used in Ruma, my Matrix homeserver and libraries. In fact, for the library components of the project, it was the last unstable feature. With the stabilization of these APIs, I'll finally be able to release versions of the libraries that work on stable Rust. This will happen later today!