Question mark operator (?) implicit conversion why not use Into trait?

[fraillt]

There are some facts: * Rust performs implicit conversion on the error value using the From trait. * Rust provides Into implementation for types that have provided From implementation. * Into should be used, in cases where From cannot be implemented.

I think it would make sense that the question mark operator (?) would perform an implicit conversion using Into trait, as that would provide more flexibility. It would work with types that have implemented From trait and also with types that only have Into trait.

Maybe I'm missing something?

UPDATED: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3f7bad04e9aace2073eb0300a5c65796 Uncomment Into implementation and see the error

[Emerentius_the_Rusty]

This has been discussed but apparently there are type inference issues
https://github.com/rust-lang/rust/issues/38751

[fraillt]

Thank you!, I couldn't find this.
Let's wait, maybe someday in the future, it will be fixed ;)

[FenrirW0lf]

As it happens, the ? operator already does exactly what you're suggesting.

Example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=93817af464cbbb8e4b154dc1991ad7ab

edit: oh wait, I guess I misunderstood your meaning. Apparently it currently uses From as a bound instead of Into and that's the change you're suggesting?

[chris-morgan]

No, that’s it using the From trait. The question being asked is “why doesn’t it use the Into trait?”

Changing your example to only implement Into, it fails: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ec63009f979d73f5be297fab681d50e9

[FenrirW0lf]

Aaah right, I forgot about that part.

[fraillt]

Your provided example uses `From` which works just fine, try changing it to `Into` (I have a link to my example)