Auto-implement `TryInto<U>` for `Option<T> where T: Into<U>`

It's important to preface this by saying that this change, although a convenient one, will most likely be a breaking change; this would overwrite some existing impls which would stop some programs/libraries from compiling. I think this is going to be the biggest reason that this never makes it into Rust, but I thought it was an idea worth discussing.

It's a simple convenience Impl; it could go something like

// Excuse the poor indentation and/or any
// errors; this is untested.
impl<T, U> TryInto<U> for Option<T> where T: Into<U> {
    type Error = Option<T>;

    fn try_into(self) -> Result<U, Self::Error> {
        if let Some(value) = self {
        } else {

As you said, this formally conflicts with impl<T, U: TryFrom<T>> TryInto<U> for T, which can't realistically be removed. But even in practice, impl<T> From<T> for Option<T> already exists and that implies that your suggested impl would apply for U = Option<T>, generating an impl<T> TryInto<Option<T>> for Option<T>, which always succeed right now (due to impl<T> From<T> for T and a bunch of other impls) but fails with your proposal. Unfortunately to me all the impls already mentioned seem more intuitive and fundamental than the one you proposed.

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What does Error = Option<T> accomplish if it's always None?


Hm, maybe it should be Option<Infallible> based on the Try implementation of Option. Any idea why it isn't simply ()?

The desired conversion can be written pretty concisely as which is also very clear as to what exactly it is doing. (I'm intentionally ignoring the Result conversion, because usually it isn't useful to convert an Option<T> into a Result<T, SomeUnitType> just because.)


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