Enum path inference with `_Variant` syntax

[scottmcm]

@160R

How can you tell when the type is contextually obvious? And do we really have a lot of situations like that in Rust to claim that skipping writing types is a big win?

Well, the whole existence of inference is about not writing types, as is method syntax.

It's interesting that it's .map(|x| x.ok()), but .map(Result::ok). If _::Variant works, though, should .map(_::ok) work too? I don't know.

Here's a previous post with a bunch of examples of places where inference already allows APIs that can do things without saying the types involved:

[Pre-RFC] Inferred Enum Type

This, in fact, already works with inference today in stable. We just don't have special syntax for it. Here's a demo: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ed060cd13f013414f43985b027053776 mod foo { mod bar { #[derive(Default, Debug)] pub struct RequestOptions { pub value: i32, pub other: i32, } pub fn make_call(url: &str, options: RequestOptions) { dbg!((url, options)); } …

But the situation that seems most clear to me is in struct literals.

Options {
    cookie_handling: CookieHandling::Skip,
    ..
}

That's pretty repetitive. Having just

```rust
Options {
    cookie_handling: .Skip,
    ..
}

seems quite reasonable to me.