Allow #[auto_enum] to be directly used on if expressions

[taiki-e]

Once rust-lang/rust#68618 is resolved, we can support this.

[Aaron1011]

This is no longer blocked.

[Boscop]

I've been using auto_enums on let x = if .. exprs and it's working. Or do you mean if exprs without let?

Btw, any idea why rustc complains about "unnecessary braces around function argument" after I updated my nightly? rust-lang/rust#71080

[taiki-e]

@Boscop

Or do you mean if exprs without let?

Yes, basically I want to annotate if expressions without depending on let.

Practically the same as the current one, it would be nice to support the following styles:

#[auto_enum]
fn a() {
    let res = {
        // This is the same as using #[auto_enum] on `let`, 
        // but maybe easier to read depending on the length of the block.
        #[auto_enum]
        if x {
            // ...
        } else {
            // ...
        }
    };
}

#[auto_enum]
fn b() -> Option<impl Foo> {
    // Equivalent to `#[auto_enum] let res = if ...` and `Some(res)`
    Some(#[auto_enum] if x { y } else { x })
}

And, these are already available on match expressions, so we can provide more consistent support.

[taiki-e]

Btw, any idea why rustc complains about "unnecessary braces around function argument" after I updated my nightly? rust-lang/rust#71080

Thanks for reporting it, I will look into it.

[taiki-e]

And, these are already available on match expressions, so we can provide more consistent support.

Oh..., this comment is not correct. The second example does not work without unstable features even if I replace if with a match (#90, if I have unstable features enabled, both will work.)
Also, the first example actually seems to be already supported. (#89)