Idea: change the desugaring of `if let` to improve borrow checking

[mark-i-m]

Related: rust-lang/rust#28449

Currently if let desugars to a match with guards. Using the example from the related post:

let mut f = Some(1);
if let Some(a) = f {
    bar();
} else if f.as_mut().is_some() {
    baz();
}

Desugars to

match f {
    Some(a) => {bar();}
    _ if f.as_mut().is_some() {baz();}
    _ => {}
}

However, this leads to f being borrowed in the else if which is truly annoying. Instead, I propose the following desugaring:

    if match f {Some(_) => true, _ => false } {
         let Some(a) = f;
         { bar(); } 
    } else if f.as_mut().is_some() {
        baz();
    }

Which (I believe) does not suffer from the same problem.

One unanswered question is: does non-lexical lifetimes solve this?

[durka]

The proposed desugaring evaluates f twice.

[mark-i-m]

Oh, that's true... But I believe it is easily fixed by just evaluating once beforehand, right?

[Ixrec]

I'm pretty sure non-lexical lifetime will solve this, since the code this desugars to is essentially the same as the classic example of trying to insert a value into a map if it's not already in the map. Niko discusses that example both on his blog way back in April and in his recent RFC for NLL.

[mark-i-m]

@Ixrec Ah, thanks. I think NLL definitely subsumes this idea...