if let semantically equivalence to if is wrong w.r.t. scope

[Havvy]

The equivalence is wrong because the scope of the expression in the predicate has to live as long as the consequent because of the ability to ref in the pattern.

Or something like that.

[eddyb]

For the record:

if let a = b { c } /* optional else */ d

Desugars to:

match b {
    a => c,
    _ => d
}

So b is in scope, not only for c but also for d (except for what a moves out).

[alercah]

There doesn't seem to be text in the current version which describes this equivalence, unless I'm looking in the wrong place.