let_chains desugaring is wrong

[Noratrieb]

I tried this code: playground

struct LoudDrop(&'static str);

impl Drop for LoudDrop {
    fn drop(&mut self) {
        println!("Dropped! {}", self.0);
    }
}

fn get_drop(str: &'static str) -> Option<LoudDrop> {
    Some(LoudDrop(str))
}

fn main() {
    println!("1");
    if get_drop("first").is_some() && get_drop("1.5").is_some() && let None = get_drop("last") {
        println!("2");
    } else {
        println!("2");
    }
    println!("3");
}

This prints

1
Dropped! 1.5
Dropped! last
2
Dropped! first
3

According to RFC 2497, it should desugar to this: playground

fn main() {
    println!("1");
    'FRESH_LABEL: {
        if get_drop("first").is_some() {
            if get_drop("1.5").is_some() {
                if let None = get_drop("last") {
                    break 'FRESH_LABEL { println!("2") }
                }
            }
        }
        { println!("2") }
    }
    println!("3");
}

But this prints

1
Dropped! first
Dropped! 1.5
Dropped! last
2
3

Meta

Wrong on the current beta

1.64.0-beta.1

(2022-08-09 56714e5332f5f21c39d7)

[Noratrieb]

This also causes issues like #99852 and #100276

[joshtriplett]

Yeah, that definitely looks wrong; we will have to roll this back and try again for 1.65.

[Noratrieb]

I've opened a zulip thread about fixing them. cc @c410-f3r, I wasn't able to find you on zulip to ping you there