Compiler crash on "if" without "else" in async fn

[guzt]

async fn f() -> Result<(), failure::Error> {
    if "a" != "b" {
        return Err("c".to_string());
    }
}

(Playground)

Triggers on playground on beta and nightly channels, not on stable.

Stacktrace: https://gist.github.com/guzt/32437a394c5c3401f8ec80a6863efe56

[guzt]

Even smaller sample (from @ComputerDruid):

async fn f() -> i32 {
    if true {
        return 0;
    }
}

[pnkfelix]

triage: P-high. Removing nomination.

[pnkfelix]

triage: P-high, removing nomination label.