Spurious semicolon inside closure is not idempotent

[Kyuuhachi]

Consider the following snippet.

fn foo() -> fn(i32) -> i32 {
    |a| {
        ;
        a
    }
}

Rustfmt starts by removing that spurious semicolon.

fn foo() -> fn(i32) -> i32 {
    |a| {
        a
    }
}

If run again, it notices that the closure fits on one line.

fn foo() -> fn(i32) -> i32 {
    |a| a
}

[rscprof]

@rustbot claim