Rustfmt misaligns comments

[AlyoshaVasilieva]

fn main() {
    let mappings = [
        (0b1110_0011, 'ε'),
        (0b1110_0100, 'μ'), // micro
        (0b1110_0101, 'σ'),
        (0b1110_0110, 'ρ'),
        (0b1110_0111, 'ɡ'), // Questionable: IPA voiced velar stop? Or is it math?
        (0b1110_1000, '√'), // sqrt
        (0b1110_1001, ' '),  // Non-Unicode: exponent -1
        (0b1110_1010, ' '),  // Very tall 'j'; non-Unicode? Can't display on my LCD anyway (TESTME)
        (0b1110_1011, ' '),
        (0b1110_1100, '¢'), // cent sign
        (0b1110_1101, '£'), // pound sign
    ];
}

Rustfmt causes two of these comments to be misaligned. Removing // Questionable: IPA voiced velar stop? Or is it math? causes the two misaligned lines be even more misaligned upon another run of rustfmt (via cargo fmt). Then removing // sqrt causes both lines to resume normal alignment.

Tested with Rust 1.43 stable (rustfmt 1.4.12-stable (a828ffea 2020-03-11)) and Rust nightly (rustfmt 1.4.14-nightly (a5cb5d26 2020-04-14))