Extra whitespace inserted between literal and line comment.

[hackguy25]

When inserting line comments in an array literal, rustfmt sometimes insert unnecessary empty space.

Simplest example:

let example_correct = [
    0, 0, // comment 1
    0, // comment 2
];

let example_incorrect = [
    0, 0x0, // comment 1
    0,   // comment 2
];

Notice how there are three spaces between the trailing comma and the comment 2 in the incorrect example.

Examples can be reproduced on the Rust Playground, which uses rustfmt 1.6.0-nighty (2023-07-10 8ca44ef), as well as with rust-analyzer v0.3.1583 VS Code extension.

[ytmimi]

@hackguy25 thanks for reaching out!

rustfmt is trying to align the comments. If you were to manually format 0x0, // comment 1 and move it to the next line you'll see that. Here's what it looks like:

let example_incorrect = [
    0,
    0x0, // comment 1
    0,   // comment 2
];

0 and 0x0, are small enough that rustfmt uses a compressed formatting style when outputting the array. however it incorrectly tries to align comments in this case, and it would probably be best not to align comments if we're not using a vertical layout.

To further observe and highlight the comment alignment you can run rustfmt and set short_array_element_width_threshold=0:

Input:

fn main() {
    let example_correct = [
        0, 0, // comment 1
        0, // comment 2
    ];

    let example_incorrect = [
        0, 0x0, // comment 1
        0,   // comment 2
    ];
}

output:

fn main() {
    let example_correct = [
        0,
        0, // comment 1
        0, // comment 2
    ];

    let example_incorrect = [
        0,
        0x0, // comment 1
        0,   // comment 2
    ];
}

Also want to note that this is probably related to #4108, although that issue is more focused on function arguments.