Configurable per-object item wrap count via `#[rustfmt::items(10)]` (for better displaying large vectors, arrays, & matrices)

[tgross35]

Hello everyone,

It was suggested on Reddit (and pretty well received) to add a macro config option that allows specifying items per row within a vector or array, and aligns the items at the comma. This would be useful for formatting large collections of numbers, especially matrices where columns and rows are significant.

Here's the quote that started the discussion:

Yeah, exactly. I love using rustfmt, but just like unsafe, people shouldn't be scared to use rustfmt::skip when necessary.

For example, dealing with math heavy code, rustfmt will always reformat any large array declaration into one item per line, but there are plenty of situations where those arrays are actually matrices and formatting them accordingly is helpful. There's no way for rustfmt to ever know this.

The current behavior appears to be to use the full line then wrap if all items are short, or one item per line if any of the items excedes a limit: Example rustfmt output:

let v1 = vec![
    1i32, 2, 3, 4, 5, -34, 78, 5, 4, 6, 7, 8, 87, 5, 43, 45, 67, 8, 76, 543, 4, 567, 8, 7,
    6543, -45, 456, 7, -4, 2, 3, 45, 64, -9, 909, -940, 185, -9849484, 984, 9898, -598,
];

let v2 = vec![
    1798798798797i64,
    268763876278189,
    -29873267368763,
    782687289780998,
    -2976173718798,
    1987897,
];

// Adding a larger number to v1 makes a 40 item long vector (truncated here)
let v3 = vec![
    ...
    7,
    6543,
    -45,
    456,
    7,
    -4,
    2,
    3,
    45,
    64,
    -9,
    909,
    -940,
    185,
    -9849484,
    984,
    9898,
    -590909023328,
];

This change would allow the user to specify that they desire N items per row:

#[rustfmt::items(10)]
let v1 = vec![
    1i32,   2,   3,   4,   5, -34,  78,   5,
       4,   6,   7,   8,  87,   5,  43,  45,
      67,   8,  76, 543,   4, 567,   8,   7,
];

#[rustfmt::items(2)]
let v2 = vec![
    1798798798797i64,  268763876278189,
     -29873267368763,  782687289780998,
      -2976173718798,          1987897,
];

Even outside of matricies, this would be a nice option to enable rustfmt to make use of more horizontal space.

[ytmimi]

Something similar was brought up in #2924. And as was suggested in the linked issue #[rustfmt::skip] could be used to achieve the same formatting that you're proposing here.

[tgross35]

Thank you for pointing that thread out! I was unable to find it.

[ytmimi]

No worries! Its not always easy tracking down existing issues in the issue tracker!

[tgross35]

Hey @ytmimi could I respectfully request that you reopen this? The linked issue #2924 only discusses #[rustfmt::skip] and has been closed a while, and I'd kind of like if the idea of per-object wrap formatting could get a bit of feedback (assuming it isn't a hard no from you)

It's just back on my mind since I brought something similar up for python's black last week psf/black#3518. No telling whether or not they'd accept it, it's just similarly relevant. I also put some thought into my answers there that could likely apply to design for this issue.

[ytmimi]

@tgross35 thanks for following up here. I discussed this with @calebcartwright and we both agree that the formatting could be improved, however we're not 100% sure if attribute driven formatting is the best approach.

I'm happy to reopen the issue, but I want to be upfront and say that this isn't high on the team's priority list, and that it's unlikely the rustfmt team will work on this directly.

PRs are always welcome, and I can certainly provide general guidance for anyone who wants to take this on.

[tgross35]

Thank you for the detailed response! I understand and appreciate it.