Lambda with long argument list should be formatted without indent_style="Visual"

[KarelPeeters]

I have a couple of lambdas with parameters with complex types, which take up a lot of space. This means rustfmt splits up the parameters over multiple lines, but it does so as if indent_style="Visual". A simple reproducing example, on 1.4.38-nightly (2022-03-19 8d60bf4) (the current rust playground formatter):

fn main() {
    let long_lambda = |x: &mut usize,
                       y: &mut usize,
                       z: &mut usize,
                       w: &mut usize,
                       v: &mut usize,
                       a: &mut usize,
                       b: &mut usize| {
        println!("{}", x);
        println!("{}", y);
        println!("{}", z);
        println!("{}", w);
        println!("{}", a);
        println!("{}", b);
    };

    long_lambda(&mut 1, &mut 2, &mut 3, &mut 4, &mut 5, &mut 6, &mut 7);
}

This is inconsistent with the rest of rustfmt formatting, and doesn't play well with things like IDE type hints.

I think it would be better if lambda parameters were formatted more like indent_style="Block", which is the default elsewhere, eg. function parameters.

fn main() {
    let long_lambda = |
        x: &mut usize,
        y: &mut usize,
        z: &mut usize,
        w: &mut usize,
        v: &mut usize,
        a: &mut usize,
        b: &mut usize
    | {
        println!("{}", x);
        println!("{}", y);
        println!("{}", z);
        println!("{}", w);
        println!("{}", a);
        println!("{}", b);
    };

    long_lambda(&mut 1, &mut 2, &mut 3, &mut 4, &mut 5, &mut 6, &mut 7);
}

[ytmimi]

Thanks for the report.

This is inconsistent with the rest of rustfmt formatting, and doesn't play well with things like IDE type hints.

I just want to point out that the rules that dictate how rustfmt is meant to work are laid out in the style guide. The relevant section on closures, does seem to indicate that the formatting should be more consistent with functions.

[ytmimi]

confirming that on rustfmt 1.4.38-nightly (63acf900 2022-03-27) given this input:

fn main() {
    let long_lambda = |
        x: &mut usize,
        y: &mut usize,
        z: &mut usize,
        w: &mut usize,
        v: &mut usize,
        a: &mut usize,
        b: &mut usize
    | {
        println!("{}", x);
        println!("{}", y);
        println!("{}", z);
        println!("{}", w);
        println!("{}", a);
        println!("{}", b);
    };

    long_lambda(&mut 1, &mut 2, &mut 3, &mut 4, &mut 5, &mut 6, &mut 7);
}

produces this output:

fn main() {
    let long_lambda = |x: &mut usize,
                       y: &mut usize,
                       z: &mut usize,
                       w: &mut usize,
                       v: &mut usize,
                       a: &mut usize,
                       b: &mut usize| {
        println!("{}", x);
        println!("{}", y);
        println!("{}", z);
        println!("{}", w);
        println!("{}", a);
        println!("{}", b);
    };

    long_lambda(&mut 1, &mut 2, &mut 3, &mut 4, &mut 5, &mut 6, &mut 7);
}

The rustfmt.toml used is the same as the one listed in the rustfmt repo:

error_on_line_overflow = true
error_on_unformatted = true
version = "Two"

[ytmimi]

Thanks again for the report. I did some more digging and it seems like this is a duplicate of #3865, and the fix for it was already implemented and merged into a different branch by #3867. This fix needs to be backported into the current mainline rustfmt master branch.

[calebcartwright]

Thank you for the report but let's go ahead and close this as a duplicate