rustfmt: formatting meta-arguments in a macro is not idempotent

[Kijewski]

Given this source:

macro_rules! new_greeter {
    () => {
        #[template(
            add = some::tokens,
            to = make::the::line::longer
        )]
        struct X;
    };
}

Call cargo fmt (tested with +stable = 1.87.0 and +nightly = 1.89.0-nightly (056f5f4f3 2025-05-09)).

The indentation level of add = ... )] is incremented by 8 spaces for every fmt invocation until the 100 character line limit is hit.

Source after multiple `cargo fmt` calls (click me)

macro_rules! new_greeter {
    () => {
        #[template(
                    add = some::tokens,
                    to = make::the::line::longer
                )]
        struct X;
    };
}

macro_rules! new_greeter {
    () => {
        #[template(
                            add = some::tokens,
                            to = make::the::line::longer
                        )]
        struct X;
    };
}

..

macro_rules! new_greeter {
    () => {
        #[template(
                                                                    add = some::tokens,
                                                                    to = make::the::line::longer
                                                                )]
        struct X;
    };
}

I would have assumed that cargo fmt is idempotent, i.e. that calling after calling it once the code remains unchanged. (IHMO my example code does not need reformatting in any case.)

It can be tested on the playground, too:

• Open https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=5f02dcbdd46ac2b63a8c764aef618407
• Click tools / rustfmt.

[ZaneErebos]

I am having the same issue: #6563

It seems that it indents by 2*tab_spaces everytime up until max_width

[workingjubilee]

@rustbot transfer rustfmt