Unexpected arrow indent change in trait methods with Rust edition 2024

[EFanZh]

With the following code:

trait Foo {
    fn fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(_: ())
        -> u32;
}

The -> is indented with 4 spaces after fn with edition 2021, but with edition 2024, rustfmt will align -> with fn:

trait Foo {
    fn fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(_: ())
    -> u32;
}

Here is a playground link for this: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9cb2cfab404be994870455658edba6e0.

Is this change intended?

[ytmimi]

What version of rustfmt are you using? When I build and run rustfmt from source (version: rustfmt 1.8.0-nightly (5619b64 2025-02-12)), I get the same output for all edition values. So I'd say this is expected.

running cargo run --bin rustfmt -- --edition={edition-value} for all editions (2015, 2018, 2021, 2024) all produce the same output:

trait Foo {
    fn fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(_: ())
    -> u32;
}

[EFanZh]

@ytmimi: rustfmt 1.8.0-stable (4d91de4e48 2025-02-17) which is provided by Rust 1.85 stable toolchain. You can reproduce the result using the playground link I provided above by selecting different editions and run the Rustfmt tool.

[ytmimi]

I realize now that setting --edition was a false negative. I should have been setting --style-edition. With that config I was able to reproduce the issue. I believe the PR that introduced that change was #3731, which has been part of the codebase for quite some time.

[ytmimi]

Based on the Style Guild section for function definitions this formatting doesn't seem correct to me. I'd expect something like this:

trait Foo {
    fn fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(
        _: ()
    ) -> u32;
}