`rustfmt` formatting of `cfg_select!` pending for Rust 1.99 diverges from style FCP

[traviscross]

In rust-lang/style-team#201 (comment), the style team FCPed a decision about how to format code within cfg_select. This included by reference the determination made in #144323 (comment). In that, we specified the following formatting:

cfg_select! {
    feature
        = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff"
    => {
        println!();
    }
    feature
        = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff"
    => {}
    anything(
        "some other long long long long long thing long long long long long long long long long long long",
    ) => {}
}

That PR was closed, and formatting for cfg_select! landed in #154202 (thanks for that!). But the current formatting, for the above, on nightly, is:

cfg_select! {
    feature = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff" =>
    {
        println!();
    }
    feature = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff" =>
        {}
    anything(
        "some other long long long long long thing long long long long long long long long long long long",
    ) => {}
}

It's also not adding trailing commas, so it formats this —

cfg_select! {
    all(target_arch = "x86_64", target_env = "gnu", not(any(target_os = "os1", target_os = "os2"))) => {}
    any(one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen) => {}
    _ => {}
}

— as:

cfg_select! {
    all(
        target_arch = "x86_64",
        target_env = "gnu",
        not(any(target_os = "os1", target_os = "os2"))
    ) => {}
    any(
        one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen,
        fourteen, fifteen, sixteen
    ) => {}
    _ => {}
}

I'd suspect we might want it to add the trailing commas there.

@ytmimi: What are you thoughts on how to handle this given the pending beta bump?

cc @rust-lang/rustfmt @rust-lang/style

[ytmimi]

@traviscross could we pull the formatting from the beta but keep it on nightly? I think we should try to adhere to the formatting rules that @rust-lang/style came up with.

Are the rules officially incorporated in the style-guide? Would be nice to have them as a reference when updating the implementation.

---

Edit: I guess to prevent this from landing on beta we could add a is_nightly_channel!() check to gate formatting

[traviscross]

That would make sense to me; thanks. I'm working on a style guide PR.

[traviscross]

Style guide PR:

• Add cfg_select! formatting rules to the style guide #160967

[ytmimi]

PR to gate cfg_select! formatting on the nightly release channel:

• rustfmt: restrict cfg_select! formatting to the nightly release channel #160998