Single line if's for 1 stmt block

[Sjael]

Fixes #5939

[ytmimi]

@Sjael thanks for the PR. More work is needed before we could consider moving forward.

First, this new feature should be gated behind an opt-in configuration option. Because of rustfmt's strong formatting guarantees we can't make single-line if formatting the default.

Second, we'll need many more test cases to show that this is implemented as expected.

Lastly, this option will need to interact well with existing configurations. single_line_if_else_max_width immediately comes to mind, but we probably also want to check how the new configuration interacts with control_brace_style

[ytmimi]

Does result contain any rewritten attributes? Would the #[allow(unused)] prevent rustfmt from writing this on a single line?

fn main() {
    #[allow(unused)]
    if true {
        println!("case 1");
    }

    #[allow(unused)]
    // What about if there's a comment here?
    if true {
        println!("case 2 with comment");
    }
}

[ytmimi]

We also don't want to allow single lining if the block contains a comment. I think the block rewriting handles that but just a call out that we'd need a test for it.

[Sjael]

result here is the condition string, so it's saying if we haven't gone to a new line due to the condition, proceed with checking if we can go for 1 line with the block. So yes, it contains rewritten attributes if I understand you correctly.

[ytmimi]

I ask because I overlooked this case when adding single_line_let_else_max_width and then we needed #5902 to correct the issue. I want to make sure that we handle this up front. It would be unfortunate if leading attributes or comments prevented the guard clause from being reformatted.

[Sjael]

@ytmimi Test files might need more cases, and names could be changed if needed. Let me know your thoughts on it.

[ytmimi]

will take another pass at this when I have some time. In the meantime take a look at the next round of comments I left.

[Sjael]

Made most of the requested changes. I also moved the test file concerning the base functionality of the feature into single_line_if/mod.rs. I didn't merge the config options, but I can do that when you decide it should be merged.