如何避免rustfmt将含trivial表达式的else代码块格式化为多行？

Great question! I’ve struggled with this exact Rustfmt behavior before—when the if branch has long, multi-line code, Rustfmt tends to split even trivial else blocks onto multiple lines, even though you want them compact like else { None }. Since single_line_if_else_maxwidth only works when the entire if-else fits within the specified width (which it won’t here, thanks to your long if block), here are your best solutions:

1. Locally Disable Rustfmt for the if-else Statement #

The most targeted fix is to tell Rustfmt to ignore just this specific if-else using a comment or attribute. This keeps your long if branch properly formatted while preserving the compact else block.

Using // rustfmt-ignore #

Add this comment directly above the if statement:

// rustfmt-ignore
if condition {
    // code that deserves to
    // be formatted as block
} else { None }

Using #[rustfmt::skip] Attribute #

If you’re assigning the result to a variable, you can apply the attribute to the entire statement:

#[rustfmt::skip]
let value = if condition {
    // code that deserves to
    // be formatted as block
} else { None };

This makes it clear exactly which code you’re opting out of formatting, and avoids accidentally ignoring surrounding code.

2. Adjust Rustfmt’s Global Configuration #

If you want this behavior to apply project-wide (for all trivial single-line else blocks), you can modify your rustfmt.toml file. Add the following line:

keep_single_line_blocks = true

This tells Rustfmt to retain single-line blocks (like { None }) as-is, instead of splitting them into multi-line blocks. Note that this will affect all similar blocks in your codebase, so make sure it aligns with your team’s formatting preferences.

3. Refactor for Implicit Return (Optional) #

If your if block is the last expression in a function, you can omit the curly braces for the else branch entirely (though this is a stylistic choice):

fn example(condition: bool) -> Option<()> {
    if condition {
        // code that deserves to
        // be formatted as block
        Some(())
    } else {
        None
    }
    // Or, more compact:
    if condition {
        // code that deserves to
        // be formatted as block
        Some(())
    } else None
}

This avoids the curly braces altogether, but it’s a matter of personal taste—some teams prefer explicit braces even for trivial expressions.

Each approach has its tradeoffs: local disabling gives you precise control, global configuration enforces consistency, and refactoring changes the syntax slightly. Pick the one that fits your project’s needs best!

内容的提问来源于stack exchange，提问作者Ingo