Add `blankline_before_or_after_brace` option.

Environment: rustfmt 1.4.37-stable (f1edd042 2021-11-29) (with no configuration file)

Currently, blank lines before the opening brace { or after the closing brace } are removed.

fn main() {

    println!("Hello, world!");

}

↓

fn main() {
    println!("Hello, world!");
}

This style is hard to read especially when the signature (plus where clause) and the body are long:

use regex::Regex;
use std::process::Command;
use std::process::Output;

fn _some_function<T, U, V, W>(
    _some_param_1: T,
    _some_param_2: &U,
    _some_param_3: Vec<V>,
    _some_param_4: &[W],
    _some_param_5: String,
) -> String
where
    T: Iterator,
    T::Item: Clone,
    U: std::fmt::Display + Sync,
    V: std::hash::Hash,
    W: PartialEq + PartialOrd,
{
    let regex = Regex::new(r"\$\((.*)\)").unwrap(); // <--- I want a blank line above this line!

    let mut s = String::new();

    match regex.captures(&s) {
        Some(c) => {
            let command = c.get(1).unwrap().as_str();
            let range = c.get(0).unwrap().range();
            let res: Output = Command::new("bash").args(["-c", command]).output().unwrap();
            let res: String = String::from_utf8(res.stdout).unwrap().trim().to_string();
            s.replace_range(range, &res);
        }
        None => (),
    }

    let a = 5;
    println!("{}", a);

    std::env::args().for_each(|e| println!("{}", e));

    String::from("hello")
}

I've checked all the stable and unstable configuration options listed in Configuring Rustfmt, but it seemed there was no option to preserve blank lines before or after braces.

So this is a feature request:

blankline_before_or_after_brace

Default value: false

Possible values: false, true, preserve

Behavior:

• false (default): No blank line before or after braces is allowed.

• true: Always insert a blank line before or after each brace.

• preserve: Preserve the style in the original code.

Start with the rustfmt configuration documentation linked in the issue and inspect the existing behavior that removes blank lines around braces. Determine how the proposed false, true, and preserve values should behave for the examples shown, then verify the resulting formatting against those examples.