Rustfmt should add enforce a single empty line before a doc comment

Input

use std::cell::RefCell;
/// Main function
fn main() {
    let a = RefCell::new(2);
    dbg!(a);
}

Output

use std::cell::RefCell;
/// Main function
fn main() {
    let a = RefCell::new(2);
    dbg!(a);
}

Expected output

use std::cell::RefCell;

/// Main function
fn main() {
    let a = RefCell::new(2);
    dbg!(a);
}

Meta

• rustfmt version: rustfmt 1.4.30-stable
• From where did you install rustfmt?: rustup

CC @GuillaumeGomez

Reproduce the issue with rustfmt 1.4.30-stable installed through rustup, using the Rust input shown in the report. Trace the formatting behavior for a doc comment and add coverage in the relevant rustfmt tests once its location is identified. Done means rustfmt inserts exactly one empty line before the doc comment without changing the other formatting.