Failed assertion with file_lines option

[scampi]

• CFG_RELEASE_CHANNEL=nightly cargo run --bin rustfmt -- --check file.rs --unstable-features --file-lines '[{"file":"file.rs","range":[2,5]}]'
• rustfmt: 1d19a08

fn foo() {
    let x =
        42;
    let y =
        42;
    let z = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    let z = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";
}

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `6`,
 right: `5`: buffer: fn foo() {
    let x = 42;
    let y = 42;
    let z = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    let z = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}
', src/formatting.rs:181:9

[scampi]

related to #3397

[topecongiro]

Possibly related to #3609.