file-lines can modify lines above the selected range

[randomPoison]

When using --file-lines to only partially format a function body, an unselected line immediately preceding the selected range can also be touched.

Summary

Consider the following example file test.rs where lines 2-4 in the function body are indented incorrectly:

fn main() {
println!("a");
println!("b");
println!("c");
}

If I format with --file-lines '[{"file":"test.rs","range":[2,2]}]' then I get the correct output, with only the code on line 2 changed:

fn main() {
    println!("a");
println!("b");
println!("c");
}

But if I attempt to format only line 4 with --file-lines '[{"file":"test.rs","range":[4,4]}]' then line 3 gets incorrectly formatted along with line 4:

fn main() {
println!("a");
    println!("b");
    println!("c");
}

Configuration

rustfmt cli options used:

rustfmt --unstable-features --file-lines '[{"file":"test.rs","range":[4,4]}]' test.rs

[randomPoison]

This appears to be specific to indentation, and might only apply to statements that are already on one line. For example, with this code:

fn stuff(val: Option<i32>) {
println!("a");
val
.map(|v| v);
println!("c");
}

If you try to format line 5 then it correctly modifies only that line:

fn stuff(val: Option<i32>) {
println!("a");
val
.map(|v| v);
    println!("c");
}

But if the preceding statement is all on one line, then it does get indented incorrectly. So for this code:

fn stuff(val: Option<i32>) {
println!("a");
val.map(|v| v);
println!("c");
}

If you format only line 4, you get:

fn stuff(val: Option<i32>) {
println!("a");
    val.map(|v| v);
    println!("c");
}

[randomPoison]

Ah, this can also impact empty statements, which can get deleted entirely. With the following:

fn main() {
;
println!("b");
;
}

If I format line 3, then it deletes both empty statements:

fn main() {
    println!("b");
}

[ytmimi]

Merged #6867