file-lines modifies unselected fn signature lines when fn body is selected

[randomPoison]

If the opening brace of a function is on a separate line, then when partially formatting the body of the function with --file-lines the opening brace can be moved to the previous line, even if neither of those lines were selected.

Summary

Consider the following file test.rs which defines a function where the opening brace of the function is on its own line:

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

I would like to use --file-lines '[{"file":"test.rs","range":[5,5]}]' to format just the last line of the function.

Expected Behavior

Only the selected line is formatted:

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

Actual Behavior

The selected line, the preceding line, and the first two lines of the function are modified:

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

I have a separate issue up for the fact that line 4 is also modified (#6863), this issue is meant to focus on the fact that lines 1-2 are modified even though they are outside the selected range.

Configuration

rustfmt cli options used:

rustfmt --unstable-features --file-lines '[{"file":"test.rs","range":[5,5]}]' test.rs

[randomPoison]

Ah, the issue here is a bit broader than I initially thought, in that the entire fn signature gets formatted even if it's not selected. For example, with this code:

fn long_fn_decl(
arg: i32,
arg2: i32,
arg3: i32,
)
    {
println!("a");
    }

If I try to format just line 7, it formats the entire function signature along with the opening brace (though the close brace is untouched):

fn long_fn_decl(arg: i32, arg2: i32, arg3: i32) {
    println!("a");
    }

[randomPoison]

Ah, there's a secondary issue in that any attempt to partially select the function signature will cause the whole signature to be rewritten: #6868. So this issue tracks that selecting the body of a function will cause the signature to be rewritten, and #6868 tracks that when the signature is selected then the whole thing gets rewritten. From what I've seen in the code, I think this issue and that one are orthogonal enough that this one can be fixed independently.

[randomPoison]

#6874 partially addresses this such that where clauses don't get formatted if entirely unselected, but the fn args still get formatted when the body is selected.