file-lines can't partially format function signatures

With the --file-lines option I'd expect to be able to partially format a function's signature, e.g. only format one argument while leaving the rest of the signature as-is. But any attempt to format the function signature formats the whole thing.

Summary

For the following code in test.rs:

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

I would like to use --file-lines '[{"file":"test.rs","range":[2,4]}]' to format just the arguments to the function, without modifying any other lines.

Expected behavior

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

Actual behavior

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

Configuration

rustfmt cli options used:

rustfmt --unstable-features --file-lines '[{"file":"test.rs","range":[2,4]}]' test.rs

Reproduce the command in the issue using the provided test.rs input and --file-lines range. Trace the rustfmt handling for partial ranges and add regression coverage for formatting only the selected function arguments; done means the expected indentation changes while the rest of the signature and body remain unchanged.