file-lines can't format trait definition contents without formatting the header

When selecting the contents of a trait definition with --file-lines, everything up to the first function definition is formatted, even when not selected. This means it's not possible to e.g. format only one function in the trait definition without touching the header portion. In this case "header" means everything from the trait keyword to the fn keyword of the first function defined in the trait (or whatever the first item in the trait is).

Summary

For the following code in test.rs:

trait WonkyTest
{
fn required_1(
arg: bool,
arg2: i32,
);
fn required_2(
arg: bool,
arg2: i32,
);
}

I would like to use --file-lines to format only lines 7-10 (just the definition of required_2), and would expect that nothing else outside of the selected lines would be changed.

Expected behavior

trait WonkyTest
{
fn required_1(
arg: bool,
arg2: i32,
);
    fn required_2(arg: bool, arg2: i32);
}

Actual behavior

trait WonkyTest {
    fn required_1(
arg: bool,
arg2: i32,
);
    fn required_2(arg: bool, arg2: i32);
}

Note that the opening { was moved and that whitespace was added before the start of fn required_1.

Configuration

rustfmt cli options used:

rustfmt --unstable-features --file-lines '[{"file":"test.rs","range":[7,10]}]' test.rs

Meta

This likely needs to be added as a blocker for https://github.com/rust-lang/rustfmt/issues/3397.

Reproduce the issue with the provided test.rs example and the rustfmt --unstable-features --file-lines command selecting lines 7-10. Trace the --file-lines handling for trait definitions, using issue #3397 as related context; done means only required_2 is reformatted while the trait header and required_1 remain unchanged.