file-lines can't partially format where clauses

With the --file-lines option I'd expect to be able to partially format a where clause, e.g. only format one or two lines of trait bounds, without modifying any unselected lines. Currently any attempt to partially format a where clause will reformat the whole thing.

Summary

For the following Rust code in test.rs:

fn generic_fn<T>() where
T: Clone,
T: Copy,
T: Default,
{
println!("a");
}

I'd like to format just the last trait bound in the where clause with rustfmt --unstable-features --file-lines '[{"file":"test.rs","range":[4,4]}]' test.rs, without modifying any other lines.

Expected behavior

fn generic_fn<T>() where
T: Clone,
T: Copy,
    T: Default,
{
println!("a");
}

Actual behavior

fn generic_fn<T>()
where
    T: Clone,
    T: Copy,
    T: Default,
{
println!("a");
}

Configuration

rustfmt cli options used:

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

Meta

Split off from https://github.com/rust-lang/rustfmt/issues/6868 because where clauses show up in other places than just function signatures. Similar to that issue, the issue here is that rewrite_where_clause doesn't currently try to handle the file-lines config at all. It'll need to be extended to check the selected lines and preserve the original text for lines outside the selected range.

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