formatter: debug panic in placement::find_pos_only_slash_offset

[davidszotten]

while working on comments inside tuple unpacking between stars and name (following #5167 (comment) )

i was wondering if function arguments could have the same issue. in trying to test that i found my test case violates this debug assert

$ cat ../../scratch.py
def foo(
    a=2 ** 3 # trailing
):
    ...

$ cargo run --bin ruff_python_formatter -- --emit stdout ../../scratch.py
   [...]
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `RParen`,
 right: `Comma`', crates/ruff_python_formatter/src/comments/placement.rs:846:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

commit: be11cae

[MichaReiser]

Thanks for reporting this issue. This is a bug in my implementation. We can fix it by either:

Changing the fallback from arguments.end() to arguments.end() - TextSize::new(1) to exclude the )

ruff/crates/ruff_python_formatter/src/comments/placement.rs

Lines 640 to 642 in be11cae

	let trivia_end = comment
	.following_node()
	.map_or(arguments.end(), |following| following.start());

Gracefully handle a ) in the find_pos_only_slash

ruff/crates/ruff_python_formatter/src/comments/placement.rs

Lines 845 to 846 in be11cae

	if let Some(comma) = tokens.next() {
	debug_assert_eq!(comma.kind(), TokenKind::Comma);

I would prefer the first fix because between_arguments_range is explicit about the range being between arguments, which isn't true for ).

[MichaReiser]

@davidszotten I tried to reproduce, but your provided snippet does not panic on main (although it gets formatted extremely weirdly):

def foo(
    a=2  # trailing
    **3,
):
    ...

[MichaReiser]

The weird formatting should be fixed in #5204

[davidszotten]

panic was fixed by #5192 (rustpython upgrade)