formatter: comment formatting instability in named expr (and dict, others)

[davidszotten]

input

if (
    # 1
    x # 2
    # 2.5
    := # 3
    # 3.5
    y # 4
):
    pass

Formatted once:
---
if (
    # 1
    x := # 2.5  # 2  # 3
    # 3.5
    y  # 4
):
    pass

---

Formatted twice:
---
if (
    # 1
    x := # 3.5  # 2.5  # 2  # 3
    y  # 4
):
    pass

i think the issue is that we use a space() around the operator (:=) and not a soft_line_break_or_space so nothing is inserting the newline before the leading ownline comment

[MichaReiser]

The use of space might be intentional to avoid breaking after the operator. But it might be necessary to emit a soft line break if the right side has a leading comment

[davidszotten]

yes sorry for being unclear. wasn't suggesting switching out the space, only thinking out loud why we hadn't seen this more elsewhere

[charliermarsh]

I'll take this one. There are similarities to with... as... so I'll wait for that to be approved before pushing anything.

[charliermarsh]

Didn't end up being that similar to with so PR here: #6634.