Fix crash with f-string docstrings

[hauntsaninja]

Fixes #4018

Python does not consider f-strings to be docstrings, so we probably shouldn't be formatting them as such. This is the easiest way to fix the issue with the AST check.

Despite its name, I think docstring_preview.py might be testing stable style, which this unfortunately does change

[github-actions]

diff-shades reports zero changes comparing this PR (c548676) to main (c54c213).

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[JelleZijlstra]

Thanks, it does look like this changes the stable style. The case it changes is rather odd, but we should still follow our policy and make the change only in the stable style.

Some options:

• Just wait two months and merge this PR in January so it can go straight into the stable style, without waiting for preview. Seems OK since the change is rather obscure.
• For now, keep formatting f-strings without any placeholders, and turn off formatting for f-strings with placeholders. This would fix the crash while preserving behavior. In the preview style we can also disable formatting for f-strings without placeholders.
• Fix the crash instead by loosening the AST safety check (which already special cases docstrings). I'd rather not do that though.

[hauntsaninja]

Option 2 would also change the stable style right?

I would also like to avoid Option 3. Note afaict that the AST safety check special cases all strings, not just docstrings, which makes me even more loathe to take Option 3.

So Option 1 it is?

[JelleZijlstra]

I thought Option 2 was safe because any change we make to f-strings with placeholders would cause an AST safety crash. Though I guess it would change the stable style under --fast? I would say that's OK but we haven't specified it explicitly.

[hauntsaninja]

Oh, it's not any f-string with placeholders. It's f-strings with leading or trailing space with placeholders adjacent to that space. I guess we could detect that specific situation and not format, but I'd prefer to have the behaviour be consistent and predictable, especially since Python does not consider f-strings to be docstrings.

[hauntsaninja]

Actually wait, there is no stability concern. The stability guarantee is just that code that was previously formatted with Black will not change, not that formatting unformatted code will lead to the same results.