Long strings not broken?

[kierdavis]

Describe the bug

With #1132 merged, I would expect strings that are longer than the line length limit and are located as the value of a keyword argument to be broken into multiple lines, but I don't see this behaviour in practice.

To Reproduce

[user@host ~] pip install 'git+https://github.com/psf/black.git#egg=proj'
...
[user@host ~] black --version
black, version 20.8b1
[user@host ~] cat test.py
foo(
    bar="a really really really really really really really really really really really really really really really really really long string",
)
[user@host ~] black test.py
All done! ✨ 🍰 ✨
1 file left unchanged.
[user@host ~] cat test.py
foo(
    bar="a really really really really really really really really really really really really really really really really really long string",
)

Expected behavior

test.py to be reformatted to look like the examples in #1132, something like:

foo(
    bar=(
        "a really really really really really really really really really"
        " really really really really really really really really long string"
    ),
)

Environment (please complete the following information):

• Version: master
• OS and Python version: RHEL 7 + Python 3.6.8

Does this bug also happen on master?

Yes (both when installed locally and on black.now.sh).

[JelleZijlstra]

This is currently behind the --experimental-string-processing flag. We'll enable that by default once the feature is stable.