Describe the bug

If you:

• have string processing enabled
• have a line that has a string literal exceeding line-length, followed by a trailing comma (to make it a one-item tuple)
• the line is NOT assigned to a variable

...then black will error.

To Reproduce

Run this code in bash:

echo '"A long string literal that is not assigned to a variable, exceeds line length when string-processing is enabled, and has a trailing comma (to make it a one-item tuple)",' | black --preview --enable-unstable-feature=string_processing -

...or, alternatively, save this to a file called string_processing_trailing_comma_error.py:

"A long string literal that is not assigned to a variable, exceeds line length when string-processing is enabled, and has a trailing comma (to make it a one-item tuple)",

...and then run:

black --preview --enable-unstable-feature=string_processing string_processing_trailing_comma_error.py

The resulting error is:

error: cannot format -: INTERNAL ERROR: Black 25.12.1.dev6+gc3cc5a95d on Python (CPython) 3.10.14 produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_jxevpxw3.log

Oh no! 💥 💔 💥
1 file failed to reformat.

Expected behavior

The string should be split up, without generating an error.

Environment

• Black's version: main (c3cc5a9) and 26.1a1
• OS and Python version: ubuntu-22.04.5 LTS, python-3.10.4 and python-3.12.11