fix(formats): keep Windows paths intact in requirements option lines

[deepspace28]

Option lines go through shlex.split, which runs in POSIX mode where a backslash escapes the next character. A native Windows path in a -e or -r line is destroyed before argparse ever sees it:

>>> shlex.split(r"-e C:\src\pkg")
["-e", "C:srcpkg"]
>>> shlex.split(r"-r C:\src\reqs.txt")
["-r", "C:srcreqs.txt"]

So pdm import of a requirements file that references a local package or a nested requirements file by absolute Windows path silently resolves the wrong location. There is no error - the path just loses its separators.

Escape backslashes on Windows before splitting, which keeps quoting behaving as it does everywhere else:

>>> RequirementParser._split_args(r"-e \"C:\src\my pkg\"")
["-e", "C:\src\my pkg"]

POSIX platforms are untouched, since a backslash there is a legitimate escape. Covered by a parametrized test that pins os.name so it runs on every platform.

[deepspace28]

Withdrawing this - I should have checked the reference implementation first.

pip does exactly the same thing, for the same reason: break_args_options hands the whole option string to shlex.split in POSIX mode.

>>> from pip._internal.req.req_file import break_args_options, build_parser
>>> # pip 26.2.1
>>> opts, _ = parser.parse_args(shlex.split(break_args_options(r"-e C:\src\pkg")[1]), defaults)
>>> opts.editables
["C:srcpkg"]

-r and --find-links behave the same. Since RequirementParser documents itself as following pip's requirements-file format, matching pip here is the correct behaviour and this patch would be a silent divergence rather than a fix. Sorry for the noise.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (14b5b9d) to head (c0103c7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3863   +/-   ##
=======================================
  Coverage   88.72%   88.73%           
=======================================
  Files         120      120           
  Lines       13278    13284    +6     
  Branches     2254     2255    +1     
=======================================
+ Hits        11781    11787    +6     
  Misses        931      931           
  Partials      566      566           

Flag	Coverage Δ
unittests	88.61% <100.00%> (+<0.01%)	⬆️

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