[FR] On Windows pyproject.toml needs pre-processing of paths as linker flags from ext-modules directive aren't handled correctly by linker

[skirpichev]

setuptools version

75.6.0

Python version

3.13

OS

Window$

Additional environment information

No response

Description

I tried to switch my project to use new ext-modules directive, but without success:
diofant/python-gmp#124

It seems that linker flags passed incorrectly. Here is a fragment from build log:

2025-01-04T05:28:32.2403885Z   "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.42.34433\bin\HostX86\x64\link.exe" \
/nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO \
/LIBPATH:.local/lib \
[...]

As you can see, setuptools pass /LIBPATH:.local/lib (that's come from library-dirs of the ext-modules directive). But linker doesn't recognize it, perhaps because it doesn't follows Windows path convention with \ separators. (But note that compiler works with added include-dirs.)

Expected behavior

Successful build:)

How to Reproduce

I did a draft pr to show problem in CI:
diofant/python-gmp#124

Output

Sorry, I can't reproduce that locally.

[abravalheri]

Thanks @skirpichev, as far as I understand the reason why the problem does not occur when using setup.py is because setup.py is programmatically choosing the right path separator depending on the OS. But if you use static strings in setup.py I believe that you are going to observe the same problem (so in this sense, the behaviour is backwards compatible).

So I am reclassifying this issue as a feature request to implement some sort of dynamic processing of paths.

[skirpichev]

setup.py is programmatically choosing the right path separator depending on the OS

Exactly.

if you use static strings in setup.py I believe that you are going to observe the same problem

Well, docs says about include_dirs: "list of directories to search for C/C++ header files (in Unix form for portability)". So, I suspect this should work in a string form too.

But the library_dirs option has no such note.