venv: Activation in fish inside msys2 (or similar) breaks PATH

[LuNoX]

Bug report

Bug description:

When running source .venv/Scripts/activate.fish on windows from a fish shell inside msys2 ucrt64 it breaks the PATH.
Equivalently to gh-103325 activating a venv inside fish (from Cygwin, MSYS or MinGW) on windows will lead to mixing Unix and Windows path styles which then has knockon effects like breaking the PATH when

cpython/Lib/venv/scripts/common/activate.fish

Line 39 in 5755d0f

set -gx PATH "$VIRTUAL_ENV/"__VENV_BIN_NAME__ $PATH

runs.

To reproduce:

• Boot a Windows partition
• Install Cygwin, MSYS2, MinGW or equivalent
• Install fish from inside Cygwin, MSYS2, MinGW or equivalent
• Install python
• cd into a python project of your choice
• Run your installed python's venv command
• Run echo $PATH
• Run source .venv/Scripts/activate.fish
• Run echo $PATH

Notice that the second PATH output contains a new broken/mismatched entry at the start which will in turn break all future uses of PATH.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-151042: venv: Pass VIRTUAL_ENV through cygpath inside fish on Windows #151043