fix(task): convert PATH to /cygdrive form for Cygwin bash tasks

[JamBalaya56562]

Problem

PR #9547 added Windows→Unix PATH conversion so mise launched from PowerShell can spawn a POSIX shell (bash -c) for a task with a usable PATH. It unconditionally emits the MSYS2 / Git Bash /c/foo form, but Cygwin expects /cygdrive/c/foo — so Cygwin users' PATH was corrupted and binaries like cygpath became command not found.

Reported in #9950. Cygwin was intentionally out of scope in #9547; this is the follow-up.

Fix

• Detect Cygwin bash by a cygwin / cygwin64 / cygwin32 path segment in the resolved bash path. Detection runs after bash resolution, so it sees the real exe (C:\cygwin64\bin\bash.exe, or a MISE_BASH_PATH pointing there). Segment-matching avoids false positives like my-cygwinish-tools.
• Thread a drive_prefix through windows_path_list_to_unix: "" for MSYS2 / Git Bash (/c/..., unchanged) and /cygdrive for Cygwin (/cygdrive/c/...).
• A non-default cygdrive mount (configured in /etc/fstab) can be supplied via MISE_CYGDRIVE_PREFIX (e.g. /mnt); mise does not parse fstab. A trailing / is trimmed, so MISE_CYGDRIVE_PREFIX=/ collapses to the MSYS /c/... form.

Pure Rust, no subprocess (no cygpath fork). Windows-only and bash-path-based; MSYSTEM is not consulted since PowerShell-launched mise inherits none.

Out of scope (intentional)

• Parsing /etc/fstab — replaced by the explicit MISE_CYGDRIVE_PREFIX escape hatch.
• Auto-adding Cygwin to bash_candidates() — would change bash auto-selection for users who have both Git Bash and Cygwin installed. Cygwin users pick their bash via MISE_BASH_PATH or PATH; this fix ensures PATH is formatted correctly for whichever bash is chosen.

Tests

• src/path.rs: Cygwin conversion (basic, forward-slash, drive-letter case, spaces, UNC / already-unix passthrough, empty, relative), custom prefix, and is_cygwin_shell detection (true/false incl. the false-positive guard).
• src/task/task_executor.rs: maybe_convert_env_for_msys_shell routes Cygwin → /cygdrive, Git Bash → /c/ (unchanged), and honors MISE_CYGDRIVE_PREFIX.
• e2e-win/task.Tests.ps1: skippable Cygwin e2e asserting the task PATH contains /cygdrive/.

28 unit tests pass locally on x86_64-pc-windows-msvc. CHANGELOG.md is git-cliff-generated, so left untouched.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces support for Cygwin bash on Windows by detecting Cygwin paths and converting the PATH environment variable to the /cygdrive/c/... format instead of the Git Bash /c/... format. It also allows overriding the default prefix via the MISE_CYGDRIVE_PREFIX environment variable, accompanied by documentation updates and tests. The review feedback focuses on performance optimizations, specifically suggesting allocation-free implementations for path segment matching in is_cygwin_shell and prefix resolution in msys_drive_prefix_for.

[greptile-apps]

Greptile Summary

This PR fixes Cygwin PATH corruption when mise is launched from PowerShell and spawns a Cygwin bash task: it detects Cygwin bash by a cygwin/cygwin64/cygwin32 path segment, then converts the task PATH to Cygwin's /cygdrive/c/... form instead of the MSYS2/Git Bash /c/... form already in use.

• windows_path_list_to_unix now accepts a drive_prefix parameter (\"\" for MSYS2, \"/cygdrive\" for Cygwin, or a custom value from MISE_CYGDRIVE_PREFIX), and msys_drive_prefix_for selects the right prefix at the spawn boundary.
• is_cygwin_shell performs whole-segment, case-insensitive matching on the resolved bash path so substrings like my-cygwinish-tools don't trigger false positives.
• MISE_CYGDRIVE_PREFIX is validated to be absolute; a relative value (e.g. mnt) is rejected with a warning and falls back to /cygdrive rather than silently producing broken PATH entries.

Confidence Score: 5/5

Safe to merge — the change is Windows-only, narrowly scoped to PATH conversion at the bash spawn boundary, and does not touch any shared non-Windows codepaths.

The detection logic is well-guarded (whole-segment matching, case-insensitive), the drive_prefix plumbing is mechanically straightforward, and MISE_CYGDRIVE_PREFIX validation correctly rejects relative values. Unit tests cover the main happy paths and edge cases. The only gap is a missing assertion for the cygwin32 segment in the detection tests, which is a minor oversight with no runtime impact.

No files require special attention.

Important Files Changed

Filename	Overview
src/path.rs	Adds drive_prefix parameter to windows_path_list_to_unix/append_single_windows_path_to_unix and new is_cygwin_shell helper; logic is correct, but cygwin32 segment detection has no unit-test coverage in the new test suite.
src/task/task_executor.rs	Adds msys_drive_prefix_for to select the right cygdrive prefix before calling windows_path_list_to_unix; task-env checked before process-env, invalid relative prefix is rejected with a warning, and edge cases (/ collapsing to MSYS form) are covered by tests.
docs/troubleshooting.md	New Cygwin section added with detection explanation, MISE_BASH_PATH/MISE_CYGDRIVE_PREFIX usage, and the absolute-prefix requirement; accurate and complete.
e2e-win/task.Tests.ps1	New Cygwin e2e test correctly skips when Cygwin is absent, saves/restores all overridden env vars, and pins MISE_CYGDRIVE_PREFIX to /cygdrive so the assertion is stable across developer machines.

_{Reviews (5): Last reviewed commit: "fix(task): address review feedback on Cy..." | Re-trigger Greptile}

[lewis-yeung]

@JamBalaya56562 A non-default cygdrive mount point is available not only in Cygwin Bash, but ALSO in Git Bash. The following code unconditionally assumes that a specific segment exists in the Bash executable path, and excludes Git Bash cases which are STILL BROKEN. :(

mise/src/path.rs

Lines 242 to 246 in 310e325

	s.split(['/', '\\']).any(|seg| {
	seg.eq_ignore_ascii_case("cygwin")
	|| seg.eq_ignore_ascii_case("cygwin64")
	|| seg.eq_ignore_ascii_case("cygwin32")
	})