Don't let-bind default-directory across switch-project-action

[natetarrh]

Summary

projectile-switch-project-by-name wraps the call to projectile-switch-project-action in (let* ((default-directory project-to-switch)) ...). Because default-directory is auto-buffer-local, the let-binding mutates the calling buffer's buffer-local default-directory for the duration of the action.

Switch-project actions that iterate (buffer-list) and read each buffer's default-directory via with-current-buffer then misclassify the originating buffer as belonging to the target project. The simplest way to hit this is setting

(setq projectile-switch-project-action #'projectile-switch-to-buffer)

…and pressing s-p p from any buffer that isn't already in the target project: that buffer shows up as a "buffer of the target project" in the picker, even though it isn't, because projectile-project-buffer-p reads its default-directory while the let-binding is in scope and finds the target root.

I hit this with projectile-switch-to-buffer from a local vterm into a /docker: TRAMP project — the local vterm's (file-remote-p default-directory) transiently equaled (file-remote-p "/docker:dev-in-docker:/src/") and the buffer got grouped with the docker project's buffers. I traced it via add-variable-watcher on default-directory; the final unlet event reverts the originating buffer's value, with the call stack rooted in projectile-switch-project-by-name.

Fix

Set default-directory on the temporary buffer the action runs in instead of let-binding it in the caller. Same effect (action body sees default-directory = target root) without the leak.

-    (let* ((default-directory project-to-switch)
-           (switched-buffer
-            (with-temp-buffer
-              (hack-dir-local-variables-non-file-buffer)
-              ...)))
+    (let ((switched-buffer
+           (with-temp-buffer
+             (setq-local default-directory project-to-switch)
+             (hack-dir-local-variables-non-file-buffer)
+             ...)))

Nothing else in projectile-switch-project-by-name reads default-directory outside the with-temp-buffer, so the change is local. All 326 existing specs pass.

Test

Added a buttercup regression test that:

1. Creates an "origin" buffer with default-directory set to a known dir.
2. Sets projectile-switch-project-action to a lambda that reads the origin buffer's default-directory while the action is running.
3. Asserts that what the action observed equals the original default-directory, not the project being switched to.

Confirmed the test fails on master (observes project/) and passes after the fix (observes origin/).

Notes

• The fix is intentionally minimal — no behaviour change for actions that depend on default-directory being the target root, since the temp buffer still carries that value.
• The let* was load-bearing only for the default-directory it set; with that moved into with-temp-buffer, it collapses to a plain let of one binding.
• I'd argue any switch-project-action that explicitly wants the caller's buffer mutated is unusual enough that this is a strict improvement; happy to be talked out of it though.