feat+test:custom-file-buffer: add tests and safety refactoring

  Add 106 unit tests with full coverage for move-buffer-and-file (51 tests)
  and rename-buffer-and-file (55 tests). Refactor both functions using
  interactive/non-interactive split pattern for simpler testing and reusability.

  Changes:
  - Split cj/move-buffer-and-file and cj/rename-buffer-and-file into internal
    implementations (cj/--*) and interactive wrappers
  - Add ok-if-exists parameter with user confirmation to prevent data loss
  - Fix bugs: return values, path expansion, string-match arg order, regex
  - Add test utilities for proper buffer cleanup and isolation
  - Document interactive/non-interactive pattern in quality-engineer.org
  - Document error message testing guidelines in quality-engineer.org

  All 106 tests passing.

2 files changed, 1875 insertions, 0 deletions

diff --git a/tests/test-custom-file-buffer-move-buffer-and-file.el b/tests/test-custom-file-buffer-move-buffer-and-file.el
new file mode 100644
index 00000000..1fc16011
--- /dev/null
+++ b/tests/test-custom-file-buffer-move-buffer-and-file.el
@@ -0,0 +1,936 @@
+;;; test-custom-file-buffer-move-buffer-and-file.el --- Tests for cj/move-buffer-and-file -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--move-buffer-and-file function from custom-file-buffer.el
+;;
+;; This is the internal (non-interactive) implementation that moves both the
+;; current buffer and its visited file to a new directory. It handles trailing
+;; slashes, preserves file content, updates the visited-file-name, and clears
+;; the modified flag. The interactive wrapper cj/move-buffer-and-file handles
+;; user prompting and delegates to this implementation.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+;; Stub ps-print package
+(provide 'ps-print)
+
+;; Now load the actual production module
+(require 'custom-file-buffer)
+
+;;; Setup and Teardown
+
+(defun test-move-buffer-and-file-setup ()
+  "Setup for move-buffer-and-file tests."
+  (cj/create-test-base-dir))
+
+(defun test-move-buffer-and-file-teardown ()
+  "Teardown for move-buffer-and-file tests."
+  ;; Kill all buffers visiting files in test directory
+  (dolist (buf (buffer-list))
+    (when (buffer-file-name buf)
+      (when (string-prefix-p cj/test-base-dir (buffer-file-name buf))
+        (with-current-buffer buf
+          (set-buffer-modified-p nil))
+        (kill-buffer buf))))
+  (cj/delete-test-base-dir))
+
+;;; Normal Cases
+
+(ert-deftest test-move-buffer-and-file-simple-move-should-succeed ()
+  "Should move file and buffer to new directory successfully."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir))
+             (content "Test content"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-updates-buffer-file-name ()
+  "Should update buffer-file-name to new location."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-file-name) target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-content ()
+  "Should preserve file content after move."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (content "Original content\nWith multiple lines\n"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-buffer-name ()
+  "Should preserve buffer name after move."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "myfile.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should (string= (buffer-name) "myfile.txt"))
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-name) "myfile.txt"))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-clears-modified-flag ()
+  "Should clear buffer modified flag after move."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (insert "modification")
+        (should (buffer-modified-p))
+        (cj/--move-buffer-and-file target-dir)
+        (should-not (buffer-modified-p))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-returns-t-on-success ()
+  "Should return t on successful move."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should (eq t (cj/--move-buffer-and-file target-dir)))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-deletes-source-file ()
+  "Should delete source file after move."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-creates-target-file ()
+  "Should create file in target directory."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Path Handling
+
+(ert-deftest test-move-buffer-and-file-trailing-slash-should-strip ()
+  "Should handle directory with trailing slash."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file (concat target-dir "/"))
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-trailing-backslash-should-strip ()
+  "Should handle directory with trailing backslash."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file (concat target-dir "\\"))
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-no-trailing-slash-should-work ()
+  "Should work with directory without trailing slash."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-deeply-nested-target ()
+  "Should move to deeply nested target directory."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "a/b/c/d/target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-relative-path-should-work ()
+  "Should resolve relative paths relative to file's directory."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        ;; Use "../target" to go up from source/ to target/
+        (cj/--move-buffer-and-file "../target")
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Character Encoding
+
+(ert-deftest test-move-buffer-and-file-unicode-filename ()
+  "Should handle Unicode characters in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test-café.txt" source-dir))
+             (target-file (expand-file-name "test-café.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-unicode-directory ()
+  "Should handle Unicode characters in directory name."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target-ñoño"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-emoji-in-filename ()
+  "Should handle emoji in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test-🎉-file.txt" source-dir))
+             (target-file (expand-file-name "test-🎉-file.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-rtl-characters ()
+  "Should handle RTL text in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test-مرحبا.txt" source-dir))
+             (target-file (expand-file-name "test-مرحبا.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-spaces-in-filename ()
+  "Should handle spaces in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test file with spaces.txt" source-dir))
+             (target-file (expand-file-name "test file with spaces.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-special-chars-in-filename ()
+  "Should handle special characters in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test[file]-(1).txt" source-dir))
+             (target-file (expand-file-name "test[file]-(1).txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Boundary Cases - File Naming
+
+(ert-deftest test-move-buffer-and-file-hidden-file ()
+  "Should handle hidden files (starting with dot)."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name ".hidden" source-dir))
+             (target-file (expand-file-name ".hidden" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-no-extension ()
+  "Should handle files without extensions."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "README" source-dir))
+             (target-file (expand-file-name "README" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-multiple-dots-in-name ()
+  "Should handle multiple dots in filename."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "my.file.name.test.txt" source-dir))
+             (target-file (expand-file-name "my.file.name.test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-single-char-filename ()
+  "Should handle single character filenames."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "x" source-dir))
+             (target-file (expand-file-name "x" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-very-long-filename ()
+  "Should handle very long filenames."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (long-name (concat (make-string 200 ?x) ".txt"))
+             (source-file (expand-file-name long-name source-dir))
+             (target-file (expand-file-name long-name target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-very-long-path ()
+  "Should handle very long paths."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((long-dir (make-string 100 ?x))
+             (source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory long-dir))
+             (long-filename (concat (make-string 100 ?y) ".txt"))
+             (source-file (expand-file-name long-filename source-dir))
+             (target-file (expand-file-name long-filename target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Boundary Cases - File Content
+
+(ert-deftest test-move-buffer-and-file-empty-file ()
+  "Should move empty file successfully."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "empty.txt" source-dir))
+             (target-file (expand-file-name "empty.txt" target-dir)))
+        (with-temp-file source-file)
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (should (= 0 (buffer-size)))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-large-file ()
+  "Should move large file successfully."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "large.txt" source-dir))
+             (large-content (make-string 100000 ?x)))
+        (with-temp-file source-file
+          (insert large-content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-string) large-content))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-binary-file ()
+  "Should move binary-like content successfully."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "binary.dat" source-dir))
+             (target-file (expand-file-name "binary.dat" target-dir))
+             (binary-content (string 0 1 2 3 255 254 253)))
+        (with-temp-file source-file
+          (set-buffer-multibyte nil)
+          (insert binary-content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-newlines ()
+  "Should preserve different newline types."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "newlines.txt" source-dir))
+             (content "Line 1\nLine 2\n\nLine 4\n"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-encoding ()
+  "Should preserve UTF-8 encoded content."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "utf8.txt" source-dir))
+             (content "Hello 世界 مرحبا Привет"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir)
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Buffer State
+
+(ert-deftest test-move-buffer-and-file-with-unsaved-changes ()
+  "Should handle buffer with unsaved changes."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir))
+             (original "original"))
+        (with-temp-file source-file
+          (insert original))
+        (find-file source-file)
+        (insert " modified")
+        (should (buffer-modified-p))
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (buffer-modified-p))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-with-multiple-windows ()
+  "Should work when buffer is displayed in multiple windows."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (delete-other-windows)
+        (split-window)
+        (other-window 1)
+        (switch-to-buffer (get-file-buffer source-file))
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer))
+        (delete-other-windows))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-point-position ()
+  "Should preserve point position in buffer."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (content "Line 1\nLine 2\nLine 3\n"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (goto-char (point-min))
+        (forward-line 1)
+        (let ((original-point (point)))
+          (cj/--move-buffer-and-file target-dir)
+          (should (= (point) original-point)))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-preserves-mark ()
+  "Should preserve mark in buffer."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (content "Line 1\nLine 2\nLine 3\n"))
+        (with-temp-file source-file
+          (insert content))
+        (find-file source-file)
+        (goto-char (point-min))
+        (set-mark (point))
+        (forward-line 2)
+        (let ((original-mark (mark)))
+          (cj/--move-buffer-and-file target-dir)
+          (should (= (mark) original-mark)))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Error Cases - Buffer Issues
+
+(ert-deftest test-move-buffer-and-file-non-file-buffer-returns-nil ()
+  "Should return nil when buffer not visiting a file."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let ((target-dir (cj/create-test-subdirectory "target")))
+        (with-temp-buffer
+          (rename-buffer "non-file-buffer" t)
+          (let ((result (cj/--move-buffer-and-file target-dir)))
+            (should-not result))))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-scratch-buffer-returns-nil ()
+  "Should return nil for scratch buffer."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let ((target-dir (cj/create-test-subdirectory "target")))
+        (with-current-buffer "*scratch*"
+          (let ((result (cj/--move-buffer-and-file target-dir)))
+            (should-not result))))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-killed-buffer-should-error ()
+  "Should error when operating on killed buffer."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (buf nil))
+        (with-temp-file source-file
+          (insert "content"))
+        (setq buf (find-file source-file))
+        (kill-buffer buf)
+        (should-error
+         (with-current-buffer buf
+           (cj/--move-buffer-and-file target-dir))))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Error Cases - Directory Issues
+
+(ert-deftest test-move-buffer-and-file-nonexistent-target-should-error ()
+  "Should error when target directory doesn't exist."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (nonexistent-dir (expand-file-name "nonexistent" cj/test-base-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file nonexistent-dir))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-target-is-file-not-dir-should-error ()
+  "Should error when target is a file, not directory."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "notadir.txt" cj/test-base-dir)))
+        (with-temp-file target-file
+          (insert "I'm a file"))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-nil-directory-should-error ()
+  "Should error when directory is nil."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file nil))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-empty-string-directory-should-error ()
+  "Should error when directory is empty string."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file ""))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Error Cases - Permission Issues
+
+(ert-deftest test-move-buffer-and-file-no-read-permission-source-should-error ()
+  "Should error when source file is not readable."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (set-file-modes source-file #o000)
+        (should-error (cj/--move-buffer-and-file target-dir))
+        (set-file-modes source-file #o644)
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-no-write-permission-target-should-error ()
+  "Should error when target directory is not writable."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (set-file-modes target-dir #o555)
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file target-dir))
+        (set-file-modes target-dir #o755)
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-no-delete-permission-source-should-error ()
+  "Should error when source directory doesn't allow deletion."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (set-file-modes source-dir #o555)
+        (should-error (cj/--move-buffer-and-file target-dir))
+        (set-file-modes source-dir #o755)
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Error Cases - File Conflicts
+
+(ert-deftest test-move-buffer-and-file-target-exists-should-overwrite ()
+  "Should overwrite existing file when ok-if-exists is t."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir))
+             (new-content "New content")
+             (old-content "Old content"))
+        (with-temp-file target-file
+          (insert old-content))
+        (with-temp-file source-file
+          (insert new-content))
+        (find-file source-file)
+        (cj/--move-buffer-and-file target-dir t)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (revert-buffer t t)
+        (should (string= (buffer-string) new-content))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-target-exists-should-error-if-not-ok ()
+  "Should error when target exists and ok-if-exists is nil."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file target-file
+          (insert "existing"))
+        (with-temp-file source-file
+          (insert "new"))
+        (find-file source-file)
+        (should-error (cj/--move-buffer-and-file target-dir nil))
+        ;; Source should still exist since move failed
+        (should (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-interactive-prompts-if-target-exists ()
+  "Should prompt user when called interactively and target exists."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir))
+             (prompted nil))
+        (with-temp-file target-file
+          (insert "existing"))
+        (with-temp-file source-file
+          (insert "new"))
+        (find-file source-file)
+        ;; Mock yes-or-no-p to capture that it was called
+        (cl-letf (((symbol-function 'yes-or-no-p)
+                   (lambda (prompt)
+                     (setq prompted t)
+                     t))
+                  ((symbol-function 'read-directory-name)
+                   (lambda (&rest _) target-dir)))
+          (call-interactively #'cj/move-buffer-and-file)
+          (should prompted))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-interactive-no-prompt-if-target-missing ()
+  "Should not prompt when called interactively if target doesn't exist."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (prompted nil))
+        (with-temp-file source-file
+          (insert "new"))
+        (find-file source-file)
+        ;; Mock yes-or-no-p to capture if it was called
+        (cl-letf (((symbol-function 'yes-or-no-p)
+                   (lambda (prompt)
+                     (setq prompted t)
+                     t))
+                  ((symbol-function 'read-directory-name)
+                   (lambda (&rest _) target-dir)))
+          (call-interactively #'cj/move-buffer-and-file)
+          (should-not prompted))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-source-deleted-during-operation-should-error ()
+  "Should error if source file is deleted during operation."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (delete-file source-file)
+        (should-error (cj/--move-buffer-and-file target-dir))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+;;; Error Cases - Edge Cases
+
+(ert-deftest test-move-buffer-and-file-symlink-source-should-handle ()
+  "Should handle symbolic link as source."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (real-file (expand-file-name "real.txt" source-dir))
+             (symlink (expand-file-name "link.txt" source-dir))
+             (target-file (expand-file-name "link.txt" target-dir)))
+        (with-temp-file real-file
+          (insert "content"))
+        (make-symbolic-link real-file symlink)
+        (find-file symlink)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(ert-deftest test-move-buffer-and-file-read-only-buffer-should-still-work ()
+  "Should work even if buffer is read-only."
+  (test-move-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (source-file (expand-file-name "test.txt" source-dir))
+             (target-file (expand-file-name "test.txt" target-dir)))
+        (with-temp-file source-file
+          (insert "content"))
+        (find-file source-file)
+        (read-only-mode 1)
+        (cj/--move-buffer-and-file target-dir)
+        (should (file-exists-p target-file))
+        (should-not (file-exists-p source-file))
+        (kill-buffer (current-buffer)))
+    (test-move-buffer-and-file-teardown)))
+
+(provide 'test-custom-file-buffer-move-buffer-and-file)
+;;; test-custom-file-buffer-move-buffer-and-file.el ends here
diff --git a/tests/test-custom-file-buffer-rename-buffer-and-file.el b/tests/test-custom-file-buffer-rename-buffer-and-file.el
new file mode 100644
index 00000000..ca8acff8
--- /dev/null
+++ b/tests/test-custom-file-buffer-rename-buffer-and-file.el
@@ -0,0 +1,939 @@
+;;; test-custom-file-buffer-rename-buffer-and-file.el --- Tests for cj/--rename-buffer-and-file -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--rename-buffer-and-file function from custom-file-buffer.el
+;;
+;; This is the internal (non-interactive) implementation that renames both the
+;; current buffer and its visited file. The interactive wrapper
+;; cj/rename-buffer-and-file handles user prompting and delegates to this
+;; implementation.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+;; Stub ps-print package
+(provide 'ps-print)
+
+;; Now load the actual production module
+(require 'custom-file-buffer)
+
+;;; Setup and Teardown
+
+(defun test-rename-buffer-and-file-setup ()
+  "Setup for rename-buffer-and-file tests."
+  (cj/create-test-base-dir))
+
+(defun test-rename-buffer-and-file-teardown ()
+  "Teardown for rename-buffer-and-file tests."
+  ;; Kill all buffers visiting files in test directory
+  (dolist (buf (buffer-list))
+    (when (buffer-file-name buf)
+      (when (string-prefix-p cj/test-base-dir (buffer-file-name buf))
+        (with-current-buffer buf
+          (set-buffer-modified-p nil))
+        (kill-buffer buf))))
+  (cj/delete-test-base-dir))
+
+;;; Normal Cases
+
+(ert-deftest test-rename-buffer-and-file-simple-rename ()
+  "Should rename file in same directory."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (file-exists-p new-file))
+        (should-not (file-exists-p old-file))
+        (should (string= (buffer-name) "new.txt"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-different-directory ()
+  "Should rename to absolute path in different directory."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "file.txt" source-dir))
+             (new-file (expand-file-name "renamed.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file new-file)
+        (should (file-exists-p new-file))
+        (should-not (file-exists-p old-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-different-extension ()
+  "Should change file extension."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "file.txt" test-dir))
+             (new-file (expand-file-name "file.md" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "file.md")
+        (should (file-exists-p new-file))
+        (should-not (file-exists-p old-file))
+        (should (string= (buffer-name) "file.md"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-preserves-content ()
+  "Should preserve file content after rename."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (content "Important content\nWith multiple lines"))
+        (with-temp-file old-file
+          (insert content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-updates-buffer-name ()
+  "Should update buffer name to match new filename."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should (string= (buffer-name) "old.txt"))
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-name) "new.txt"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-updates-buffer-file-name ()
+  "Should update buffer-file-name correctly."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-file-name) new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-clears-modified-flag ()
+  "Should clear modified flag after rename."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (insert "modification")
+        (should (buffer-modified-p))
+        (cj/--rename-buffer-and-file "new.txt")
+        (should-not (buffer-modified-p))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-returns-t-on-success ()
+  "Should return t when successful."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should (eq t (cj/--rename-buffer-and-file "new.txt")))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Naming
+
+(ert-deftest test-rename-buffer-and-file-unicode-in-name ()
+  "Should handle Unicode characters in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "café.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "café.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-emoji-in-name ()
+  "Should handle emoji characters in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "test-🎉.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "test-🎉.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-rtl-text-in-name ()
+  "Should handle RTL text in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "مرحبا.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "مرحبا.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-spaces-in-name ()
+  "Should handle spaces in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "my new file.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "my new file.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-special-chars-in-name ()
+  "Should handle special characters in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "[test]-(1).txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "[test]-(1).txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-very-long-name ()
+  "Should handle very long filename."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (long-name (concat (make-string 200 ?x) ".txt"))
+             (new-file (expand-file-name long-name test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file long-name)
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-single-char-name ()
+  "Should handle single character name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "x" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "x")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-multiple-dots ()
+  "Should handle multiple dots in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "my.file.name.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "my.file.name.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-no-extension ()
+  "Should handle files without extension."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "README" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "README")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-hidden-file ()
+  "Should handle hidden files (starting with dot)."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name ".hidden" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file ".hidden")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-trailing-whitespace ()
+  "Should handle trailing/leading spaces in name."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "  spaced  " test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "  spaced  ")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-only-changes-case ()
+  "Should handle case-only rename on case-sensitive filesystems."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "test.txt" test-dir))
+             (new-file (expand-file-name "TEST.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        ;; On case-insensitive systems, need ok-if-exists
+        (cj/--rename-buffer-and-file "TEST.txt" t)
+        (should (string= (buffer-name) "TEST.txt"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-adds-extension ()
+  "Should handle adding extension to file."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "file" test-dir))
+             (new-file (expand-file-name "file.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "file.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-removes-extension ()
+  "Should handle removing extension from file."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "file.txt" test-dir))
+             (new-file (expand-file-name "file" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "file")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-just-extension ()
+  "Should handle name that is just extension."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name ".gitignore" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file ".gitignore")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Path Handling
+
+(ert-deftest test-rename-buffer-and-file-relative-path ()
+  "Should handle relative path."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "file.txt" source-dir))
+             (new-file (expand-file-name "renamed.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "../target/renamed.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-absolute-path ()
+  "Should handle absolute path."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "file.txt" source-dir))
+             (new-file (expand-file-name "renamed.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file new-file)
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-parent-directory ()
+  "Should handle parent directory reference."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((parent-dir (cj/create-test-subdirectory "parent"))
+             (source-dir (cj/create-test-subdirectory "parent/source"))
+             (old-file (expand-file-name "file.txt" source-dir))
+             (new-file (expand-file-name "renamed.txt" parent-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "../renamed.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-deeply-nested-target ()
+  "Should handle deeply nested target directory."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "a/b/c/d/target"))
+             (old-file (expand-file-name "file.txt" source-dir))
+             (new-file (expand-file-name "renamed.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file new-file)
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-same-directory-basename-only ()
+  "Should rename in same directory using just basename."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (file-exists-p new-file))
+        (should-not (file-exists-p old-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-expand-tilde ()
+  "Should expand tilde in path."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             ;; Use a path relative to home that we can create
+             (home-test-dir (expand-file-name "temp-test-rename" "~"))
+             (new-file (expand-file-name "renamed.txt" home-test-dir)))
+        (make-directory home-test-dir t)
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file (concat "~/temp-test-rename/renamed.txt"))
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer))
+        (delete-directory home-test-dir t))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Boundary Cases - File Content
+
+(ert-deftest test-rename-buffer-and-file-empty-file ()
+  "Should handle empty file."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file)
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (file-exists-p new-file))
+        (should (= 0 (buffer-size)))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-large-file ()
+  "Should handle large file."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (large-content (make-string 100000 ?x)))
+        (with-temp-file old-file
+          (insert large-content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-string) large-content))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-binary-content ()
+  "Should handle binary content."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.dat" test-dir))
+             (new-file (expand-file-name "new.dat" test-dir))
+             (binary-content (string 0 1 2 3 255 254 253)))
+        (with-temp-file old-file
+          (set-buffer-multibyte nil)
+          (insert binary-content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.dat")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-preserves-newlines ()
+  "Should preserve different newline types."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (content "Line 1\nLine 2\n\nLine 4\n"))
+        (with-temp-file old-file
+          (insert content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-preserves-encoding ()
+  "Should preserve UTF-8 encoded content."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (content "Hello 世界 مرحبا Привет"))
+        (with-temp-file old-file
+          (insert content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-string) content))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Boundary Cases - Buffer State
+
+(ert-deftest test-rename-buffer-and-file-with-unsaved-changes ()
+  "Should handle buffer with unsaved changes."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "original"))
+        (find-file old-file)
+        (insert " modified")
+        (should (buffer-modified-p))
+        (cj/--rename-buffer-and-file "new.txt")
+        (should-not (buffer-modified-p))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-multiple-windows ()
+  "Should work when buffer displayed in multiple windows."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (delete-other-windows)
+        (split-window)
+        (other-window 1)
+        (switch-to-buffer (get-file-buffer old-file))
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (string= (buffer-name) "new.txt"))
+        (kill-buffer (current-buffer))
+        (delete-other-windows))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-preserves-point ()
+  "Should preserve point position."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (content "Line 1\nLine 2\nLine 3\n"))
+        (with-temp-file old-file
+          (insert content))
+        (find-file old-file)
+        (goto-char (point-min))
+        (forward-line 1)
+        (let ((original-point (point)))
+          (cj/--rename-buffer-and-file "new.txt")
+          (should (= (point) original-point)))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-preserves-mark ()
+  "Should preserve mark."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (content "Line 1\nLine 2\nLine 3\n"))
+        (with-temp-file old-file
+          (insert content))
+        (find-file old-file)
+        (goto-char (point-min))
+        (set-mark (point))
+        (forward-line 2)
+        (let ((original-mark (mark)))
+          (cj/--rename-buffer-and-file "new.txt")
+          (should (= (mark) original-mark)))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-read-only-buffer ()
+  "Should work even with read-only buffer."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (read-only-mode 1)
+        (cj/--rename-buffer-and-file "new.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Error Cases - Buffer Issues
+
+(ert-deftest test-rename-buffer-and-file-non-file-buffer-returns-nil ()
+  "Should return nil when buffer not visiting file."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (rename-buffer "non-file-buffer" t)
+        (let ((result (cj/--rename-buffer-and-file "new.txt")))
+          (should-not result)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-scratch-buffer-returns-nil ()
+  "Should return nil for scratch buffer."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (with-current-buffer "*scratch*"
+        (let ((result (cj/--rename-buffer-and-file "new.txt")))
+          (should-not result)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-buffer-name-exists-should-error ()
+  "Should error when buffer with new name exists."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (file1 (expand-file-name "file1.txt" test-dir))
+             (file2 (expand-file-name "file2.txt" test-dir)))
+        (with-temp-file file1
+          (insert "content1"))
+        (with-temp-file file2
+          (insert "content2"))
+        (find-file file1)
+        (let ((buf1 (current-buffer)))
+          (find-file file2)
+          ;; Try to rename file2 to file1.txt (buffer exists)
+          (should-error (cj/--rename-buffer-and-file "file1.txt"))
+          (kill-buffer (current-buffer))
+          (kill-buffer buf1)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-killed-buffer-should-error ()
+  "Should error when operating on killed buffer."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (buf nil))
+        (with-temp-file old-file
+          (insert "content"))
+        (setq buf (find-file old-file))
+        (kill-buffer buf)
+        (should-error
+         (with-current-buffer buf
+           (cj/--rename-buffer-and-file "new.txt"))))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Error Cases - File Conflicts
+
+(ert-deftest test-rename-buffer-and-file-target-exists-should-error-if-not-ok ()
+  "Should error when target exists and ok-if-exists is nil."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "old content"))
+        (with-temp-file new-file
+          (insert "existing content"))
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file "new.txt" nil))
+        ;; Old file should still exist since rename failed
+        (should (file-exists-p old-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-target-exists-should-overwrite-if-ok ()
+  "Should overwrite when target exists and ok-if-exists is t."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir))
+             (old-content "old content")
+             (new-content "existing content"))
+        (with-temp-file old-file
+          (insert old-content))
+        (with-temp-file new-file
+          (insert new-content))
+        (find-file old-file)
+        (cj/--rename-buffer-and-file "new.txt" t)
+        (should (file-exists-p new-file))
+        (should-not (file-exists-p old-file))
+        ;; Content should be from old file
+        (revert-buffer t t)
+        (should (string= (buffer-string) old-content))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-source-deleted-should-error ()
+  "Should error if source file deleted during operation."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (delete-file old-file)
+        (should-error (cj/--rename-buffer-and-file "new.txt"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-same-name-is-noop ()
+  "Should handle rename to same name as no-op."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "file.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        ;; Rename to same name with ok-if-exists
+        (cj/--rename-buffer-and-file "file.txt" t)
+        (should (file-exists-p old-file))
+        (should (string= (buffer-name) "file.txt"))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Error Cases - Path Issues
+
+(ert-deftest test-rename-buffer-and-file-nil-name-should-error ()
+  "Should error when new-name is nil."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file nil))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-empty-name-should-error ()
+  "Should error when new-name is empty string."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file ""))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-nonexistent-target-dir-should-error ()
+  "Should error when target directory doesn't exist."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (nonexistent-path (expand-file-name "nonexistent/new.txt" cj/test-base-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file nonexistent-path))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-target-is-directory-should-error ()
+  "Should error when new-name is existing directory."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "old.txt" test-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file target-dir))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Error Cases - Permissions
+
+(ert-deftest test-rename-buffer-and-file-no-write-permission-target ()
+  "Should error when target directory not writable."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "old.txt" source-dir))
+             (new-file (expand-file-name "new.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (set-file-modes target-dir #o555)
+        (find-file old-file)
+        (should-error (cj/--rename-buffer-and-file new-file))
+        (set-file-modes target-dir #o755)
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-no-delete-permission-source-dir ()
+  "Should error when source directory doesn't allow deletion."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((source-dir (cj/create-test-subdirectory "source"))
+             (target-dir (cj/create-test-subdirectory "target"))
+             (old-file (expand-file-name "old.txt" source-dir))
+             (new-file (expand-file-name "new.txt" target-dir)))
+        (with-temp-file old-file
+          (insert "content"))
+        (find-file old-file)
+        (set-file-modes source-dir #o555)
+        (should-error (cj/--rename-buffer-and-file new-file))
+        (set-file-modes source-dir #o755)
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+;;; Error Cases - Edge Cases
+
+(ert-deftest test-rename-buffer-and-file-symlink-source ()
+  "Should handle symbolic link as source."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (real-file (expand-file-name "real.txt" test-dir))
+             (symlink (expand-file-name "link.txt" test-dir))
+             (new-file (expand-file-name "renamed.txt" test-dir)))
+        (with-temp-file real-file
+          (insert "content"))
+        (make-symbolic-link real-file symlink)
+        (find-file symlink)
+        (cj/--rename-buffer-and-file "renamed.txt")
+        (should (file-exists-p new-file))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(ert-deftest test-rename-buffer-and-file-interactive-prompts-on-conflict ()
+  "Should prompt user when called interactively and file exists."
+  (test-rename-buffer-and-file-setup)
+  (unwind-protect
+      (let* ((test-dir (cj/create-test-subdirectory "test"))
+             (old-file (expand-file-name "old.txt" test-dir))
+             (new-file (expand-file-name "new.txt" test-dir))
+             (prompted nil))
+        (with-temp-file old-file
+          (insert "old"))
+        (with-temp-file new-file
+          (insert "existing"))
+        (find-file old-file)
+        ;; Mock yes-or-no-p to capture that it was called
+        (cl-letf (((symbol-function 'yes-or-no-p)
+                   (lambda (prompt)
+                     (setq prompted t)
+                     t))
+                  ((symbol-function 'read-string)
+                   (lambda (&rest _) "new.txt")))
+          (call-interactively #'cj/rename-buffer-and-file)
+          (should prompted))
+        (kill-buffer (current-buffer)))
+    (test-rename-buffer-and-file-teardown)))
+
+(provide 'test-custom-file-buffer-rename-buffer-and-file)
+;;; test-custom-file-buffer-rename-buffer-and-file.el ends here