feat(custom-buffer-file): make C-; b p dispatch by major-mode, with tests

Old behavior: `C-; b p' called `cj/copy-path-to-buffer-file-as-kill',
which only worked in file-visiting buffers and errored otherwise.
That meant the most useful "give me a clickable handle on this
buffer" key did nothing in eww, elfeed, dired (file-at-point ≠
buffer's default-directory), and other browsing-shaped modes.

Replace with a `major-mode'-aware dispatch:

- `cj/buffer-source-functions' alist maps major-mode → thunk
  returning a string (or nil to fall through).
- `cj/copy-buffer-source-as-kill' looks up the current mode,
  calls the thunk, falls back to `buffer-file-name', errors only
  when both yield nil.
- `cj/copy-path-to-buffer-file-as-kill' kept as a `defalias' for
  backwards compat (the old name is referenced in adjacent tests).

First-batch dispatches:
- eww-mode         -> (eww-current-url)
- elfeed-show-mode -> (elfeed-entry-link elfeed-show-entry)
- dired-mode       -> (dired-get-filename nil t)
- dirvish-mode     -> same
- doc-view / pdf-view: covered by the buffer-file-name fallback
  (they already set buffer-file-name correctly).

10 new ERT tests cover the dispatch paths, the
buffer-file-name fallback, the user-error on nil source, the alias
target, and the `C-; b p' keymap entry.

which-key label flipped from "copy file path" to "copy buffer
source" to match.

Deferred to a follow-up task: mu4e-view-mode, org-mode at a
heading, help-mode, Info-mode, magit-log/commit/status, xref/grep/
compilation, image-mode, archive-mode -- each needs a format
decision before implementation.

1 files changed, 144 insertions, 0 deletions

diff --git a/tests/test-custom-buffer-file-copy-buffer-source.el b/tests/test-custom-buffer-file-copy-buffer-source.el
new file mode 100644
index 00000000..c4e073ae
--- /dev/null
+++ b/tests/test-custom-buffer-file-copy-buffer-source.el
@@ -0,0 +1,144 @@
+;;; test-custom-buffer-file-copy-buffer-source.el --- Tests for cj/copy-buffer-source-as-kill -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests the dispatch behavior of `cj/copy-buffer-source-as-kill', the
+;; major-mode-aware extension of the old
+;; `cj/copy-path-to-buffer-file-as-kill' (which is kept as a
+;; `defalias' for backwards compat).
+;;
+;; Dispatch tiers:
+;;   eww-mode         -> (eww-current-url)
+;;   elfeed-show-mode -> (elfeed-entry-link elfeed-show-entry)
+;;   dired-mode       -> (dired-get-filename nil t)
+;;   dirvish-mode     -> (dired-get-filename nil t)
+;;   doc-view-mode    -> buffer-file-name
+;;   pdf-view-mode    -> buffer-file-name
+;;   anything else    -> buffer-file-name fallback (errors if nil)
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'custom-buffer-file)
+
+;; Top-level defvars so let-bindings reach the dynamic var under
+;; lexical scope (and stand-ins for vars defined in modes we don't
+;; require here).
+(defvar elfeed-show-entry nil)
+
+;;; Fallback to buffer-file-name
+
+(ert-deftest test-copy-buffer-source-falls-back-to-buffer-file-name ()
+  "Normal: a file-visiting buffer with no special dispatch falls back
+to `buffer-file-name'."
+  (let (kill-ring)
+    (with-temp-buffer
+      (setq buffer-file-name "/tmp/notes.txt")
+      (fundamental-mode)
+      (cl-letf (((symbol-function 'message) #'ignore))
+        (cj/copy-buffer-source-as-kill))
+      (should (equal (car kill-ring) "/tmp/notes.txt")))))
+
+(ert-deftest test-copy-buffer-source-errors-on-nil-source ()
+  "Error: a buffer with no file and no mode dispatch signals a user-error."
+  (with-temp-buffer
+    (fundamental-mode)
+    (setq buffer-file-name nil)
+    (should-error (cj/copy-buffer-source-as-kill) :type 'user-error)))
+
+;;; eww-mode dispatch
+
+(ert-deftest test-copy-buffer-source-eww-copies-current-url ()
+  "Normal: in eww-mode, copy `(eww-current-url)'."
+  (let (kill-ring)
+    (cl-letf (((symbol-function 'eww-current-url)
+               (lambda () "https://example.org/article")))
+      (with-temp-buffer
+        (setq major-mode 'eww-mode)
+        (cl-letf (((symbol-function 'message) #'ignore))
+          (cj/copy-buffer-source-as-kill))
+        (should (equal (car kill-ring) "https://example.org/article"))))))
+
+;;; elfeed-show-mode dispatch
+
+(ert-deftest test-copy-buffer-source-elfeed-copies-entry-link ()
+  "Normal: in elfeed-show-mode, copy the entry's link."
+  (let ((kill-ring nil)
+        (elfeed-show-entry (list :link "https://feed.test/post/42")))
+    (cl-letf (((symbol-function 'elfeed-entry-link)
+               (lambda (entry) (plist-get entry :link))))
+      (with-temp-buffer
+        (setq major-mode 'elfeed-show-mode)
+        (cl-letf (((symbol-function 'message) #'ignore))
+          (cj/copy-buffer-source-as-kill))
+        (should (equal (car kill-ring) "https://feed.test/post/42"))))))
+
+;;; dired-mode dispatch
+
+(ert-deftest test-copy-buffer-source-dired-copies-file-at-point ()
+  "Normal: in dired-mode, copy the absolute path of the file at point
+(not the dired buffer's `default-directory')."
+  (let (kill-ring)
+    (cl-letf (((symbol-function 'dired-get-filename)
+               (lambda (&optional _localp _no-error)
+                 "/home/u/projects/notes.org")))
+      (with-temp-buffer
+        (setq major-mode 'dired-mode)
+        (cl-letf (((symbol-function 'message) #'ignore))
+          (cj/copy-buffer-source-as-kill))
+        (should (equal (car kill-ring) "/home/u/projects/notes.org"))))))
+
+(ert-deftest test-copy-buffer-source-dirvish-copies-file-at-point ()
+  "Normal: dirvish-mode dispatches identically to dired-mode."
+  (let (kill-ring)
+    (cl-letf (((symbol-function 'dired-get-filename)
+               (lambda (&optional _localp _no-error)
+                 "/home/u/books/the-book.epub")))
+      (with-temp-buffer
+        (setq major-mode 'dirvish-mode)
+        (cl-letf (((symbol-function 'message) #'ignore))
+          (cj/copy-buffer-source-as-kill))
+        (should (equal (car kill-ring) "/home/u/books/the-book.epub"))))))
+
+;;; doc-view / pdf-view dispatch
+
+(ert-deftest test-copy-buffer-source-doc-view-copies-buffer-file ()
+  "Normal: doc-view-mode copies the underlying file path
+(`buffer-file-name')."
+  (let (kill-ring)
+    (with-temp-buffer
+      (setq buffer-file-name "/home/u/papers/paper.pdf"
+            major-mode 'doc-view-mode)
+      (cl-letf (((symbol-function 'message) #'ignore))
+        (cj/copy-buffer-source-as-kill))
+      (should (equal (car kill-ring) "/home/u/papers/paper.pdf")))))
+
+(ert-deftest test-copy-buffer-source-pdf-view-copies-buffer-file ()
+  "Normal: pdf-view-mode copies the underlying file path."
+  (let (kill-ring)
+    (with-temp-buffer
+      (setq buffer-file-name "/home/u/books/manual.pdf"
+            major-mode 'pdf-view-mode)
+      (cl-letf (((symbol-function 'message) #'ignore))
+        (cj/copy-buffer-source-as-kill))
+      (should (equal (car kill-ring) "/home/u/books/manual.pdf")))))
+
+;;; Backwards-compat alias
+
+(ert-deftest test-copy-path-old-name-aliases-new-command ()
+  "Backwards compat: `cj/copy-path-to-buffer-file-as-kill' still
+resolves to the new `cj/copy-buffer-source-as-kill' via defalias."
+  (should (eq (symbol-function 'cj/copy-path-to-buffer-file-as-kill)
+              'cj/copy-buffer-source-as-kill)))
+
+;;; Keybinding
+
+(ert-deftest test-copy-buffer-source-bound-on-b-p ()
+  "Normal: `p' in `cj/buffer-and-file-map' invokes the new command."
+  (should (eq (keymap-lookup cj/buffer-and-file-map "p")
+              #'cj/copy-buffer-source-as-kill)))
+
+(provide 'test-custom-buffer-file-copy-buffer-source)
+;;; test-custom-buffer-file-copy-buffer-source.el ends here