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, 41 insertions, 11 deletions

diff --git a/modules/custom-buffer-file.el b/modules/custom-buffer-file.el
index ef4fc945..8c6014d3 100644
--- a/modules/custom-buffer-file.el
+++ b/modules/custom-buffer-file.el
@@ -214,16 +214,46 @@ When called interactively, prompts for confirmation if target file exists."
       (kill-new file-path)
       (message "Copied file link to kill ring: %s" file-path))))
 
-(defun cj/copy-path-to-buffer-file-as-kill ()
-  "Copy the full path of the current buffer's file to the kill ring.
-Signal an error if the buffer is not visiting a file."
+(defvar cj/buffer-source-functions
+  '((eww-mode         . (lambda () (eww-current-url)))
+    (elfeed-show-mode . (lambda () (elfeed-entry-link elfeed-show-entry)))
+    (dired-mode       . (lambda () (dired-get-filename nil t)))
+    (dirvish-mode     . (lambda () (dired-get-filename nil t))))
+  "Alist mapping major-mode -> thunk returning the buffer's \"source\".
+
+Each thunk is called with no arguments and should return a string
+to be copied to the kill ring, or nil to fall through to
+`buffer-file-name'.  Modes not listed here also fall through to
+`buffer-file-name'.
+
+Used by `cj/copy-buffer-source-as-kill' (`C-; b p').  Doc-view and
+PDF-view modes intentionally aren't listed -- their
+`buffer-file-name' already points at the underlying file, so the
+fallback handles them.")
+
+(defun cj/copy-buffer-source-as-kill ()
+  "Copy the current buffer's \"source\" to the kill ring.
+
+Source means the URL, file path, or other clickable reference that
+identifies what the buffer represents.  Dispatches by `major-mode'
+via `cj/buffer-source-functions'; falls back to `buffer-file-name'
+for modes without a dispatch entry.
+
+Signals `user-error' when no source can be determined."
   (interactive)
-  (let ((path (buffer-file-name)))
-    (if (not path)
-        (user-error "Current buffer is not visiting a file")
-      (kill-new path)
-      (message "Copied file path: %s" path)
-      path)))
+  (let* ((handler (alist-get major-mode cj/buffer-source-functions))
+         (source (or (and handler (funcall handler))
+                     (buffer-file-name))))
+    (unless source
+      (user-error "Buffer has no copyable source"))
+    (kill-new source)
+    (message "Copied: %s" source)
+    source))
+
+;; Backwards-compat alias.  The old name predates the dispatch
+;; extension and several test files still reference it; keep the
+;; alias so external callers and existing tests continue to work.
+(defalias 'cj/copy-path-to-buffer-file-as-kill 'cj/copy-buffer-source-as-kill)
 
 (defun cj/copy-whole-buffer ()
   "Copy the entire contents of the current buffer to the kill ring.
@@ -445,7 +475,7 @@ Signals an error if:
   :doc "Keymap for buffer and file operations."
   "m" #'cj/move-buffer-and-file
   "r" #'cj/rename-buffer-and-file
-  "p" #'cj/copy-path-to-buffer-file-as-kill
+  "p" #'cj/copy-buffer-source-as-kill
   "d" #'cj/delete-buffer-and-file
   "D" #'cj/diff-buffer-with-file
   "c" cj/copy-buffer-content-map
@@ -480,7 +510,7 @@ Signals an error if:
     "C-; b" "buffer and file menu"
     "C-; b m" "move file"
     "C-; b r" "rename file"
-    "C-; b p" "copy file path"
+    "C-; b p" "copy buffer source"
     "C-; b d" "delete file"
     "C-; b D" "diff buffer with file"
     "C-; b c" "buffer copy menu"