Support transforming the dirname used by uniquify

By transforming the buffer's directory name, we can add
additional information to use during uniquifying.  A basic
one: uniquifying buffer names based on the project name.
* lisp/progmodes/project.el (project-uniquify-dirname-transform): Add.
* lisp/uniquify.el (uniquify-dirname-transform-default)
(uniquify-dirname-transform): Add.  (Bug#62621)
(uniquify-rationalize-file-buffer-names, uniquify-buffer-file-name):
Use 'uniquify-dirname-transform'.
* test/lisp/uniquify-tests.el (uniquify-home)
(uniquify-project-transform): Add tests.

lisp/progmodes/project.el

@@ -1887,5 +1887,22 @@ to directory DIR."
     (let ((project-current-directory-override dir))
       (call-interactively command))))
 
+;;;###autoload
+(defun project-uniquify-dirname-transform (dirname)
+  "Include `project-name' in DIRNAME if in a project.
+
+If you set `uniquify-dirname-transform' to this function,
+slash-separated components from `project-name' will be added to
+the buffer's name when buffers from two different projects would
+otherwise have the same name."
+  (if-let (proj (project-current nil dirname))
+      (let ((root (project-root proj)))
+        (expand-file-name
+         (file-name-concat
+          (file-name-directory root)
+          (project-name proj)
+          (file-relative-name dirname root))))
+    dirname))
+
 (provide 'project)
 ;;; project.el ends here

lisp/uniquify.el

@@ -168,6 +168,31 @@ This can be handy when you have deep parallel hierarchies."
 That means that when `buffer-file-name' is set to nil, `list-buffers-directory'
 contains the name of the directory which the buffer is visiting.")
 
+(defcustom uniquify-dirname-transform #'identity
+  "Function to transform buffer's directory for uniquifying its name.
+
+If `uniquify-buffer-name-style' is non-nil and a buffer's name
+would be the same as some other buffer, then components from the
+buffer's directory name are added to the buffer's name until the
+buffer's name is unique.
+
+This function is used to transform the buffer's directory name
+before the uniquifying process, allowing the unique buffer name
+to include components from other sources.  The default is
+`identity', so only the buffer's directory name is used for
+uniquifying.  This function is called with the buffer's directory
+name and should return a file name (which does not need to
+actually exist in the filesystem) to use components from.
+
+To include components from `project-name', set this variable to
+`project-uniquify-dirname-transform'."
+  :type '(choice (function-item :tag "Don't change the dirname" identity)
+                 (function-item :tag "Include project name in dirname"
+                                #'project-uniquify-dirname-transform)
+                 function)
+  :version "30.1"
+  :group 'uniquify)
+
 ;;; Utilities
 
 ;; uniquify-fix-list data structure
@@ -209,7 +234,8 @@ this rationalization."
   ;; this buffer.
   (with-current-buffer newbuf (setq uniquify-managed nil))
   (when dirname
-    (setq dirname (expand-file-name (directory-file-name dirname)))
+    (setq dirname (funcall uniquify-dirname-transform
+                           (expand-file-name (directory-file-name dirname))))
     (let ((fix-list (list (uniquify-make-item base dirname newbuf
                                               nil)))
 	  items)
@@ -268,10 +294,11 @@ in `uniquify-list-buffers-directory-modes', otherwise returns nil."
 	       (if (memq major-mode uniquify-list-buffers-directory-modes)
 		   list-buffers-directory))))
       (when filename
-	(directory-file-name
-	 (file-name-directory
-	  (expand-file-name
-	   (directory-file-name filename))))))))
+	 (funcall uniquify-dirname-transform
+	          (directory-file-name
+	          (file-name-directory
+	           (expand-file-name
+	            (directory-file-name filename)))))))))
 
 (defun uniquify-rerationalize-w/o-cb (fix-list)
   "Re-rationalize the buffers in FIX-LIST, but ignoring `current-buffer'."

test/lisp/uniquify-tests.el

@@ -88,6 +88,21 @@
                        '("a/dir/" "b/dir/")))
         (mapc #'kill-buffer bufs)))))
 
+(ert-deftest uniquify-home ()
+  "uniquify works, albeit confusingly, in the presence of directories named \"~\""
+  (let (bufs)
+    (save-excursion
+      (push (find-file-noselect "~") bufs)
+      (push (find-file-noselect "./~") bufs)
+      (should (equal (mapcar #'buffer-name bufs)
+                     '("~<test>" "~<>")))
+      (push (find-file-noselect "~/foo") bufs)
+      (push (find-file-noselect "./~/foo") bufs)
+      (should (equal (mapcar #'buffer-name bufs)
+                     '("foo<~>" "foo</nonexistent>" "~<test>" "~<>")))
+      (while bufs
+        (kill-buffer (pop bufs))))))
+
 (ert-deftest uniquify-rename-to-dir ()
   "Giving a buffer a name which matches a directory doesn't rename the buffer"
   (let ((uniquify-buffer-name-style 'forward)
@@ -125,5 +140,23 @@ uniquify-trailing-separator-p is ignored"
     (should (equal (buffer-name) "| foo"))
     (kill-buffer)))
 
+(require 'project)
+(ert-deftest uniquify-project-transform ()
+  "`project-uniquify-dirname-transform' works"
+  (let ((uniquify-dirname-transform #'project-uniquify-dirname-transform)
+        (project-vc-name "foo1/bar")
+        bufs)
+    (save-excursion
+      (should (file-exists-p "../README"))
+      (push (find-file-noselect "../README") bufs)
+      (push (find-file-noselect "other/README") bufs)
+      (should (equal (mapcar #'buffer-name bufs)
+                     '("README<other>" "README<bar>")))
+      (push (find-file-noselect "foo2/bar/README") bufs)
+      (should (equal (mapcar #'buffer-name bufs)
+                     '("README<foo2/bar>" "README<other>" "README<foo1/bar>")))
+      (while bufs
+        (kill-buffer (pop bufs))))))
+
 (provide 'uniquify-tests)
 ;;; uniquify-tests.el ends here