diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 85f3907..becccbb 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -154,6 +154,15 @@ project--find-in-directory
     vc-directory-exclusion-list)
    grep-find-ignored-files))
 
+(cl-defgeneric project--find-file-in (filename dirs project)
+  "Complete FILENAME (a string or nil) in directories DIRS; visit the file.
+If non-nil, DIRS is a list of absolute directories; it should be some
+subset of the project roots and external roots. If nil, the backend
+uses all the directories it knows about.
+PROJECT is used to find the project ignores and other project meta-data."
+  ;; dispatch on PROJECT
+  )
+
 (defgroup project-vc nil
   "Project implementation using the VC package."
   :version "25.1"
@@ -312,31 +321,34 @@ project--find-regexp-in
     (xref--show-xrefs xrefs nil)))
 
 ;;;###autoload
-(defun project-find-file ()
-  "Visit a file in the current project's roots.
-
-This is like `find-file', but it limits the file-name completion
-candidates to the files within the current project roots."
-  (interactive)
+(defun project-find-file (filename)
+  "Visit FILENAME in the current project's roots.
+FILENAME defaults to the filename at point (nil if none
+recognized).
+FILENAME is completed with the files within the current project
+roots."
+  (interactive (list (when current-prefix-arg (thing-at-point 'filename))))
   (let* ((pr (project-current t))
          (dirs (project-roots pr)))
-    (project--find-file-in dirs pr)))
+    (project--find-file-in filename dirs pr)))
 
 ;;;###autoload
-(defun project-or-external-find-file ()
-  "Visit a file in the current project's roots or external roots.
-
-This is like `find-file', but it limits the file-name completion
-candidates to the files within the current project roots and external roots."
-  (interactive)
+(defun project-or-external-find-file (filename)
+  "Visit FILENAME in the current project's roots or external roots.
+FILENAME defaults to the filename at point (nil if none
+recognized).
+FILENAME is completed with the files within the current project
+roots and external roots."
+  (interactive (list (when current-prefix-arg (thing-at-point 'filename))))
   (let* ((pr (project-current t))
          (dirs (append
                 (project-roots pr)
                 (project-external-roots pr))))
-    (project--find-file-in dirs pr)))
+    (project--find-file-in filename dirs pr)))
 
-;; FIXME: Uniquely abbreviate the roots?
-(defun project--find-file-in (dirs project)
+  ;; FIXME: Uniquely abbreviate the roots?
+(cl-defmethod project--find-file-in (filename dirs project)
+  "Default implementation using `find-program'."
   (require 'xref)
   (let* ((all-files
           (cl-mapcan
@@ -357,7 +369,7 @@ project--find-file-in
                    (t
                     (complete-with-action action all-files string pred))))))
     (find-file
-     (completing-read "Find file: " table nil t))))
+     (completing-read "Find file: " table nil t filename))))
 
 (provide 'project)
 ;;; project.el ends here
diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 267853d..ad7d6bb 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -948,6 +948,9 @@ xref--rgrep-command
    (xref--find-ignores-arguments ignores dir)))
 
 (defun xref--find-ignores-arguments (ignores dir)
+  "Convert IGNORES and DIR to a list of arguments for 'find'.
+IGNORES is a list of glob patterns.  DIR is an absolute
+directory, used as the root of the ignore globs."
   ;; `shell-quote-argument' quotes the tilde as well.
   (cl-assert (not (string-match-p "\\`~" dir)))
   (when ignores