feat(corfu): more CAPFs and ergonomy changes

Add CAPFs from cape:
- `cape-dabbrev`;
- `cape-elisp-block`;
- `cape-file`;
Fix some CAPFs via cape:
- Make non-exclusive, purified and silent `pcomplete-completions-at-point`;
- Make non-exclusive and non-interruptable `lsp-completion-at-point`;
- Make non-exclusive `eglot-completion-at-point`;
- Make non-exclusive `comint-completion-at-point`;
Fix and improve keybindings:
- Smart `DEL`;
Add depth to CAPFs, allowing ordering to be adjustable.

modules/completion/corfu/README.org

@@ -26,6 +26,9 @@ utilities for fine-tuning. Only some of common behaviors are documented here.
 - +orderless ::
   Pull in [[doom-package:orderless]] if necessary and apply multi-component
   completion (still needed if [[doom-module::completion vertico]] is active).
+- +dabbrev ::
+  Enable and configure [[doom-package:dabbrev]] as a close-to-universal CAPF
+  fallback.
 
 ** Packages
 - [[doom-package:corfu]]
@@ -190,15 +193,18 @@ A few variables may be set to change behavior of this module:
   Enables commiting with [[RET]] when the popup is visible. Default is ~t~, may be set to
   ~'minibuffer~ if you want to commit both the completion and the minibuffer when
   active. When ~nil~, it is always passed-through.
+- [[var:+corfu-buffer-scanning-size-limit]]  ::
+  Sets the maximum buffer size to be scanned by ~cape-dabbrev~. Defaults to 1 MB.
+  Set this if you are having performance problems using the CAPF.
 
 ** Adding CAPFs to a mode
 To add other CAPFs on a mode-per-mode basis, put either of the following in your
 ~config.el~:
 
 #+begin_src emacs-lisp
-(add-hook! some-mode (add-to-list 'completion-at-point-functions #'some-capf))
+(add-hook! some-mode (add-hook 'completion-at-point-functions #'some-capf depth t))
 ;; OR, but note the different call signature
-(add-hook 'some-mode-hook (lambda () (add-to-list 'completion-at-point-functions #'some-capf)))
+(add-hook 'some-mode-hook (lambda () (add-hook 'completion-at-point-functions #'some-capf depth t)))
 #+end_src
 
 ~DEPTH~ above is an integer between -100, 100, and defaults to 0 if nil. Also
@@ -221,6 +227,17 @@ all CAPFs are interactive to be called this way, in which case you can use
 * Troubleshooting
 [[doom-report:][Report an issue?]]
 
+If you have performance issues with ~cape-dabbrev~, the first thing I recommend
+doing is to look at the list of buffers Dabbrev is scanning:
+
+#+begin_src emacs-lisp
+(dabbrev--select-buffers) ; => (#<buffer README.org> #<buffer config.el<3>> #<buffer cape.el> ...)
+(length (dabbrev--select-buffers)) ; => 37
+#+end_src
+
+... and modify ~dabbrev-ignored-buffer-regexps~ or ~dabbrev-ignored-buffer-modes~
+accordingly.
+
 * Frequently asked questions
 /This module has no FAQs yet./ [[doom-suggest-faq:][Ask one?]]
 

modules/completion/corfu/config.el

@@ -8,6 +8,9 @@ Possible values are:
               and immediatelly exit if in the minibuffer;
 - nil: Pass-through without inserting.")
 
+(defvar +corfu-buffer-scanning-size-limit (* 1 1024 1024) ; 1 MB
+  "Size limit for a buffer to be scanned by `cape-dabbrev'.")
+
 ;;
 ;;; Packages
 (use-package! corfu
@@ -50,10 +53,27 @@ Possible values are:
   :init
   (add-hook! prog-mode
     (defun +corfu-add-cape-file-h ()
-      (add-to-list 'completion-at-point-functions #'cape-file)))
+      (add-hook 'completion-at-point-functions #'cape-file -10 t)))
   (add-hook! (org-mode markdown-mode)
     (defun +corfu-add-cape-elisp-block-h ()
       (add-hook 'completion-at-point-functions #'cape-elisp-block 0 t)))
+  ;; Enable Dabbrev completion basically everywhere as a fallback.
+  (when (modulep! +dabbrev)
+    (setq cape-dabbrev-check-other-buffers t)
+    ;; Set up `cape-dabbrev' options.
+    (defun +dabbrev-friend-buffer-p (other-buffer)
+      (< (buffer-size other-buffer) +corfu-buffer-scanning-size-limit))
+    (add-hook! (prog-mode text-mode conf-mode comint-mode minibuffer-setup
+                          eshell-mode)
+      (defun +corfu-add-cape-dabbrev-h ()
+        (add-hook 'completion-at-point-functions #'cape-dabbrev 20 t)))
+    (after! dabbrev
+      (setq dabbrev-friend-buffer-function #'+dabbrev-friend-buffer-p
+            dabbrev-ignored-buffer-regexps
+            '("^ "
+              "\\(TAGS\\|tags\\|ETAGS\\|etags\\|GTAGS\\|GRTAGS\\|GPATH\\)\\(<[0-9]+>\\)?")
+            dabbrev-upcase-means-case-search t)
+      (add-to-list 'dabbrev-ignored-buffer-modes 'pdf-view-mode)))
 
   ;; Make these capfs composable.
   (advice-add #'lsp-completion-at-point :around #'cape-wrap-noninterruptible)

modules/config/default/config.el

@@ -468,7 +468,13 @@ Continues comments if executed from a commented line. Consults
          [backtab] #'corfu-previous
          "TAB" #'corfu-next
          [tab] #'corfu-next))
-  (let ((cmds-ret
+  (let ((cmds-del
+         `(menu-item "Reset completion" corfu-reset
+           :filter ,(lambda (cmd)
+                      (when (and (>= corfu--index 0)
+                                 (eq corfu-preview-current 'insert))
+                        cmd))))
+         (cmds-ret
           `(menu-item "Insert completion DWIM" corfu-insert
              :filter ,(lambda (cmd)
                         (interactive)
@@ -488,6 +494,8 @@ Continues comments if executed from a commented line. Consults
                               (t cmd))))))
     (map! :when (modulep! :completion corfu)
           :map corfu-map
+          [backspace] cmds-del
+          "DEL" cmds-del
           :gi [return] cmds-ret
           :gi "RET" cmds-ret))