orderless-expand-prefix: Support the values `prefix`, `substring` and `nil`

Using `completion-substring-try-completion` can be costly in comparison to
prefix completion only via `try-completion`, in particular in the context of
timer-based auto completion. As a compromise, default to `prefix` completion
only. Performance should not be affected, while expansion is still provided.
Depending on preference the expansion setting can be escalated to `substring` or
even disabled, in case the restrictive Orderless (non-)expansion behavior is
preferred.

1 files changed, 46 insertions, 40 deletions

diff --git a/orderless.el b/orderless.el
index 187fd38..4e731fb 100644
--- a/orderless.el
+++ b/orderless.el
@@ -212,14 +212,17 @@ is determined by the values of `completion-ignore-case',
 `read-buffer-completion-ignore-case', as usual for completion."
   :type 'boolean)
 
-(defcustom orderless-expand-substring t
+(defcustom orderless-expand-substring 'prefix
   "Whether to perform literal substring expansion.
-If enabled, `orderless-try-completion' will first attempt literal
-substring expansion via `completion-substring-try-completion'.
-Otherwise expansion is only performed for single unique matches.
 This configuration option affects the behavior of some completion
-interfaces when pressing TAB."
-  :type 'boolean)
+interfaces when pressing TAB.  If enabled `orderless-try-completion'
+will first attempt literal substring expansion.  If disabled,
+expansion is only performed for single unique matches.  For
+performance reasons only `prefix' expansion is enabled by default.
+Set the variable to `substring' for full substring expansion."
+  :type '(choice (const :tag "No expansion" nil)
+                 (const :tag "Substring" substring)
+                 (const :tag "Prefix (efficient)" prefix)))
 
 ;;; Matching styles
 
@@ -563,40 +566,43 @@ match, it completes to that match.  If there are no matches, it
 returns nil.  In any other case it \"completes\" STRING to
 itself, without moving POINT.
 This function is part of the `orderless' completion style."
-  (or (and orderless-expand-substring
-           (completion-substring-try-completion string table pred point))
-      (catch 'orderless--many
-        (pcase-let ((`(,prefix ,regexps ,ignore-case ,pred)
-                     (orderless--compile string table pred))
-                    (one nil))
-          ;; Abuse all-completions/orderless--filter as a fast search loop.
-          ;; Should be almost allocation-free since our "predicate" is not
-          ;; called more than two times.
-          (orderless--filter
-           prefix regexps ignore-case table
-           (orderless--predicate-normalized-and
-            pred
-            (lambda (arg)
-              ;; Check if there is more than a single match (= many).
-              (when (and one (not (equal one arg)))
-                (throw 'orderless--many (cons string point)))
-              (setq one arg)
-              t)))
-          (when one
-            ;; Prepend prefix if the candidate does not already have the same
-            ;; prefix.  This workaround is needed since the predicate may either
-            ;; receive an unprefixed or a prefixed candidate as argument.  Most
-            ;; completion tables consistently call the predicate with unprefixed
-            ;; candidates, for example `completion-file-name-table'.  In contrast,
-            ;; `completion-table-with-context' calls the predicate with prefixed
-            ;; candidates.  This could be an unintended bug or oversight in
-            ;; `completion-table-with-context'.
-            (unless (or (equal prefix "")
-                        (and (string-prefix-p prefix one)
-                             (test-completion one table pred)))
-              (setq one (concat prefix one)))
-            (or (equal string one) ;; Return t for unique exact match
-                (cons one (length one))))))))
+  (or
+   (pcase orderless-expand-substring
+     ('nil nil)
+     ('prefix (completion-emacs21-try-completion string table pred point))
+     (_ (completion-substring-try-completion string table pred point)))
+   (catch 'orderless--many
+     (pcase-let ((`(,prefix ,regexps ,ignore-case ,pred)
+                  (orderless--compile string table pred))
+                 (one nil))
+       ;; Abuse all-completions/orderless--filter as a fast search loop.
+       ;; Should be almost allocation-free since our "predicate" is not
+       ;; called more than two times.
+       (orderless--filter
+        prefix regexps ignore-case table
+        (orderless--predicate-normalized-and
+         pred
+         (lambda (arg)
+           ;; Check if there is more than a single match (= many).
+           (when (and one (not (equal one arg)))
+             (throw 'orderless--many (cons string point)))
+           (setq one arg)
+           t)))
+       (when one
+         ;; Prepend prefix if the candidate does not already have the same
+         ;; prefix.  This workaround is needed since the predicate may either
+         ;; receive an unprefixed or a prefixed candidate as argument.  Most
+         ;; completion tables consistently call the predicate with unprefixed
+         ;; candidates, for example `completion-file-name-table'.  In contrast,
+         ;; `completion-table-with-context' calls the predicate with prefixed
+         ;; candidates.  This could be an unintended bug or oversight in
+         ;; `completion-table-with-context'.
+         (unless (or (equal prefix "")
+                     (and (string-prefix-p prefix one)
+                          (test-completion one table pred)))
+           (setq one (concat prefix one)))
+         (or (equal string one) ;; Return t for unique exact match
+             (cons one (length one))))))))
 
 ;;;###autoload
 (add-to-list 'completion-styles-alist