New variable 'completion-preview-is-calling'

* lisp/completion-preview.el (completion-preview-is-calling):
New variable.
(completion-preview--capf-wrapper): Bind it to t when calling
the CAPF.
* etc/NEWS: Announce it.

etc/NEWS

@@ -4489,6 +4489,11 @@ the minibuffer, instead of waiting with 'sit-for' and then clearing it.
 This makes 'minibuffer-message' usable in Lisp programs which want to
 print a message and then continue to perform work.
 
+---
+** New variable 'completion-preview-is-calling'.
+Completion functions (in 'completion-at-point-functions') can check this
+variable to tell if they are being called by Completion Preview mode.
+
 ** Special Events
 
 +++

lisp/completion-preview.el

@@ -523,9 +523,20 @@ candidates or if there are multiple matching completions and
             (setq sorted (cdr sorted)))
           (list (substring string 0 base) common suffixes))))))
 
+(defvar completion-preview-is-calling nil
+  "Non-nil while Completion Preview mode is calling a completion function.
+
+Completion functions (in `completion-at-point-functions') can check this
+variable and adjust their behavior for completion preview.
+For example, a completion function can skip calculating annotations for
+candidates it produces for display in the completion preview, which does
+not make use of such annotations.")
+
 (defun completion-preview--capf-wrapper (capf)
   "Translate return value of CAPF to properties for completion preview overlay."
-  (let ((res (ignore-errors (funcall capf))))
+  (let ((res (ignore-errors
+               (let ((completion-preview-is-calling t))
+                 (funcall capf)))))
     (and (consp res)
          (not (functionp res))
          (seq-let (beg end table &rest plist) res