Merge pull request from tzz/custom-set-default

GitHub-reference: https://github.com/jwiegley/use-package/issues/855

lisp/use-package/use-package-core.el

@@ -1563,7 +1563,9 @@ this file.  Usage:
 :load-path       Add to the `load-path' before attempting to load the package.
 :diminish        Support for diminish.el (if installed).
 :delight         Support for delight.el (if installed).
-:custom          Call `customize-set-variable' with each variable definition.
+:custom          Call `custom-set' or `set-default' with each variable
+                 definition without modifying the Emacs `custom-file'.
+                 (compare with `custom-set-variables').
 :custom-face     Call `customize-set-faces' with each face definition.
 :ensure          Loads the package using package.el if necessary.
 :pin             Pin the package to an archive."

test/lisp/use-package/use-package-tests.el

@@ -1133,7 +1133,11 @@
   (match-expansion
    (use-package foo :custom (foo bar))
    `(progn
-      (customize-set-variable 'foo bar "Customized with use-package foo")
+      (funcall
+       (or
+        (get 'foo 'custom-set)
+        (function set-default))
+       'foo bar)
       (require 'foo nil nil))))
 
 (ert-deftest use-package-test/:custom-face-1 ()