Further improve button.el support for help-echo

The last change to forward-button added support for help-echo values
that are functions.  This patch fixes the arguments passed to such
functions and further adds support for help-echo values that are
forms (bug#37515).
* doc/lispref/display.texi (Button Properties): Fix description of
help-echo button property.
* lisp/button.el (button--help-echo): New function.
(forward-button): Use it.
(backward-button): Clarify help-echo reference in docstring.
* test/lisp/button-tests.el (button--help-echo-string)
(button--help-echo-form, button--help-echo-function): New tests.

1 files changed, 16 insertions, 10 deletions

diff --git a/lisp/button.el b/lisp/button.el
index 32efc2f95be..04e77ca904f 100644
--- a/lisp/button.el
+++ b/lisp/button.el
@@ -467,13 +467,22 @@ return t."
         (button-activate button use-mouse-action)
         t))))
 
+(defun button--help-echo (button)
+  "Evaluate BUTTON's `help-echo' property and return its value."
+  (let ((help (button-get button 'help-echo)))
+    (if (functionp help)
+        (let ((obj (if (overlayp button) button (current-buffer))))
+          (funcall help (selected-window) obj (button-start button)))
+      (eval help lexical-binding))))
+
 (defun forward-button (n &optional wrap display-message no-error)
   "Move to the Nth next button, or Nth previous button if N is negative.
 If N is 0, move to the start of any button at point.
 If WRAP is non-nil, moving past either end of the buffer continues from the
 other end.
-If DISPLAY-MESSAGE is non-nil, the button's help-echo string is displayed.
-Any button with a non-nil `skip' property is skipped over.
+If DISPLAY-MESSAGE is non-nil, the button's `help-echo' property
+is displayed.  Any button with a non-nil `skip' property is
+skipped over.
 
 If NO-ERROR, return nil if no further buttons could be found
 instead of erroring out.
@@ -506,13 +515,9 @@ Returns the button found."
             (unless (button-get button 'skip)
               (setq n (1- n)))))))
     (if (null button)
-        (if no-error
-            nil
+        (unless no-error
           (user-error (if wrap "No buttons!" "No more buttons")))
-      (let ((msg (and display-message (button-get button 'help-echo))))
-        (when (functionp msg)
-          (setq msg (funcall msg (selected-window) (current-buffer)
-                             (button-start button))))
+      (let ((msg (and display-message (button--help-echo button))))
         (when msg
           (message "%s" msg)))
       button)))
@@ -522,8 +527,9 @@ Returns the button found."
 If N is 0, move to the start of any button at point.
 If WRAP is non-nil, moving past either end of the buffer continues from the
 other end.
-If DISPLAY-MESSAGE is non-nil, the button's help-echo string is displayed.
-Any button with a non-nil `skip' property is skipped over.
+If DISPLAY-MESSAGE is non-nil, the button's `help-echo' property
+is displayed.  Any button with a non-nil `skip' property is
+skipped over.
 
 If NO-ERROR, return nil if no further buttons could be found
 instead of erroring out.