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.

doc/lispref/display.texi

@@ -6607,14 +6607,23 @@ in the variable @code{button-map}, which defines @key{RET} and
 The button type.  @xref{Button Types}.
 
 @item help-echo
-@kindex help-index @r{(button property)}
-A string displayed by the Emacs tool-tip help system; by default,
-@code{"mouse-2, RET: Push this button"}.
+@kindex help-echo @r{(button property)}
+A string displayed by the Emacs tooltip help system; by default,
+@code{"mouse-2, RET: Push this button"}.  Alternatively, a function
+that returns, or a form that evaluates to, a string to be displayed or
+@code{nil}.  For details see @ref{Text help-echo}.
+
+The function is called with three arguments, @var{window},
+@var{object}, and @var{pos}.  The second argument, @var{object}, is
+either the overlay that had the property (for overlay buttons), or the
+buffer containing the button (for text property buttons).  The other
+arguments have the same meaning as for the special text property
+@code{help-echo}.
 
 @item follow-link
 @kindex follow-link @r{(button property)}
-The follow-link property, defining how a @key{mouse-1} click behaves
-on this button, @xref{Clickable Text}.
+The @code{follow-link} property, defining how a @key{mouse-1} click
+behaves on this button, @xref{Clickable Text}.
 
 @item button
 @kindex button @r{(button property)}

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.

test/lisp/button-tests.el

@@ -37,4 +37,60 @@
     (widget-create 'link "link widget")
     (should-not (button-at (1- (point))))))
 
+(ert-deftest button--help-echo-string ()
+  "Test `button--help-echo' with strings."
+  (with-temp-buffer
+    ;; Text property buttons.
+    (let ((button (insert-text-button "text" 'help-echo "text help")))
+      (should (equal (button--help-echo button) "text help")))
+    ;; Overlay buttons.
+    (let ((button (insert-button "overlay" 'help-echo "overlay help")))
+      (should (equal (button--help-echo button) "overlay help")))))
+
+(ert-deftest button--help-echo-form ()
+  "Test `button--help-echo' with forms."
+  (with-temp-buffer
+    ;; Test text property buttons with dynamic scoping.
+    (let* ((help   (make-symbol "help"))
+           (form   `(funcall (let ((,help "lexical form"))
+                               (lambda () ,help))))
+           (button (insert-text-button "text" 'help-echo form)))
+      (set help "dynamic form")
+      (should (equal (button--help-echo button) "dynamic form")))
+    ;; Test overlay buttons with lexical scoping.
+    (setq lexical-binding t)
+    (let* ((help   (make-symbol "help"))
+           (form   `(funcall (let ((,help "lexical form"))
+                               (lambda () ,help))))
+           (button (insert-button "overlay" 'help-echo form)))
+      (set help "dynamic form")
+      (should (equal (button--help-echo button) "lexical form")))))
+
+(ert-deftest button--help-echo-function ()
+  "Test `button--help-echo' with functions."
+  (with-temp-buffer
+    ;; Text property buttons.
+    (let* ((owin   (selected-window))
+           (obuf   (current-buffer))
+           (opos   (point))
+           (help   (lambda (win obj pos)
+                     (should (eq win owin))
+                     (should (eq obj obuf))
+                     (should (=  pos opos))
+                     "text function"))
+           (button (insert-text-button "text" 'help-echo help)))
+      (should (equal (button--help-echo button) "text function"))
+      ;; Overlay buttons.
+      (setq help (lambda (win obj pos)
+                   (should (eq win owin))
+                   (should (overlayp obj))
+                   (should (eq obj button))
+                   (should (eq (overlay-buffer obj) obuf))
+                   (should (= (overlay-start obj) opos))
+                   (should (= pos opos))
+                   "overlay function"))
+      (setq opos (point))
+      (setq button (insert-button "overlay" 'help-echo help))
+      (should (equal (button--help-echo button) "overlay function")))))
+
 ;;; button-tests.el ends here