Implement default help-echo for mode line.

* lisp/bindings.el: Remove explicit help-echo from format-mode-line.
(mode-line-front-space, mode-line-end-spaces)
(mode-line-misc-info): New variables.
(mode-line-modes, mode-line-position): Move the default value to
the variable definition.
(mode-line-default-help-echo): New defcustom.

* src/buffer.c (buffer_local_value_1): New function, split from
Fbuffer_local_value; can return Qunbound.
(Fbuffer_local_value): Use it.
(Vmode_line_format): Docstring tweaks.

* src/xdisp.c (calc_pixel_width_or_height): Use Fbuffer_local_value.
(note_mode_line_or_margin_highlight): If there is no help echo,
use mode-line-default-help-echo.  Handle the case where the mouse
position is past the end of the mode line string.

* doc/lispref/modes.texi (Mode Line Data): Use "mode line construct"
terminology for consistency.

4 files changed, 121 insertions, 50 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index b90db0b7fa9..c0098445cb8 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,15 @@
+2012-06-03  Chong Yidong  <cyd@gnu.org>
+
+        * xdisp.c (calc_pixel_width_or_height): Use Fbuffer_local_value.
+        (note_mode_line_or_margin_highlight): If there is no help echo,
+        use mode-line-default-help-echo.  Handle the case where the mouse
+        position is past the end of the mode line string.
+
+        * buffer.c (buffer_local_value_1): New function, split from
+        Fbuffer_local_value; can return Qunbound.
+        (Fbuffer_local_value): Use it.
+        (Vmode_line_format): Docstring tweaks.
+
 2012-06-02  Paul Eggert  <eggert@cs.ucla.edu>
 
         * sysdep.c (system_process_attributes): Improve comment.
diff --git a/src/buffer.c b/src/buffer.c
index 20260d5a5d8..386d9a78153 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -933,6 +933,21 @@ If VARIABLE does not have a buffer-local binding in BUFFER, the value
 is the default binding of the variable. */)
   (register Lisp_Object variable, register Lisp_Object buffer)
 {
+  register Lisp_Object result = buffer_local_value_1 (variable, buffer);
+
+  if (EQ (result, Qunbound))
+    xsignal1 (Qvoid_variable, variable);
+
+  return result;
+}
+
+
+/* Like Fbuffer_local_value, but return Qunbound if the variable is
+   locally unbound.  */
+
+Lisp_Object
+buffer_local_value_1 (Lisp_Object variable, Lisp_Object buffer)
+{
   register struct buffer *buf;
   register Lisp_Object result;
   struct Lisp_Symbol *sym;
@@ -985,10 +1000,7 @@ is the default binding of the variable. */)
     default: abort ();
     }
 
-  if (!EQ (result, Qunbound))
-    return result;
-
-  xsignal1 (Qvoid_variable, variable);
+  return result;
 }
 
 /* Return an alist of the Lisp-level buffer-local bindings of
@@ -5329,31 +5341,40 @@ the mode line appears at the bottom.  */);
   DEFVAR_PER_BUFFER ("mode-line-format", &BVAR (current_buffer, mode_line_format),
                      Qnil,
                      doc: /* Template for displaying mode line for current buffer.
-Each buffer has its own value of this variable.
-Value may be nil, a string, a symbol or a list or cons cell.
+
+The value may be nil, a string, a symbol or a list.
+
 A value of nil means don't display a mode line.
-For a symbol, its value is used (but it is ignored if t or nil).
- A string appearing directly as the value of a symbol is processed verbatim
- in that the %-constructs below are not recognized.
- Note that unless the symbol is marked as a `risky-local-variable', all
- properties in any strings, as well as all :eval and :propertize forms
- in the value of that symbol will be ignored.
-For a list of the form `(:eval FORM)', FORM is evaluated and the result
- is used as a mode line element.  Be careful--FORM should not load any files,
- because that can cause an infinite recursion.
-For a list of the form `(:propertize ELT PROPS...)', ELT is displayed
- with the specified properties PROPS applied.
-For a list whose car is a symbol, the symbol's value is taken,
- and if that is non-nil, the cadr of the list is processed recursively.
- Otherwise, the caddr of the list (if there is one) is processed.
-For a list whose car is a string or list, each element is processed
- recursively and the results are effectively concatenated.
-For a list whose car is an integer, the cdr of the list is processed
-  and padded (if the number is positive) or truncated (if negative)
-  to the width specified by that number.
+
+For any symbol other than t or nil, the symbol's value is processed as
+ a mode line construct.  As a special exception, if that value is a
+ string, the string is processed verbatim, without handling any
+ %-constructs (see below).  Also, unless the symbol has a non-nil
+ `risky-local-variable' property, all properties in any strings, as
+ well as all :eval and :propertize forms in the value, are ignored.
+
+A list whose car is a string or list is processed by processing each
+ of the list elements recursively, as separate mode line constructs,
+ and concatenating the results.
+
+A list of the form `(:eval FORM)' is processed by evaluating FORM and
+ using the result as a mode line construct.  Be careful--FORM should
+ not load any files, because that can cause an infinite recursion.
+
+A list of the form `(:propertize ELT PROPS...)' is processed by
+ processing ELT as the mode line construct, and adding the text
+ properties PROPS to the result.
+
+A list whose car is a symbol is processed by examining the symbol's
+ value, and, if that value is non-nil, processing the cadr of the list
+ recursively; and if that value is nil, processing the caddr of the
+ list recursively.
+
+A list whose car is an integer is processed by processing the cadr of
+ the list, and padding (if the number is positive) or truncating (if
+ negative) to the width specified by that number.
+
 A string is printed verbatim in the mode line except for %-constructs:
-  (%-constructs are allowed when the string is the entire mode-line-format
-   or when it is found in a cons-cell or a list)
   %b -- print buffer name.      %f -- print visited file name.
   %F -- print frame name.
   %* -- print %, * or hyphen.   %+ -- print *, % or hyphen.
diff --git a/src/buffer.h b/src/buffer.h
index 97d891f044b..3aa4b11c450 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -912,6 +912,7 @@ extern void validate_region (Lisp_Object *, Lisp_Object *);
 extern void set_buffer_internal (struct buffer *);
 extern void set_buffer_internal_1 (struct buffer *);
 extern void set_buffer_temp (struct buffer *);
+extern Lisp_Object buffer_local_value_1 (Lisp_Object, Lisp_Object);
 extern void record_buffer (Lisp_Object);
 extern void buffer_slot_type_mismatch (Lisp_Object, int) NO_RETURN;
 extern void fix_overlays_before (struct buffer *, ptrdiff_t, ptrdiff_t);
diff --git a/src/xdisp.c b/src/xdisp.c
index 0763fc19c73..6eb7f50c6fc 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -751,6 +751,7 @@ static struct glyph_slice null_glyph_slice = { 0, 0, 0, 0 };
 int redisplaying_p;
 
 static Lisp_Object Qinhibit_free_realized_faces;
+static Lisp_Object Qmode_line_default_help_echo;
 
 /* If a string, XTread_socket generates an event to display that string.
    (The display is done in read_char.)  */
@@ -22091,7 +22092,9 @@ calc_pixel_width_or_height (double *res, struct it *it, Lisp_Object prop,
             return OK_PIXELS (WINDOW_SCROLL_BAR_AREA_WIDTH (it->w));
         }
 
-      prop = Fbuffer_local_value (prop, it->w->buffer);
+      prop = buffer_local_value_1 (prop, it->w->buffer);
+      if (EQ (prop, Qunbound))
+        prop = Qnil;
     }
 
   if (INTEGERP (prop) || FLOATP (prop))
@@ -22141,7 +22144,9 @@ calc_pixel_width_or_height (double *res, struct it *it, Lisp_Object prop,
               return OK_PIXELS (pixels);
             }
 
-          car = Fbuffer_local_value (car, it->w->buffer);
+          car = buffer_local_value_1 (car, it->w->buffer);
+          if (EQ (car, Qunbound))
+            car = Qnil;
         }
 
       if (INTEGERP (car) || FLOATP (car))
@@ -27035,7 +27040,6 @@ note_mode_line_or_margin_highlight (Lisp_Object window, int x, int y,
               if (!NILP (help))
                 {
                   help_echo_string = help;
-                  /* Is this correct?  ++kfs */
                   XSETWINDOW (help_echo_window, w);
                   help_echo_object = w->buffer;
                   help_echo_pos = charpos;
@@ -27048,46 +27052,77 @@ note_mode_line_or_margin_highlight (Lisp_Object window, int x, int y,
 #endif  /* HAVE_WINDOW_SYSTEM */
 
   if (STRINGP (string))
+    pos = make_number (charpos);
+
+  /* Set the help text and mouse pointer.  If the mouse is on a part
+     of the mode line without any text (e.g. past the right edge of
+     the mode line text), use the default help text and pointer.  */
+  if (STRINGP (string) || area == ON_MODE_LINE)
     {
-      pos = make_number (charpos);
-      /* If we're on a string with `help-echo' text property, arrange
-         for the help to be displayed.  This is done by setting the
-         global variable help_echo_string to the help string.  */
+      /* Arrange to display the help by setting the global variables
+         help_echo_string, help_echo_object, and help_echo_pos.  */
       if (NILP (help))
         {
-          help = Fget_text_property (pos, Qhelp_echo, string);
-          if (!NILP (help))
+          if (STRINGP (string))
+            help = Fget_text_property (pos, Qhelp_echo, string);
+
+          if (STRINGP (help))
             {
               help_echo_string = help;
               XSETWINDOW (help_echo_window, w);
               help_echo_object = string;
               help_echo_pos = charpos;
             }
+          else if (area == ON_MODE_LINE)
+            {
+              Lisp_Object default_help
+                = buffer_local_value_1 (Qmode_line_default_help_echo,
+                                        w->buffer);
+
+              if (STRINGP (default_help))
+                {
+                  help_echo_string = default_help;
+                  XSETWINDOW (help_echo_window, w);
+                  help_echo_object = Qnil;
+                  help_echo_pos = -1;
+                }
+            }
         }
 
 #ifdef HAVE_WINDOW_SYSTEM
+      /* Change the mouse pointer according to what is under it.  */
       if (FRAME_WINDOW_P (f))
         {
           dpyinfo = FRAME_X_DISPLAY_INFO (f);
-          cursor  = FRAME_X_OUTPUT (f)->nontext_cursor;
-          if (NILP (pointer))
-            pointer = Fget_text_property (pos, Qpointer, string);
-
-          /* Change the mouse pointer according to what is under X/Y.  */
-          if (NILP (pointer)
-              && ((area == ON_MODE_LINE) || (area == ON_HEADER_LINE)))
+          if (STRINGP (string))
             {
-              Lisp_Object map;
-              map = Fget_text_property (pos, Qlocal_map, string);
-              if (!KEYMAPP (map))
-                map = Fget_text_property (pos, Qkeymap, string);
-              if (!KEYMAPP (map))
-                cursor = dpyinfo->vertical_scroll_bar_cursor;
+              cursor = FRAME_X_OUTPUT (f)->nontext_cursor;
+
+              if (NILP (pointer))
+                pointer = Fget_text_property (pos, Qpointer, string);
+
+              /* Change the mouse pointer according to what is under X/Y.  */
+              if (NILP (pointer)
+                  && ((area == ON_MODE_LINE) || (area == ON_HEADER_LINE)))
+                {
+                  Lisp_Object map;
+                  map = Fget_text_property (pos, Qlocal_map, string);
+                  if (!KEYMAPP (map))
+                    map = Fget_text_property (pos, Qkeymap, string);
+                  if (!KEYMAPP (map))
+                    cursor = dpyinfo->vertical_scroll_bar_cursor;
+                }
             }
+          else
+            /* Default mode-line pointer.  */
+            cursor = FRAME_X_DISPLAY_INFO (f)->vertical_scroll_bar_cursor;
         }
 #endif
+    }
 
-     /* Change the mouse face according to what is under X/Y.  */
+  /* Change the mouse face according to what is under X/Y.  */
+  if (STRINGP (string))
+    {
       mouse_face = Fget_text_property (pos, Qmouse_face, string);
       if (!NILP (mouse_face)
           && ((area == ON_MODE_LINE) || (area == ON_HEADER_LINE))
@@ -28428,6 +28463,8 @@ syms_of_xdisp (void)
   Vmode_line_unwind_vector = Qnil;
   staticpro (&Vmode_line_unwind_vector);
 
+  DEFSYM (Qmode_line_default_help_echo, "mode-line-default-help-echo");
+
   help_echo_string = Qnil;
   staticpro (&help_echo_string);
   help_echo_object = Qnil;