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.

1 files changed, 1 insertions, 0 deletions

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);