(Major Mode Conventions, Mode Line Variables): `mode-name' need not be a

string.  xref to "Mode Line Data" for details, and to "Emulating Mode Line"
for computing a string value.

doc/lispref/ChangeLog

+2008-10-15  Eli Zaretskii  <eliz@gnu.org>
+
+	* modes.texi (Major Mode Conventions, Mode Line Variables):
+	`mode-name' need not be a string.  xref to "Mode Line Data" for
+	details, and to "Emulating Mode Line" for computing a string
+	value.
+
 2008-10-14  Eli Zaretskii  <eliz@gnu.org>
 
 	* processes.texi (System Processes): New section.

doc/lispref/modes.texi

@@ -317,8 +317,9 @@ which documentation to print.
 
 @item
 The major mode command should set the variable @code{mode-name} to the
-``pretty'' name of the mode, as a string.  This string appears in the
-mode line.
+``pretty'' name of the mode, usually a string (but see @ref{Mode Line
+Data}, for other possible forms).  The name of the mode appears
+in the mode line.
 
 @item
 @cindex functions in modes
@@ -1865,8 +1866,12 @@ effect.
 
 @defvar mode-name
 This buffer-local variable holds the ``pretty'' name of the current
-buffer's major mode.  Each major mode should set this variable so that the
-mode name will appear in the mode line.
+buffer's major mode.  Each major mode should set this variable so that
+the mode name will appear in the mode line.  The value does not have
+to be a string, but can use any of the data types valid in a mode-line
+construct (@pxref{Mode Line Data}).  To compute the string that will
+identify the mode name in the mode line, use @code{format-mode-line}
+(@pxref{Emulating Mode Line}).
 @end defvar
 
 @defvar mode-line-process

etc/NEWS

@@ -1068,6 +1068,7 @@ checking/manipulating elements directly, use the new functions
 `byte-compile-warning-enabled-p', `byte-compile-disable-warning', and
 `byte-compile-enable-warning.'
 
++++
 ** `mode-name' is no longer guaranteed to be a string.
 Use `(format-mode-line mode-name)' to ensure a string value.