3 files changed, 12 insertions, 5 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 6e3412c3d05..c6f9eb28e25 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5620,6 +5620,8 @@ file I/O and the behavior of various editing commands.
 
 This variable is buffer-local but you cannot set it directly;
 use the function `set-buffer-multibyte' to change a buffer's representation.
+To prevent any attempts to set it or make it buffer-local, Emacs will
+signal an error in those cases.
 See also Info node `(elisp)Text Representations'.  */);
   make_symbol_constant (intern_c_string ("enable-multibyte-characters"));
 
diff --git a/src/data.c b/src/data.c
index b4f6fd5c652..d54c46d72bf 100644
--- a/src/data.c
+++ b/src/data.c
@@ -3880,12 +3880,14 @@ syms_of_data (void)
   set_symbol_function (Qwholenump, XSYMBOL (Qnatnump)->u.s.function);
 
   DEFVAR_LISP ("most-positive-fixnum", Vmost_positive_fixnum,
-	       doc: /* The largest value that is representable in a Lisp integer.  */);
+	       doc: /* The largest value that is representable in a Lisp integer.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vmost_positive_fixnum = make_number (MOST_POSITIVE_FIXNUM);
   make_symbol_constant (intern_c_string ("most-positive-fixnum"));
 
   DEFVAR_LISP ("most-negative-fixnum", Vmost_negative_fixnum,
-	       doc: /* The smallest value that is representable in a Lisp integer.  */);
+	       doc: /* The smallest value that is representable in a Lisp integer.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vmost_negative_fixnum = make_number (MOST_NEGATIVE_FIXNUM);
   make_symbol_constant (intern_c_string ("most-negative-fixnum"));
 
diff --git a/src/font.c b/src/font.c
index 51625b49fa8..f7cebdce78c 100644
--- a/src/font.c
+++ b/src/font.c
@@ -5421,19 +5421,22 @@ gets the repertory information by an opened font and ENCODING.  */);
 	       doc: /*  Vector of valid font weight values.
 Each element has the form:
     [NUMERIC-VALUE SYMBOLIC-NAME ALIAS-NAME ...]
-NUMERIC-VALUE is an integer, and SYMBOLIC-NAME and ALIAS-NAME are symbols. */);
+NUMERIC-VALUE is an integer, and SYMBOLIC-NAME and ALIAS-NAME are symbols.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_weight_table = BUILD_STYLE_TABLE (weight_table);
   make_symbol_constant (intern_c_string ("font-weight-table"));
 
   DEFVAR_LISP_NOPRO ("font-slant-table", Vfont_slant_table,
 	       doc: /*  Vector of font slant symbols vs the corresponding numeric values.
-See `font-weight-table' for the format of the vector. */);
+See `font-weight-table' for the format of the vector.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_slant_table = BUILD_STYLE_TABLE (slant_table);
   make_symbol_constant (intern_c_string ("font-slant-table"));
 
   DEFVAR_LISP_NOPRO ("font-width-table", Vfont_width_table,
 	       doc: /*  Alist of font width symbols vs the corresponding numeric values.
-See `font-weight-table' for the format of the vector. */);
+See `font-weight-table' for the format of the vector.
+This variable cannot be set; trying to do so will signal an error.  */);
   Vfont_width_table = BUILD_STYLE_TABLE (width_table);
   make_symbol_constant (intern_c_string ("font-width-table"));