c, objc: Add -Wunterminated-string-initialization

Warn about the following:

    char  s[3] = "foo";

Initializing a char array with a string literal of the same length as
the size of the array is usually a mistake.  Rarely is the case where
one wants to create a non-terminated character sequence from a string
literal.

In some cases, for writing faster code, one may want to use arrays
instead of pointers, since that removes the need for storing an array of
pointers apart from the strings themselves.

    char  *log_levels[]   = { "info", "warning", "err" };
vs.
    char  log_levels[][7] = { "info", "warning", "err" };

This forces the programmer to specify a size, which might change if a
new entry is later added.  Having no way to enforce null termination is
very dangerous, however, so it is useful to have a warning for this, so
that the compiler can make sure that the programmer didn't make any
mistakes.  This warning catches the bug above, so that the programmer
will be able to fix it and write:

    char  log_levels[][8] = { "info", "warning", "err" };

This warning already existed as part of -Wc++-compat, but this patch
allows enabling it separately.  It is also included in -Wextra, since
it may not always be desired (when unterminated character sequences are
wanted), but it's likely to be desired in most cases.

Since Wc++-compat now includes this warning, the test has to be modified
to expect the text of the new warning too, in <gcc.dg/Wcxx-compat-14.c>.

Link: https://lists.gnu.org/archive/html/groff/2022-11/msg00059.html
Link: https://lists.gnu.org/archive/html/groff/2022-11/msg00063.html
Link: https://inbox.sourceware.org/gcc/36da94eb-1cac-5ae8-7fea-ec66160cf413@gmail.com/T/

	PR c/115185

gcc/c-family/ChangeLog:

	* c.opt: Add -Wunterminated-string-initialization.

gcc/c/ChangeLog:

	* c-typeck.cc (digest_init): Separate warnings about character
	arrays being initialized as unterminated character sequences
	with string literals, from -Wc++-compat, into a new warning,
	-Wunterminated-string-initialization.

gcc/ChangeLog:

	* doc/invoke.texi: Document the new
	-Wunterminated-string-initialization.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wcxx-compat-14.c: Adapt the test to match the new text
	of the warning, which doesn't say anything about C++ anymore.
	* gcc.dg/Wunterminated-string-initialization.c: New test.

Acked-by: Doug McIlroy <douglas.mcilroy@dartmouth.edu>
Acked-by: Mike Stump <mikestump@comcast.net>
Reviewed-by: Sandra Loosemore <sloosemore@baylibre.com>
Reviewed-by: Martin Uecker <uecker@tugraz.at>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Marek Polacek <polacek@redhat.com>

gcc/c-family/c.opt

@@ -1464,6 +1464,10 @@ Wunsuffixed-float-constants
 C ObjC Var(warn_unsuffixed_float_constants) Warning
 Warn about unsuffixed float constants.
 
+Wunterminated-string-initialization
+C ObjC Var(warn_unterminated_string_initialization) Warning LangEnabledBy(C ObjC,Wextra || Wc++-compat)
+Warn about character arrays initialized as unterminated character sequences with a string literal.
+
 Wunused
 C ObjC C++ ObjC++ LangEnabledBy(C ObjC C++ ObjC++,Wall)
 ; documented in common.opt

gcc/c/c-typeck.cc

@@ -8878,11 +8878,11 @@ digest_init (location_t init_loc, tree type, tree init, tree origtype,
 		pedwarn_init (init_loc, 0,
 			      ("initializer-string for array of %qT "
 			       "is too long"), typ1);
-	      else if (warn_cxx_compat
+	      else if (warn_unterminated_string_initialization
 		       && compare_tree_int (TYPE_SIZE_UNIT (type), len) < 0)
-		warning_at (init_loc, OPT_Wc___compat,
+		warning_at (init_loc, OPT_Wunterminated_string_initialization,
 			    ("initializer-string for array of %qT "
-			     "is too long for C++"), typ1);
+			     "is too long"), typ1);
 	      if (compare_tree_int (TYPE_SIZE_UNIT (type), len) < 0)
 		{
 		  unsigned HOST_WIDE_INT size

gcc/doc/invoke.texi

@@ -415,7 +415,9 @@ Objective-C and Objective-C++ Dialects}.
 -Wsystem-headers  -Wtautological-compare  -Wtrampolines  -Wtrigraphs
 -Wtrivial-auto-var-init  -Wno-tsan  -Wtype-limits  -Wundef
 -Wuninitialized  -Wunknown-pragmas
--Wunsuffixed-float-constants  -Wunused
+-Wunsuffixed-float-constants
+-Wunterminated-string-initialization
+-Wunused
 -Wunused-but-set-parameter  -Wunused-but-set-variable
 -Wunused-const-variable  -Wunused-const-variable=@var{n}
 -Wunused-function  -Wunused-label  -Wunused-local-typedefs
@@ -6496,6 +6498,7 @@ name is still supported, but the newer name is more descriptive.)
 -Wstring-compare
 -Wtype-limits
 -Wuninitialized
+-Wunterminated-string-initialization
 -Wunused-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)}
 -Wunused-but-set-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)}}
 
@@ -8664,6 +8667,21 @@ arithmetic that may yield out of bounds values. This warning level may
 give a larger number of false positives and is deactivated by default.
 @end table
 
+@opindex Wunterminated-string-initialization
+@opindex Wno-unterminated-string-initialization
+@item -Wunterminated-string-initialization @r{(C and Objective-C only)}
+Warn about character arrays
+initialized as unterminated character sequences
+with a string literal.
+For example:
+
+@smallexample
+char arr[3] = "foo";
+@end smallexample
+
+This warning is enabled by @option{-Wextra} and @option{-Wc++-compat}.
+In C++, such initializations are an error.
+
 @opindex Warray-compare
 @opindex Wno-array-compare
 @item -Warray-compare

gcc/testsuite/gcc.dg/Wcxx-compat-14.c

@@ -2,5 +2,5 @@
 /* { dg-options "-Wc++-compat" } */
 
 char a1[] = "a";
-char a2[1] = "a";	/* { dg-warning "C\[+\]\[+\]" } */
+char a2[1] = "a";	/* { dg-warning "initializer-string for array of 'char' is too long" } */
 char a3[2] = "a";

gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c

@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-Wunterminated-string-initialization" } */
+
+char a1[] = "a";
+char a2[1] = "a";	/* { dg-warning "initializer-string for array of 'char' is too long" } */
+char a3[2] = "a";