c: C2x attributes fixes and updates

Implement some changes to the currently supported C2x standard
attributes that have been made to the specification since they were
first implemented in GCC, and some consequent changes:

* maybe_unused is now supported on labels.  In fact that was already
  accidentally supported in GCC as a result of sharing the
  implementation with __attribute__ ((unused)), but needed to be
  covered in the tests.

* As part of the support for maybe_unused on labels, its
  __has_c_attribute value changed.

* The issue of maybe_unused accidentally being already supported on
  labels showed up the lack of tests for other standard attributes
  being incorrectly applied to labels; add such tests.

* Use of fallthrough or nodiscard attributes on labels already
  properly resulted in a pedwarn.  For the deprecated attribute,
  however, there was only a warning, and the wording "'deprecated'
  attribute ignored for 'void'" included an unhelpful "for 'void'".
  Arrange for the case of the deprecated attribute on a label to be
  checked for separately and result in a pedwarn.  As with
  inappropriate uses of fallthrough (see commit
  6c80b1b56dec2691436f3e2676e3d1b105b01b89), it seems reasonable for
  this pedwarn to apply regardless of whether [[]] or __attribute__
  was used and regardless of whether C or C++ is being compiled.

* Attributes on case or default labels (the standard syntax supports
  attributes on all kinds of labels) were quietly ignored, whether or
  not appropriate for use in such a context, because they weren't
  passed to decl_attributes at all.  (Note where I'm changing the
  do_case prototype that such a function is actually only defined in
  the C front end, not for C++, despite the declaration being in
  c-common.h.)

* A recent change as part of the editorial review in preparation for
  the C2x CD ballot has changed the __has_c_attribute value for
  fallthrough to 201910 to reflect when that attribute was actually
  voted into the working draft.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c-family/
	* c-attribs.cc (handle_deprecated_attribute): Check and pedwarn
	for LABEL_DECL.
	* c-common.cc (c_add_case_label): Add argument ATTRS.  Call
	decl_attributes.
	* c-common.h (do_case, c_add_case_label): Update declarations.
	* c-lex.cc (c_common_has_attribute): For C, produce a result of
	201910 for fallthrough and 202106 for maybe_unused.

gcc/c/
	* c-parser.cc (c_parser_label): Pass attributes to do_case.
	* c-typeck.cc (do_case): Add argument ATTRS.  Pass it to
	c_add_case_label.

gcc/testsuite/
	* gcc.dg/c2x-attr-deprecated-2.c, gcc.dg/c2x-attr-fallthrough-2.c,
	gcc.dg/c2x-attr-maybe_unused-1.c, gcc.dg/c2x-attr-nodiscard-2.c:
	Add tests of attributes on labels.
	* gcc.dg/c2x-has-c-attribute-2.c: Update expected results for
	maybe_unused and fallthrough.

gcc/c-family/c-attribs.cc

@@ -4163,6 +4163,13 @@ handle_deprecated_attribute (tree *node, tree name,
 	  || TREE_CODE (decl) == CONST_DECL
 	  || objc_method_decl (TREE_CODE (decl)))
 	TREE_DEPRECATED (decl) = 1;
+      else if (TREE_CODE (decl) == LABEL_DECL)
+	{
+	  pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
+		   name);
+	  *no_add_attrs = true;
+	  return NULL_TREE;
+	}
       else
 	warn = 1;
     }

gcc/c-family/c-common.cc

@@ -5068,11 +5068,12 @@ case_compare (splay_tree_key k1, splay_tree_key k2)
    CASES is a tree containing all the case ranges processed so far;
    COND is the condition for the switch-statement itself.
    Returns the CASE_LABEL_EXPR created, or ERROR_MARK_NODE if no
-   CASE_LABEL_EXPR is created.  */
+   CASE_LABEL_EXPR is created.  ATTRS are the attributes to be applied
+   to the label.  */
 
 tree
 c_add_case_label (location_t loc, splay_tree cases, tree cond,
-		  tree low_value, tree high_value)
+		  tree low_value, tree high_value, tree attrs)
 {
   tree type;
   tree label;
@@ -5081,6 +5082,7 @@ c_add_case_label (location_t loc, splay_tree cases, tree cond,
 
   /* Create the LABEL_DECL itself.  */
   label = create_artificial_label (loc);
+  decl_attributes (&label, attrs, 0);
 
   /* If there was an error processing the switch condition, bail now
      before we get more confused.  */

gcc/c-family/c-common.h

@@ -1018,7 +1018,7 @@ extern void c_parse_final_cleanups (void);
 /* True iff TYPE is cv decltype(nullptr).  */
 #define NULLPTR_TYPE_P(TYPE) (TREE_CODE (TYPE) == NULLPTR_TYPE)
 
-extern tree do_case (location_t, tree, tree);
+extern tree do_case (location_t, tree, tree, tree);
 extern tree build_stmt (location_t, enum tree_code, ...);
 extern tree build_real_imag_expr (location_t, enum tree_code, tree);
 
@@ -1046,7 +1046,8 @@ extern tree boolean_increment (enum tree_code, tree);
 
 extern int case_compare (splay_tree_key, splay_tree_key);
 
-extern tree c_add_case_label (location_t, splay_tree, tree, tree, tree);
+extern tree c_add_case_label (location_t, splay_tree, tree, tree, tree,
+			      tree = NULL_TREE);
 extern bool c_switch_covers_all_cases_p (splay_tree, tree);
 extern bool c_block_may_fallthru (const_tree);
 

gcc/c-family/c-lex.cc

@@ -381,12 +381,14 @@ c_common_has_attribute (cpp_reader *pfile, bool std_syntax)
 	    }
 	  else
 	    {
-	      if (is_attribute_p ("deprecated", attr_name)
-		  || is_attribute_p ("maybe_unused", attr_name)
-		  || is_attribute_p ("fallthrough", attr_name))
+	      if (is_attribute_p ("deprecated", attr_name))
 		result = 201904;
+	      else if (is_attribute_p ("fallthrough", attr_name))
+		result = 201910;
 	      else if (is_attribute_p ("nodiscard", attr_name))
 		result = 202003;
+	      else if (is_attribute_p ("maybe_unused", attr_name))
+		result = 202106;
 	    }
 	  if (result)
 	    attr_name = NULL_TREE;

gcc/c/c-parser.cc

@@ -5912,14 +5912,14 @@ c_parser_label (c_parser *parser, tree std_attrs)
       if (c_parser_next_token_is (parser, CPP_COLON))
 	{
 	  c_parser_consume_token (parser);
-	  label = do_case (loc1, exp1, NULL_TREE);
+	  label = do_case (loc1, exp1, NULL_TREE, std_attrs);
 	}
       else if (c_parser_next_token_is (parser, CPP_ELLIPSIS))
 	{
 	  c_parser_consume_token (parser);
 	  exp2 = c_parser_expr_no_commas (parser, NULL).value;
 	  if (c_parser_require (parser, CPP_COLON, "expected %<:%>"))
-	    label = do_case (loc1, exp1, exp2);
+	    label = do_case (loc1, exp1, exp2, std_attrs);
 	}
       else
 	c_parser_error (parser, "expected %<:%> or %<...%>");
@@ -5928,7 +5928,7 @@ c_parser_label (c_parser *parser, tree std_attrs)
     {
       c_parser_consume_token (parser);
       if (c_parser_require (parser, CPP_COLON, "expected %<:%>"))
-	label = do_case (loc1, NULL_TREE, NULL_TREE);
+	label = do_case (loc1, NULL_TREE, NULL_TREE, std_attrs);
     }
   else
     {

gcc/c/c-typeck.cc

@@ -11172,10 +11172,10 @@ c_start_switch (location_t switch_loc,
   return add_stmt (cs->switch_stmt);
 }
 
-/* Process a case label at location LOC.  */
+/* Process a case label at location LOC, with attributes ATTRS.  */
 
 tree
-do_case (location_t loc, tree low_value, tree high_value)
+do_case (location_t loc, tree low_value, tree high_value, tree attrs)
 {
   tree label = NULL_TREE;
 
@@ -11211,7 +11211,7 @@ do_case (location_t loc, tree low_value, tree high_value)
 
   label = c_add_case_label (loc, c_switch_stack->cases,
 			    SWITCH_STMT_COND (c_switch_stack->switch_stmt),
-			    low_value, high_value);
+			    low_value, high_value, attrs);
   if (label == error_mark_node)
     label = NULL_TREE;
   return label;

gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c

@@ -3,7 +3,8 @@
 /* { dg-options "-std=c2x -pedantic-errors" } */
 
 /* This attribute is not valid in most cases on types other than their
-   definitions, or on statements, or as an attribute-declaration.  */
+   definitions, or on labels, or on statements, or as an
+   attribute-declaration.  */
 
 [[deprecated]]; /* { dg-error "ignored" } */
 
@@ -21,4 +22,10 @@ f (void)
   int a;
   [[deprecated]]; /* { dg-error "ignored" } */
   [[deprecated]] a = 1; /* { dg-error "ignored" } */
+  [[deprecated]] label: ; /* { dg-error "ignored" } */
+  switch (var)
+    {
+    [[deprecated]] case 1: ; /* { dg-error "ignored" } */
+    [[deprecated]] default: ; /* { dg-error "ignored" } */
+    }
 }

gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c

@@ -33,6 +33,10 @@ f (int a)
     case 5:
       b += 5;
       break;
+    [[fallthrough]] case 6: break; /* { dg-error "ignored" } */
+    [[fallthrough]] default: break; /* { dg-error "ignored" } */
     }
   [[fallthrough]] return b; /* { dg-error "ignored" } */
+ [[fallthrough]] label: ; /* { dg-error "ignored" } */
+  goto label;
 }

gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c

@@ -14,7 +14,9 @@ g ([[maybe_unused]] int x, int y)
   [[maybe_unused]] int a;
   int b [[__maybe_unused__]];
   int c [[maybe_unused]];
+ [[__maybe_unused__]] label1:
   c = y;
+ [[maybe_unused]] label2:
   return y;
 }
 
@@ -29,3 +31,14 @@ union [[maybe_unused]] u { int x; };
 enum [[maybe_unused]] eu { E2 };
 
 union u2 { [[maybe_unused]] int a; int b [[maybe_unused]]; } y;
+
+void
+g2 (int x)
+{
+  switch (x)
+    {
+      [[maybe_unused]] case 1: ;
+      [[__maybe_unused__]] case 2: ;
+      [[maybe_unused]] default: ;
+    }
+}

gcc/testsuite/gcc.dg/c2x-attr-nodiscard-2.c

@@ -39,4 +39,10 @@ f (void)
   [[nodiscard ("reason")]] int b = 1; /* { dg-error "can only be applied" } */
   [[nodiscard]]; /* { dg-error "ignored" } */
   [[nodiscard]] a = 1; /* { dg-error "ignored" } */
+  [[nodiscard]] label: ; /* { dg-error "can only be applied" } */
+  switch (var)
+    {
+      [[nodiscard]] case 1: ; /* { dg-error "can only be applied" } */
+      [[nodiscard]] default: ; /* { dg-error "can only be applied" } */
+    }
 }

gcc/testsuite/gcc.dg/c2x-has-c-attribute-2.c

@@ -10,11 +10,11 @@
 #error "bad result for __nodiscard__"
 #endif
 
-#if __has_c_attribute(maybe_unused) != 201904L
+#if __has_c_attribute(maybe_unused) != 202106L
 #error "bad result for maybe_unused"
 #endif
 
-#if __has_c_attribute(__maybe_unused__) != 201904L
+#if __has_c_attribute(__maybe_unused__) != 202106L
 #error "bad result for __maybe_unused__"
 #endif
 
@@ -26,11 +26,11 @@
 #error "bad result for __deprecated__"
 #endif
 
-#if __has_c_attribute (fallthrough) != 201904L
+#if __has_c_attribute (fallthrough) != 201910L
 #error "bad result for fallthrough"
 #endif
 
-#if __has_c_attribute (__fallthrough__) != 201904L
+#if __has_c_attribute (__fallthrough__) != 201910L
 #error "bad result for __fallthrough__"
 #endif