c: add -Wmissing-variable-declarations [PR65213]

Resolves:
PR c/65213 - Extend -Wmissing-declarations to variables [i.e. add
-Wmissing-variable-declarations]

gcc/c-family/ChangeLog:

	PR c/65213
	* c.opt (-Wmissing-variable-declarations): New option.

gcc/c/ChangeLog:

	PR c/65213
	* c-decl.cc (start_decl): Handle
	-Wmissing-variable-declarations.

gcc/ChangeLog:

	PR c/65213
	* doc/invoke.texi (-Wmissing-variable-declarations): Document
	new option.

gcc/testsuite/ChangeLog:

	PR c/65213
	* gcc.dg/Wmissing-variable-declarations.c: New test.

Signed-off-by: Hamza Mahfooz <someguy@effective-light.com>

4 files changed, 55 insertions, 3 deletions

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 4abdc8d..0ed87fc 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1048,6 +1048,10 @@ Wmissing-prototypes
 C ObjC Var(warn_missing_prototypes) Warning
 Warn about global functions without prototypes.
 
+Wmissing-variable-declarations
+C ObjC Var(warn_missing_variable_declarations) Warning
+Warn about global variables without previous declarations.
+
 Wmudflap
 C ObjC C++ ObjC++ WarnRemoved
 
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index ecd10eb..1f9eb44 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -5340,6 +5340,7 @@ start_decl (struct c_declarator *declarator, struct c_declspecs *declspecs,
 	    location_t *lastloc /* = NULL */)
 {
   tree decl;
+  tree old_decl;
   tree tem;
   tree expr = NULL_TREE;
   enum deprecated_states deprecated_state = DEPRECATED_NORMAL;
@@ -5360,7 +5361,9 @@ start_decl (struct c_declarator *declarator, struct c_declspecs *declspecs,
   if (!decl || decl == error_mark_node)
     return NULL_TREE;
 
-  if (tree lastdecl = lastloc ? lookup_last_decl (decl) : NULL_TREE)
+  old_decl = lookup_last_decl (decl);
+
+  if (tree lastdecl = lastloc ? old_decl : NULL_TREE)
     if (lastdecl != error_mark_node)
       *lastloc = DECL_SOURCE_LOCATION (lastdecl);
 
@@ -5372,6 +5375,11 @@ start_decl (struct c_declarator *declarator, struct c_declspecs *declspecs,
       && TREE_PUBLIC (decl))
     warning (OPT_Wmain, "%q+D is usually a function", decl);
 
+  if (warn_missing_variable_declarations && VAR_P (decl)
+      && !DECL_EXTERNAL (decl) && TREE_PUBLIC (decl) && old_decl == NULL_TREE)
+    warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wmissing_variable_declarations,
+		"no previous declaration for %qD", decl);
+
   if (initialized)
     /* Is it valid for this decl to have an initializer at all?
        If not, set INITIALIZED to zero, which will indirectly
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c43260f..104766f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -498,8 +498,8 @@ Objective-C and Objective-C++ Dialects}.
 
 @item C and Objective-C-only Warning Options
 @gccoptlist{-Wbad-function-cast  -Wmissing-declarations
--Wmissing-parameter-type  -Wmissing-prototypes  -Wnested-externs
--Wold-style-declaration  -Wold-style-definition
+-Wmissing-parameter-type -Wmissing-prototypes -Wmissing-variable-declarations
+-Wnested-externs -Wold-style-declaration  -Wold-style-definition
 -Wstrict-prototypes  -Wtraditional  -Wtraditional-conversion
 -Wdeclaration-after-statement  -Wpointer-sign}
 
@@ -9611,6 +9611,13 @@ provide prototypes and a non-matching declaration declares an
 overload rather than conflict with an earlier declaration.
 Use @option{-Wmissing-declarations} to detect missing declarations in C++.
 
+@opindex Wmissing-variable-declarations
+@opindex Wno-missing-variable-declarations
+@item -Wmissing-variable-declarations @r{(C and Objective-C only)}
+Warn if a global variable is defined without a previous declaration.
+Use this option to detect global variables that do not have a matching
+extern declaration in a header file.
+
 @opindex Wmissing-declarations
 @opindex Wno-missing-declarations
 @item -Wmissing-declarations
diff --git a/gcc/testsuite/gcc.dg/Wmissing-variable-declarations.c b/gcc/testsuite/gcc.dg/Wmissing-variable-declarations.c
new file mode 100644
index 0000000..b292dbe
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wmissing-variable-declarations.c
@@ -0,0 +1,33 @@
+/* { dg-do compile } */
+/* { dg-options "-Wmissing-variable-declarations" } */
+
+int b0; /* { dg-warning "no previous declaration for 'b0'" } */
+
+int b1 = 1; /* { dg-warning "no previous declaration for 'b1'" } */
+
+int b2; /* { dg-warning "no previous declaration for 'b2'" } */
+int b2 = 2; 
+
+struct {
+    int g0;
+} b3; /* { dg-warning "no previous declaration for 'b3'" } */
+
+int b4; /* { dg-warning "no previous declaration for 'b4'" } */
+int b4 = 3;
+extern int b4;
+
+static int g1;
+
+void g2(void);
+
+extern int g3;
+int g3;
+int g3 = 4;
+
+struct g4 {
+    int g5;
+};
+
+void g6(void) {
+    int g7;
+}