DeadCode: Add MISRA C 2012 Rule 2.4

Add a query for finding unused tags. This query uses `TypeMention`s to
identify tags that are never referenced. Note: this query tries to
exclude tags generated from macros, but this is not currently working due
to a bug in the CodeQL CLI 2.9.4.

Author: lcartey

c/misra/src/rules/RULE-2-4/UnusedTagDeclaration.ql

@@ -0,0 +1,33 @@
+/**
+ * @id c/misra/unused-tag-declaration
+ * @name RULE-2-4: A project should not contain unused tag declarations
+ * @description Unused tag declarations are either redundant or indicate a possible mistake on the
+ *              part of the programmer.
+ * @kind problem
+ * @precision very-high
+ * @problem.severity warning
+ * @tags external/misra/id/rule-2-4
+ *       readability
+ *       maintainability
+ *       external/misra/obligation/advisory
+ */
+
+import cpp
+import codingstandards.c.misra
+import codingstandards.cpp.TypeUses
+
+from UserType s
+where
+  not isExcluded(s, DeadCodePackage::unusedTagDeclarationQuery()) and
+  // ignore structs without a tag name
+  not s.getName() = "struct <unnamed>" and
+  // typedefs do not have a "tag" name, so this rule does not apply to them
+  not s instanceof TypedefType and
+  // Not mentioned anywhere
+  not exists(TypeMention tm | tm.getMentionedType() = s) and
+  // Exclude any struct that is fully generated from a macro expansion, as it may be used in other
+  // expansions of the same macro.
+  // Note: due to a bug in the CodeQL CLI version 2.9.4, this will currently have no effect, because
+  // `isInMacroExpansion` is broken for `UserType`s.
+  not s.isInMacroExpansion()
+select s, "struct " + s.getName() + " has an unused tag."

c/misra/test/rules/RULE-2-4/UnusedTagDeclaration.expected

@@ -0,0 +1,7 @@
+| test.c:4:8:4:9 | S2 | struct S2 has an unused tag. |
+| test.c:7:16:7:17 | S3 | struct S3 has an unused tag. |
+| test.c:17:6:17:7 | E1 | struct E1 has an unused tag. |
+| test.c:31:10:31:11 | S7 | struct S7 has an unused tag. |
+| test.c:50:8:50:10 | S10 | struct S10 has an unused tag. |
+| test.c:66:3:66:14 | S13 | struct S13 has an unused tag. |
+| test.c:79:8:79:10 | s14 | struct s14 has an unused tag. |

c/misra/test/rules/RULE-2-4/UnusedTagDeclaration.qlref

@@ -0,0 +1 @@
+rules/RULE-2-4/UnusedTagDeclaration.ql

c/misra/test/rules/RULE-2-4/test.c

@@ -0,0 +1,80 @@
+struct S1 { // COMPLIANT
+  int x;
+};
+struct S2 { // NON_COMPLIANT
+  int x;
+};
+typedef struct S3 { // NON_COMPLIANT
+  int x;
+} T1;
+typedef struct S4 { // COMPLIANT
+  int x;
+} T2;
+struct S5 { // COMPLIANT
+  int x;
+};
+typedef struct S5 T3;
+enum E1 { state1, state2 }; // NON_COMPLIANT
+enum E2 { state3, state4 }; // COMPLIANT
+struct {                    // COMPLIANT - no tag
+  int x;
+} s6;
+
+void test() {
+  struct S1 s1;
+  T1 t1;
+  t1.x = 0; // Field access on struct S3
+  T2 t2;
+  struct S4 s4;
+  int x = state1; // enum access on E1
+  enum E2 e2;
+  struct S7 { // NON_COMPLIANT
+    int x
+  } s7;
+  struct S8 { // COMPLIANT
+    int x
+  } s8;
+  struct S8 s8_2;
+
+  struct S11 { // COMPLIANT
+    int x;
+  } foo(struct S11 s);
+}
+
+struct S9 { // COMPLIANT
+  int x;
+} test_2() {
+  return (struct S9){0};
+}
+
+struct S10 { // NON_COMPLIANT
+  int x;
+} * test_3() {
+  return 0;
+}
+
+struct S12 { // COMPLIANT
+  int x;
+} foo(struct S12 s);
+
+#define STRUCT_MACRO                                                           \
+  struct S13 {                                                                 \
+    int x;                                                                     \
+  };
+
+void testMacroNameUsed() {
+  STRUCT_MACRO // COMPLIANT[FALSE_POSITIVE] - although the struct generated by
+               // the macro is never used in this expansion, it may be used in
+               // other expansions, so we don't want to report it as unused
+}
+
+void testMacroNameNotUsed() {
+  STRUCT_MACRO // COMPLIANT - S13 is used in this expansion
+      struct S13 s13_2;
+}
+
+#define PARTIAL                                                                \
+  { int x; }
+
+struct s14 PARTIAL; // NON_COMPLIANT - affected by macro, but not fully
+                    // generated, so fair to report as unused

c/misra/test/rules/RULE-2-4/test.ql

@@ -0,0 +1,6 @@
+import cpp
+
+// from UserType ut
+// where ut.isAffectedByMacro()
+// select ut
+select any(Location l).toString() as s, count(Location l | l.toString() = s)