C++: Factor out isFromMacroDefinition predicate

This trick for excluding elements from macro bodies but not macro
arguments looks promising and should probably be used much more. With
this commit, it's now easy to use from any query.

Performance is still good because the new predicate gets appropriately
magiced.

Author: jbj

cpp/ql/src/Likely Bugs/Arithmetic/PointlessSelfComparison.ql

@@ -13,18 +13,13 @@
 
 import cpp
 import PointlessSelfComparison
+import semmle.code.cpp.commons.Exclusions
 
 from ComparisonOperation cmp
 where
   pointlessSelfComparison(cmp) and
   not nanTest(cmp) and
   not overflowTest(cmp) and
   not cmp.isFromTemplateInstantiation(_) and
-  not exists(MacroInvocation mi |
-    // cmp is in mi
-    mi.getAnExpandedElement() = cmp and
-    // and cmp was apparently not passed in as a macro parameter
-    cmp.getLocation().getStartLine() = mi.getLocation().getStartLine() and
-    cmp.getLocation().getStartColumn() = mi.getLocation().getStartColumn()
-  )
+  not isFromMacroDefinition(cmp)
 select cmp, "Self comparison."

cpp/ql/src/semmle/code/cpp/commons/Exclusions.qll

@@ -79,3 +79,26 @@ predicate functionContainsPreprocCode(Function f) {
     pbdStartLine >= fBlockStartLine
   )
 }
+
+/**
+ * Holds if `e` is completely or partially from a macro definition, as opposed
+ * to being passed in as an argument.
+ *
+ * In the following example, the call to `f` is from a macro definition,
+ * while `y`, `+`, `1`, and `;` are not. This assumes that no identifier apart
+ * from `M` refers to a macro.
+ * ```
+ * #define M(x) f(x)
+ * ...
+ *   M(y + 1);
+ * ```
+ */
+predicate isFromMacroDefinition(Element e) {
+  exists(MacroInvocation mi |
+    // e is in mi
+    mi.getAnExpandedElement() = e and
+    // and e was apparently not passed in as a macro parameter
+    e.getLocation().getStartLine() = mi.getLocation().getStartLine() and
+    e.getLocation().getStartColumn() = mi.getLocation().getStartColumn()
+  )
+}

cpp/ql/test/library-tests/exclusions/exclusions.cpp

@@ -0,0 +1,9 @@
+// This is the example from the QLDoc of `isFromMacroDefinition`.
+
+void f(int);
+
+#define M(x) f(x)
+
+void useM(int y) {
+  M(y + 1);
+}

cpp/ql/test/library-tests/exclusions/exclusions.expected

@@ -0,0 +1 @@
+| exclusions.cpp:8:3:8:10 | call to f |

cpp/ql/test/library-tests/exclusions/exclusions.ql

@@ -0,0 +1,5 @@
+import semmle.code.cpp.commons.Exclusions
+
+from Element e
+where isFromMacroDefinition(e)
+select e