QL: Add "implicit this" query

Author: tausbn

ql/src/queries/style/ImplicitThis.ql

@@ -0,0 +1,34 @@
+/**
+ * @name Using implicit `this`
+ * @description Writing member predicate calls with an implicit `this` can be confusing
+ * @kind problem
+ * @problem.severity warning
+ * @precision very-high
+ * @id ql/implicit-this
+ * @tags maintainability
+ */
+
+import ql
+
+MemberCall explicitThisCallInFile(File f) {
+  result.getLocation().getFile() = f and
+  result.getBase() instanceof ThisAccess and
+  // Exclude `this.(Type).whatever(...)`, as some files have that as their only instance of `this`.
+  not result = any(InlineCast c).getBase()
+}
+
+PredicateCall implicitThisCallInFile(File f) {
+  result.getLocation().getFile() = f and
+  exists(result.getTarget().getDeclaringType().getASuperType()) and
+  // Exclude `SomeModule::whatever(...)`
+  not exists(result.getQualifier())
+}
+
+PredicateCall confusingImplicitThisCall(File f) {
+  result = implicitThisCallInFile(f) and
+  exists(explicitThisCallInFile(f))
+}
+
+from PredicateCall c
+where c = confusingImplicitThisCall(_)
+select c, "Use of implicit `this`."