Add OmittableExists QL-for-QL query

Author: atorralba

ql/ql/src/queries/style/OmittableExists.ql

@@ -0,0 +1,22 @@
+/**
+ * @name Omittable 'exists' argument
+ * @description Writing 'exists(x | pred(x))' is bad practice and can be omitted.
+ * @kind problem
+ * @problem.severity warning
+ * @precision high
+ * @id ql/omittable-exists
+ * @tags maintainability
+ */
+
+import ql
+
+from VarDecl existsArgument, VarAccess use
+where
+  existsArgument = any(Exists e).getAnArgument() and
+  strictcount(existsArgument.getAnAccess()) = 1 and
+  use = existsArgument.getAnAccess() and
+  exists(Call c, int argPos | c.getArgument(argPos) = use |
+    not existsArgument.getType().getASuperType+() = c.getTarget().getParameterType(argPos)
+  )
+select existsArgument, "This exists argument can be omitted by using a don't-care expression $@.",
+  use, "in this argument"

ql/ql/test/queries/style/OmittableExists/OmittableExists.expected

@@ -0,0 +1 @@
+| Test.qll:10:10:10:14 | i | This exists argument can be omitted by using a don't-care expression $@. | Test.qll:10:29:10:29 | i | in this argument |

ql/ql/test/queries/style/OmittableExists/OmittableExists.qlref

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+queries/style/OmittableExists.ql

ql/ql/test/queries/style/OmittableExists/Test.qll

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+predicate aPredicate(int i) { none() }
+
+predicate anotherPredicate(int i) { none() }
+
+class SmallInt extends int {
+  SmallInt() { this = [0 .. 10] }
+}
+
+predicate test() {
+  exists(int i | aPredicate(i)) // BAD
+  or
+  exists(int i | aPredicate(i) or anotherPredicate(i)) // BAD [NOT DETECTED]
+  or
+  exists(int i | aPredicate(i) and i.abs() = 0) // GOOD
+  or
+  exists(int i | aPredicate(i) and exists(int i2 | i = i2)) // GOOD
+  or
+  exists(SmallInt i | aPredicate(i)) // GOOD
+}