Python: Add metrics query for type annotations

Adds a query that counts the number of type annotations of various
kinds. Intended to be used with something like MRVA to inform our
modelling decisions.

Currently the query counts the following "interesting" types in addition
to the total number of types:
- Built-in types (which are less likely to be interesting from a
modelling perspective)
- Forward declarations (i.e. annotations inside strings) which will
require a fair bit of QL machinery to interpret.
- Simple types (stuff like `foo` or `foo.bar.baz`)
- Optional types (stuff like `Optional[foo]` which from a modelling
perspective should likely be treated the same as `foo`)
- Complex types (anything that contains more complex type constructions
such as instantiations of generic types)

python/ql/src/Metrics/Internal/TypeAnnotations.ql

@@ -0,0 +1,108 @@
+/**
+ * @name Type metrics
+ * @description Counts of various kinds of type annotations in Python code.
+ * @kind table
+ * @id py/type-metrics
+ */
+
+import python
+
+class BuiltinType extends Name {
+  BuiltinType() { this.getId() in ["int", "float", "str", "bool", "bytes", "None"] }
+}
+
+newtype TAnnotatable =
+  TAnnotatedFunction(FunctionExpr f) { exists(f.getReturns()) } or
+  TAnnotatedParameter(Parameter p) { exists(p.getAnnotation()) } or
+  TAnnotatedAssignment(AnnAssign a) { exists(a.getAnnotation()) }
+
+abstract class Annotatable extends TAnnotatable {
+  string toString() { result = "Annotatable" }
+
+  abstract Expr getAnnotation();
+}
+
+class AnnotatedFunction extends TAnnotatedFunction, Annotatable {
+  FunctionExpr function;
+
+  AnnotatedFunction() { this = TAnnotatedFunction(function) }
+
+  override Expr getAnnotation() { result = function.getReturns() }
+}
+
+class AnnotatedParameter extends TAnnotatedParameter, Annotatable {
+  Parameter parameter;
+
+  AnnotatedParameter() { this = TAnnotatedParameter(parameter) }
+
+  override Expr getAnnotation() { result = parameter.getAnnotation() }
+}
+
+class AnnotatedAssignment extends TAnnotatedAssignment, Annotatable {
+  AnnAssign assignment;
+
+  AnnotatedAssignment() { this = TAnnotatedAssignment(assignment) }
+
+  override Expr getAnnotation() { result = assignment.getAnnotation() }
+}
+
+/** Holds if `e` is a forward declaration of a type. */
+predicate is_forward_declaration(Expr e) { e instanceof StringLiteral }
+
+/** Holds if `e` is a type that may be difficult to analyze. */
+predicate is_complex_type(Expr e) {
+  e instanceof Subscript and not is_optional_type(e)
+  or
+  e instanceof Tuple
+  or
+  e instanceof List
+}
+
+/** Holds if `e` is a type of the form `Optional[...]`. */
+predicate is_optional_type(Subscript e) { e.getObject().(Name).getId() = "Optional" }
+
+/** Holds if `e` is a simple type, that is either an identifier (excluding built-in types) or an attribute of a simple type. */
+predicate is_simple_type(Expr e) {
+  e instanceof Name and not e instanceof BuiltinType
+  or
+  is_simple_type(e.(Attribute).getObject())
+}
+
+/** Holds if `e` is a built-in type. */
+predicate is_builtin_type(Expr e) { e instanceof BuiltinType }
+
+predicate type_count(
+  string kind, int total, int built_in_count, int forward_declaration_count, int simple_type_count,
+  int complex_type_count, int optional_type_count
+) {
+  kind = "Parameter annotation" and
+  total = count(AnnotatedParameter p) and
+  built_in_count = count(AnnotatedParameter p | is_builtin_type(p.getAnnotation())) and
+  forward_declaration_count =
+    count(AnnotatedParameter p | is_forward_declaration(p.getAnnotation())) and
+  simple_type_count = count(AnnotatedParameter p | is_simple_type(p.getAnnotation())) and
+  complex_type_count = count(AnnotatedParameter p | is_complex_type(p.getAnnotation())) and
+  optional_type_count = count(AnnotatedParameter p | is_optional_type(p.getAnnotation()))
+  or
+  kind = "Return type annotation" and
+  total = count(AnnotatedFunction f) and
+  built_in_count = count(AnnotatedFunction f | is_builtin_type(f.getAnnotation())) and
+  forward_declaration_count = count(AnnotatedFunction f | is_forward_declaration(f.getAnnotation())) and
+  simple_type_count = count(AnnotatedFunction f | is_simple_type(f.getAnnotation())) and
+  complex_type_count = count(AnnotatedFunction f | is_complex_type(f.getAnnotation())) and
+  optional_type_count = count(AnnotatedFunction f | is_optional_type(f.getAnnotation()))
+  or
+  kind = "Annotated assignment" and
+  total = count(AnnotatedAssignment a) and
+  built_in_count = count(AnnotatedAssignment a | is_builtin_type(a.getAnnotation())) and
+  forward_declaration_count =
+    count(AnnotatedAssignment a | is_forward_declaration(a.getAnnotation())) and
+  simple_type_count = count(AnnotatedAssignment a | is_simple_type(a.getAnnotation())) and
+  complex_type_count = count(AnnotatedAssignment a | is_complex_type(a.getAnnotation())) and
+  optional_type_count = count(AnnotatedAssignment a | is_optional_type(a.getAnnotation()))
+}
+
+from
+  string message, int total, int built_in, int forward_decl, int simple, int complex, int optional
+where type_count(message, total, built_in, forward_decl, simple, complex, optional)
+select message, total, built_in, forward_decl, simple, complex, optional