Rust: Restrict type propagation into arguments

rust/ql/lib/codeql/rust/internal/Type.qll

@@ -51,6 +51,7 @@ newtype TType =
   TSliceType() or
   TNeverType() or
   TPtrType() or
+  TContextType() or
   TTupleTypeParameter(int arity, int i) { exists(TTuple(arity)) and i in [0 .. arity - 1] } or
   TTypeParamTypeParameter(TypeParam t) or
   TAssociatedTypeTypeParameter(TypeAlias t) { any(TraitItemNode trait).getAnAssocItem() = t } or
@@ -371,6 +372,30 @@ class PtrType extends Type, TPtrType {
   override Location getLocation() { result instanceof EmptyLocation }
 }
 
+/**
+ * A special pseudo type used to indicate that the actual type may have to be
+ * inferred from a context.
+ *
+ * For example, a call like `Default::default()` is assigned this type, which
+ * means that the actual type is to be inferred from the context in which the call
+ * occurs.
+ *
+ * Context types are not restricted to root types, for example in a call like
+ * `Vec::new()` we assign this type at the type path corresponding to the type
+ * parameter of `Vec`.
+ *
+ * Context types are used to restrict when type information is allowed to flow
+ * into call arguments (including method call receivers), in order to avoid
+ * combinatorial explosions.
+ */
+class ContextType extends Type, TContextType {
+  override TypeParameter getPositionalTypeParameter(int i) { none() }
+
+  override string toString() { result = "(context typed)" }
+
+  override Location getLocation() { result instanceof EmptyLocation }
+}
+
 /** A type parameter. */
 abstract class TypeParameter extends Type {
   override TypeParameter getPositionalTypeParameter(int i) { none() }