diff --git a/syntax/improper.rs b/syntax/improper.rs
new file mode 100644
index 0000000..6fd3162
--- /dev/null
+++ b/syntax/improper.rs
@@ -0,0 +1,36 @@
+use self::ImproperCtype::*;
+use crate::syntax::atom::Atom::{self, *};
+use crate::syntax::{Type, Types};
+use proc_macro2::Ident;
+
+pub enum ImproperCtype<'a> {
+    Definite(bool),
+    Depends(&'a Ident),
+}
+
+impl<'a> Types<'a> {
+    // yes, no, maybe
+    pub fn determine_improper_ctype(&self, ty: &Type) -> ImproperCtype<'a> {
+        match ty {
+            Type::Ident(ident) => {
+                let ident = &ident.rust;
+                if let Some(atom) = Atom::from(ident) {
+                    Definite(atom == RustString)
+                } else if let Some(strct) = self.structs.get(ident) {
+                    Depends(&strct.name.rust) // iterate to fixed-point
+                } else {
+                    Definite(self.rust.contains(ident))
+                }
+            }
+            Type::RustBox(_)
+            | Type::RustVec(_)
+            | Type::Str(_)
+            | Type::Fn(_)
+            | Type::Void(_)
+            | Type::Slice(_)
+            | Type::SliceRefU8(_) => Definite(true),
+            Type::UniquePtr(_) | Type::CxxVector(_) => Definite(false),
+            Type::Ref(ty) => self.determine_improper_ctype(&ty.inner),
+        }
+    }
+}