diff --git a/src/extern_type.rs b/src/extern_type.rs
index 6701ef5..951bc6c 100644
--- a/src/extern_type.rs
+++ b/src/extern_type.rs
@@ -104,7 +104,28 @@
     /// # }
     /// ```
     type Id;
+
+    /// Either `kind::Opaque` or `kind::Trivial`. If in doubt, use
+    /// `kind::Opaque`.
+    type Kind;
+}
+
+pub(crate) mod kind {
+
+    /// An opaque type which can't be passed or held by value within Rust.
+    /// For example, a C++ type with a destructor, or a non-trivial move
+    /// constructor. Rust's strict move semantics mean that we can't own
+    /// these by value in Rust, but they can still be owned by a
+    /// `UniquePtr`...
+    pub struct Opaque;
+
+    /// A type with trivial move constructors and no destructor, which
+    /// can therefore be owned and moved around in Rust code directly.
+    pub struct Trivial;
 }
 
 #[doc(hidden)]
 pub fn verify_extern_type<T: ExternType<Id = Id>, Id>() {}
+
+#[doc(hidden)]
+pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind>() {}
diff --git a/src/lib.rs b/src/lib.rs
index 9efac51..2f21de9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -395,6 +395,8 @@
 pub use crate::cxx_string::CxxString;
 pub use crate::cxx_vector::CxxVector;
 pub use crate::exception::Exception;
+pub use crate::extern_type::kind::Opaque;
+pub use crate::extern_type::kind::Trivial;
 pub use crate::extern_type::ExternType;
 pub use crate::unique_ptr::UniquePtr;
 pub use cxxbridge_macro::bridge;
@@ -422,6 +424,7 @@
 #[doc(hidden)]
 pub mod private {
     pub use crate::cxx_vector::VectorElement;
+    pub use crate::extern_type::verify_extern_kind;
     pub use crate::extern_type::verify_extern_type;
     pub use crate::function::FatFunction;
     pub use crate::opaque::Opaque;