diff --git a/src/cast.rs b/src/cast.rs
index 9e6974f..2d7fe19 100644
--- a/src/cast.rs
+++ b/src/cast.rs
@@ -454,6 +454,9 @@ impl<T: NumCast> NumCast for Wrapping<T> {
 
 /// A generic interface for casting between machine scalars with the
 /// `as` operator, which admits narrowing and precision loss.
+/// Implementers of this trait AsPrimitive should behave like a primitive
+/// numeric type (e.g. a newtype around another primitive), and the
+/// intended conversion must never fail.
 ///
 /// # Examples
 ///