Add safety notice in AsPrimitive docs

src/cast.rs

@@ -462,6 +462,28 @@ impl<T: NumCast> NumCast for Wrapping<T> {
 /// let three: i32 = (3.14159265f32).as_();
 /// assert_eq!(three, 3);
 /// ```
+/// 
+/// # Safety
+/// 
+/// Currently, some uses of the `as` operator are not entirely safe.
+/// In particular, it is undefined behavior if:
+/// 
+/// - A truncated floating point value cannot fit in the target integer
+///   type ([#10184](https://github.com/rust-lang/rust/issues/10184));
+/// 
+/// ```ignore
+/// # use num_traits::AsPrimitive;
+/// let x: u8 = (1.04E+17).as_(); // UB
+/// ```
+/// 
+/// - Or a floating point value does not fit in another floating
+///   point type ([#15536](https://github.com/rust-lang/rust/issues/15536)).
+///
+/// ```ignore
+/// # use num_traits::AsPrimitive;
+/// let x: f32 = (1e300f64).as_(); // UB
+/// ```
+/// 
 pub trait AsPrimitive<T>: 'static + Copy
 where
     T: 'static + Copy