NumCast: document when `from` can return `None`
@@ -639,7 +639,8 @@ pub fn cast<T: NumCast, U: NumCast>(n: T) -> Option<U> {
/// An interface for casting between machine scalars.
pub trait NumCast: Sized + ToPrimitive {
/// Creates a number from another value that can be converted into
- /// a primitive via the `ToPrimitive` trait.
+ /// a primitive via the `ToPrimitive` trait. If the source value cannot be
+ /// represented by the target type, then `None` is returned.
fn from<T: ToPrimitive>(n: T) -> Option<Self>;
}
