Accommodate arith-overflow in `rand` and `std::rand`.

Regarding the `rand` changes: It is unfortunate that Wrapping(T) does
not support the `+=` operator.  We may want to try to fix that before
1.0 to make porting code like this palatable.

Regarding `std::rand`, just arith-overflow in first example from
`std::rand::random()` doc.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/rand/mod.rs b/src/libstd/rand/mod.rs
index 5c891441198..ac7622fc7f7 100644
--- a/src/libstd/rand/mod.rs
+++ b/src/libstd/rand/mod.rs
@@ -386,8 +386,8 @@ impl Rng for ThreadRng {
 /// ```
 /// use std::rand;
 ///
-/// let x = rand::random();
-/// println!("{}", 2u8 * x);
+/// let x: u8 = rand::random();
+/// println!("{}", 2 * x as u16);
 ///
 /// let y = rand::random::<f64>();
 /// println!("{}", y);