Add example and test: distribution usage with type-erased Rng

Author: dhardy

src/distributions/mod.rs

@@ -173,6 +173,18 @@ impl<'a, T, D: Distribution<T>> Distribution<T> for &'a D {
 /// println!("f32 from [0,1): {}", val);
 /// ```
 ///
+/// With dynamic dispatch (type erasure of `Rng`):
+/// 
+/// ```rust
+/// use rand::{thread_rng, Rng, RngCore};
+/// use rand::distributions::Uniform;
+///
+/// let mut rng = thread_rng();
+/// let mut erased_rng: &mut RngCore = &mut rng;
+/// let val: f32 = erased_rng.sample(Uniform);
+/// println!("f32 from [0,1): {}", val);
+/// ```
+///
 /// [`Open01`]: struct.Open01.html
 /// [`Closed01`]: struct.Closed01.html
 /// [`Exp1`]: struct.Exp1.html

src/lib.rs

@@ -1418,6 +1418,7 @@ mod test {
     #[test]
     #[cfg(any(feature="std", feature="alloc"))]
     fn test_rng_trait_object() {
+        use distributions::{Distribution, Uniform};
         let mut rng = rng(109);
         {
             let mut r = &mut rng as &mut RngCore;
@@ -1428,6 +1429,7 @@ mod test {
             let b: &[_] = &[1, 1, 1];
             assert_eq!(v, b);
             assert_eq!(r.gen_range(0, 1), 0);
+            let _c: u8 = Uniform.sample(&mut r);
         }
         {
             let mut r = Box::new(rng) as Box<RngCore>;
@@ -1438,6 +1440,7 @@ mod test {
             let b: &[_] = &[1, 1, 1];
             assert_eq!(v, b);
             assert_eq!(r.gen_range(0, 1), 0);
+            let _c: u8 = Uniform.sample(&mut r);
         }
     }