std::rand: Make Rng.next_u32 non-default, waiting for rust-lang#7771.

src/libstd/rand/isaac.rs

@@ -363,6 +363,12 @@ impl Isaac64Rng {
 }
 
 impl Rng for Isaac64Rng {
+    // FIXME #7771: having next_u32 like this should be unnecessary
+    #[inline]
+    fn next_u32(&mut self) -> u32 {
+        self.next_u64() as u32
+    }
+
     #[inline]
     fn next_u64(&mut self) -> u64 {
         if self.cnt == 0 {

src/libstd/rand/mod.rs

@@ -99,12 +99,8 @@ pub trait Rng {
     /// Return the next random u32. This rarely needs to be called
     /// directly, prefer `r.gen()` to `r.next_u32()`.
     ///
-    /// By default this is implemented in terms of `next_u64`. An
-    /// implementation of this trait must provide at least one of
-    /// these two methods.
-    fn next_u32(&mut self) -> u32 {
-        self.next_u64() as u32
-    }
+    // FIXME #7771: Should be implemented in terms of next_u64
+    fn next_u32(&mut self) -> u32;
 
     /// Return the next random u64. This rarely needs to be called
     /// directly, prefer `r.gen()` to `r.next_u64()`.

src/libstd/rand/rand_impls.rs

@@ -209,6 +209,9 @@ mod tests {
     use rand::Rng;
     struct ConstantRng(u64);
     impl Rng for ConstantRng {
+        fn next_u32(&mut self) -> u32 {
+            (**self) as u32
+        }
         fn next_u64(&mut self) -> u64 {
             **self
         }