libcore: Remove mutable fields from rand.

src/libstd/tempfile.rs

@@ -13,7 +13,7 @@
 use core::rand::RngUtil;
 
 pub fn mkdtemp(tmpdir: &Path, suffix: &str) -> Option<Path> {
-    let r = rand::rng();
+    let mut r = rand::rng();
     for 1000.times {
         let p = tmpdir.push(r.gen_str(16) + suffix);
         if os::make_dir(&p, 0x1c0) { // 700

src/libstd/test.rs

@@ -688,7 +688,7 @@ pub mod bench {
         // not met, it may run as long as the Go algorithm.
         pub fn auto_bench(&mut self, f: &fn(&mut BenchHarness)) -> ~[f64] {
 
-            let rng = rand::rng();
+            let mut rng = rand::rng();
             let mut magnitude = 10;
             let mut prev_madp = 0.0;