Fix ToPrimitive for f64 -> f32 conversion.

It should use the destination type and not the source type to check if
the conversion would be to a value that's in range.

Previously, too large f64 values would produce inf f32 values.

Author: bluss

traits/src/cast.rs

@@ -223,7 +223,7 @@ macro_rules! impl_to_primitive_float_to_float {
             Some($slf as $DstT)
         } else {
             let n = $slf as f64;
-            let max_value: $SrcT = ::std::$SrcT::MAX;
+            let max_value: $DstT = ::std::$DstT::MAX;
             if -max_value as f64 <= n && n <= max_value as f64 {
                 Some($slf as $DstT)
             } else {
@@ -431,3 +431,12 @@ impl_num_cast!(i64,   to_i64);
 impl_num_cast!(isize, to_isize);
 impl_num_cast!(f32,   to_f32);
 impl_num_cast!(f64,   to_f64);
+
+
+#[test]
+fn to_primitive_float() {
+    let f32_max = ::std::f32::MAX;
+    let f32_toolarge = 1e39f64;
+    assert_eq!((f32_max as f64).to_f32(), Some(f32_max));
+    assert_eq!(f32_toolarge.to_f32(), None);
+}