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

src/traits.rs

@@ -1216,7 +1216,7 @@ 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 {
@@ -2550,3 +2550,11 @@ fn from_str_radix_unwrap() {
     let f: f32 = Num::from_str_radix("0.0", 10).unwrap();
     assert_eq!(f, 0.0);
 }
+
+#[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);
+}