Add must_use on copysign

Added a #[must_use] annotation on copysign, per review feedback.

Author: raphlinus

src/libstd/f32.rs

@@ -221,6 +221,7 @@ impl f32 {
     /// assert!(f32::NAN.copysign(1.0).is_nan());
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature="copysign", issue="55169")]
     pub fn copysign(self, y: f32) -> f32 {
         unsafe { intrinsics::copysignf32(self, y) }

src/libstd/f64.rs

@@ -199,6 +199,7 @@ impl f64 {
     /// assert!(f64::NAN.copysign(1.0).is_nan());
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature="copysign", issue="55169")]
     pub fn copysign(self, y: f64) -> f64 {
         unsafe { intrinsics::copysignf64(self, y) }