Float copysign does not exist in libcore yet

Author: dtolnay

serde/src/de/impls.rs

@@ -187,12 +187,12 @@ macro_rules! num_as_copysign_self {
         where
             E: Error,
         {
-            #[cfg(no_float_copysign)]
+            #[cfg(any(no_float_copysign, not(feature = "std")))]
             {
                 Ok(v as Self::Value)
             }
 
-            #[cfg(not(no_float_copysign))]
+            #[cfg(all(not(no_float_copysign), feature = "std"))]
             {
                 // Preserve sign of NaN. The `as` produces a nondeterministic sign.
                 let sign = if v.is_sign_positive() { 1.0 } else { -1.0 };