Ensure f32 deserialized from f64 and vice versa preserve NaN sign

Author: dtolnay

serde/build.rs

@@ -27,6 +27,12 @@ fn main() {
         println!("cargo:rustc-cfg=no_relaxed_trait_bounds");
     }
 
+    // f32::copysign and f64::copysign stabilized in Rust 1.35.
+    // https://blog.rust-lang.org/2019/05/23/Rust-1.35.0.html#copy-the-sign-of-a-floating-point-number-onto-another
+    if minor < 35 {
+        println!("cargo:rustc-cfg=no_float_copysign");
+    }
+
     // Current minimum supported version of serde_derive crate is Rust 1.56.
     if minor < 56 {
         println!("cargo:rustc-cfg=no_serde_derive");

serde/src/de/impls.rs

@@ -180,6 +180,28 @@ macro_rules! num_as_self {
     };
 }
 
+macro_rules! num_as_copysign_self {
+    ($ty:ident : $visit:ident) => {
+        #[inline]
+        fn $visit<E>(self, v: $ty) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            #[cfg(no_float_copysign)]
+            {
+                Ok(v as Self::Value)
+            }
+
+            #[cfg(not(no_float_copysign))]
+            {
+                // Preserve sign of NaN. The `as` produces a nondeterministic sign.
+                let sign = if v.is_sign_positive() { 1.0 } else { -1.0 };
+                Ok((v as Self::Value).copysign(sign))
+            }
+        }
+    };
+}
+
 macro_rules! int_to_int {
     ($ty:ident : $visit:ident) => {
         #[inline]
@@ -351,15 +373,15 @@ impl_deserialize_num! {
 impl_deserialize_num! {
     f32, deserialize_f32
     num_self!(f32:visit_f32);
-    num_as_self!(f64:visit_f64);
+    num_as_copysign_self!(f64:visit_f64);
     num_as_self!(i8:visit_i8 i16:visit_i16 i32:visit_i32 i64:visit_i64);
     num_as_self!(u8:visit_u8 u16:visit_u16 u32:visit_u32 u64:visit_u64);
 }
 
 impl_deserialize_num! {
     f64, deserialize_f64
     num_self!(f64:visit_f64);
-    num_as_self!(f32:visit_f32);
+    num_as_copysign_self!(f32:visit_f32);
     num_as_self!(i8:visit_i8 i16:visit_i16 i32:visit_i32 i64:visit_i64);
     num_as_self!(u8:visit_u8 u16:visit_u16 u32:visit_u32 u64:visit_u64);
 }