feat: Serialize NaN without sign (serde only)

crates/toml/src/value.rs

@@ -921,12 +921,14 @@ impl ser::Serializer for ValueSerializer {
     }
 
     fn serialize_f32(self, value: f32) -> Result<Value, crate::ser::Error> {
-        // Preserve sign of NaN. The `as` produces a nondeterministic sign.
-        let sign = if value.is_sign_positive() { 1.0 } else { -1.0 };
-        self.serialize_f64((value as f64).copysign(sign))
+        self.serialize_f64(value as f64)
     }
 
-    fn serialize_f64(self, value: f64) -> Result<Value, crate::ser::Error> {
+    fn serialize_f64(self, mut value: f64) -> Result<Value, crate::ser::Error> {
+        // Discard sign of NaN. See ValueSerializer::serialize_f64.
+        if value.is_nan() {
+            value = value.copysign(1.0);
+        }
         Ok(Value::Float(value))
     }
 

crates/toml/tests/testsuite/float.rs

@@ -47,7 +47,7 @@ macro_rules! float_inf_tests {
         assert!(inf.sf5.is_nan());
         assert!(inf.sf5.is_sign_positive());
         assert!(inf.sf6.is_nan());
-        assert!(inf.sf6.is_sign_negative());
+        assert!(inf.sf6.is_sign_negative()); // NOTE: serializes to just `nan`
 
         assert_eq!(inf.sf7, 0.0);
         assert!(inf.sf7.is_sign_positive());
@@ -63,7 +63,7 @@ sf2 = inf
 sf3 = -inf
 sf4 = nan
 sf5 = nan
-sf6 = -nan
+sf6 = nan
 sf7 = 0.0
 sf8 = -0.0
 "

crates/toml_edit/src/ser/value.rs

@@ -105,12 +105,20 @@ impl serde::ser::Serializer for ValueSerializer {
     }
 
     fn serialize_f32(self, v: f32) -> Result<Self::Ok, Self::Error> {
-        // Preserve sign of NaN. The `as` produces a nondeterministic sign.
-        let sign = if v.is_sign_positive() { 1.0 } else { -1.0 };
-        self.serialize_f64((v as f64).copysign(sign))
-    }
-
-    fn serialize_f64(self, v: f64) -> Result<Self::Ok, Self::Error> {
+        self.serialize_f64(v as f64)
+    }
+
+    fn serialize_f64(self, mut v: f64) -> Result<Self::Ok, Self::Error> {
+        // Discard sign of NaN when serialized using Serde.
+        //
+        // In all likelihood the sign of NaNs is not meaningful in the user's
+        // program. Ending up with `-nan` in the TOML document would usually be
+        // surprising and undesirable, when the sign of the NaN was not
+        // intentionally controlled by the caller, or may even be
+        // nondeterministic if it comes from arithmetic operations or a cast.
+        if v.is_nan() {
+            v = v.copysign(1.0);
+        }
         Ok(v.into())
     }
 

crates/toml_edit/src/value.rs

@@ -284,6 +284,7 @@ impl From<i64> for Value {
 
 impl From<f64> for Value {
     fn from(f: f64) -> Self {
+        // Preserve sign of NaN. It may get written to TOML as `-nan`.
         Value::Float(Formatted::new(f))
     }
 }