Eliminate f32-to-f64 casting in arbitrary_precision mode

Author: dtolnay

src/number.rs

@@ -279,6 +279,24 @@ impl Number {
         }
     }
 
+    pub(crate) fn from_f32(f: f32) -> Option<Number> {
+        if f.is_finite() {
+            let n = {
+                #[cfg(not(feature = "arbitrary_precision"))]
+                {
+                    N::Float(f as f64)
+                }
+                #[cfg(feature = "arbitrary_precision")]
+                {
+                    ryu::Buffer::new().format_finite(f).to_owned()
+                }
+            };
+            Some(Number { n })
+        } else {
+            None
+        }
+    }
+
     #[cfg(feature = "arbitrary_precision")]
     /// Not public API. Only tests use this.
     #[doc(hidden)]

src/value/from.rs

@@ -40,7 +40,7 @@ impl From<f32> for Value {
     /// let x: Value = f.into();
     /// ```
     fn from(f: f32) -> Self {
-        From::from(f as f64)
+        Number::from_f32(f).map_or(Value::Null, Value::Number)
     }
 }
 

src/value/ser.rs

@@ -1,6 +1,5 @@
 use crate::error::{Error, ErrorCode, Result};
 use crate::map::Map;
-use crate::number::Number;
 use crate::value::{to_value, Value};
 use alloc::borrow::ToOwned;
 use alloc::string::{String, ToString};
@@ -149,13 +148,13 @@ impl serde::Serializer for Serializer {
     }
 
     #[inline]
-    fn serialize_f32(self, value: f32) -> Result<Value> {
-        self.serialize_f64(value as f64)
+    fn serialize_f32(self, float: f32) -> Result<Value> {
+        Ok(Value::from(float))
     }
 
     #[inline]
-    fn serialize_f64(self, value: f64) -> Result<Value> {
-        Ok(Number::from_f64(value).map_or(Value::Null, Value::Number))
+    fn serialize_f64(self, float: f64) -> Result<Value> {
+        Ok(Value::from(float))
     }
 
     #[inline]

tests/regression/issue1004.rs

@@ -5,5 +5,8 @@ fn test() {
     let float = 5.55f32;
     let value = serde_json::to_value(&float).unwrap();
     let json = serde_json::to_string(&value).unwrap();
-    assert_eq!(json, "5.550000190734863"); // FIXME
+
+    // If the f32 were cast to f64 by Value before serialization, then this
+    // would incorrectly serialize as 5.550000190734863.
+    assert_eq!(json, "5.55");
 }