docs: Explain why ConstF64 is not PartialEq (#1829)

hugr-core/src/std_extensions/arithmetic/float_types.rs

@@ -38,6 +38,18 @@ pub fn float64_type() -> Type {
 
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
 /// A floating-point value.
+///
+/// This constant type does **not** implement equality. Any two instances of
+/// `ConstF64` are considered different.
+//
+// The main problem for equality checking is comparisons of serialized values
+// with different precision in `CustomSerialized`.
+// For example, `3.3508025818765467e243 /= 3.350802581876547e243` for serde,
+// but they would be equal after loaded as a `ConstF64` type.
+//
+// `serde_json` provides some options to overcome this issue, but since
+// custom values are encoded inside `serde_json::Value`s they are not directly
+// reachable by these solutions.
 pub struct ConstF64 {
     /// The value.
     value: f64,