message: Seal `HostFunctionKind`
diff: @@ -1105,7 +1105,7 @@ mod inner {
  /// the trait system to automatically generate the appropriate
  /// host functions.
  #[doc(hidden)]
  - pub trait HostFunctionKind {}
  + pub trait HostFunctionKind: private::HostFunctionKindSealed {}
  /// An empty struct to help Rust typing to determine
  /// when a `HostFunction` does have an environment.
  