Use a private uninhabited type for Export impls without hints

This makes it no longer a breaking change to add hints for these types
later, since the type can't be constructed, and can't be named except
as an associated type, so it could not be used in a way that would break
if we replace it with something else in safe code.

gdnative-core/src/nativescript/init/property.rs

@@ -19,6 +19,12 @@ use accessor::{Getter, InvalidGetter, InvalidSetter, RawGetter, RawSetter, Sette
 /// Trait for exportable types.
 pub trait Export: ToVariant {
     /// A type-specific hint type that is valid for the type being exported.
+    ///
+    /// If this type shows up as `NoHint`, a private, uninhabitable type indicating
+    /// that there are no hints available for the time being, users *must* use `None`
+    /// for properties of this type. This ensures that it will not be a breaking change
+    /// to add a hint for the type later, since it supports no operations and cannot
+    /// be named directly in user code.
     type Hint;
 
     /// Returns `ExportInfo` given an optional typed hint.
@@ -319,6 +325,13 @@ impl Usage {
 mod impl_export {
     use super::*;
 
+    /// Hint type indicating that there are no hints available for the time being.
+    ///
+    /// This is an inhabitable type that cannot be constructed. As a result, users
+    /// *must* use `None` for the property hint when exporting types using this as
+    /// the hint.
+    pub enum NoHint {}
+
     macro_rules! impl_export_for_int {
         ($ty:ident) => {
             impl Export for $ty {
@@ -382,7 +395,7 @@ mod impl_export {
     macro_rules! impl_export_for_core_type_without_hint {
         ($ty:ty: $variant_ty:ident) => {
             impl Export for $ty {
-                type Hint = ();
+                type Hint = NoHint;
                 #[inline]
                 fn export_info(_hint: Option<Self::Hint>) -> ExportInfo {
                     ExportInfo::new(VariantType::$variant_ty)
@@ -430,7 +443,7 @@ mod impl_export {
     where
         T: GodotObject,
     {
-        type Hint = ();
+        type Hint = NoHint;
         #[inline]
         fn export_info(_hint: Option<Self::Hint>) -> ExportInfo {
             ExportInfo::resource_type::<T>()
@@ -442,7 +455,7 @@ mod impl_export {
         T: NativeClass,
         Instance<T, Shared>: ToVariant,
     {
-        type Hint = ();
+        type Hint = NoHint;
         #[inline]
         fn export_info(_hint: Option<Self::Hint>) -> ExportInfo {
             ExportInfo::resource_type::<T::Base>()