Use fn() -> T instead of *mut T in PhantomData

Avoids unessicarrily ruining our Sync/Send impl for TypeId
and FunctionDefinition

Author: Techcable

src/funcs.rs

@@ -21,9 +21,9 @@ pub struct FunctionDeclaration<'a, R = (), Args = ()> {
     /// Unlike the [PhantomData], this is actually retained at runtime.
     pub signature: SignatureDef<'a>,
     /// PhantomData: The return type of the function 
-    pub return_type: PhantomData<*mut R>,
+    pub return_type: PhantomData<fn() -> R>,
     /// PhantomData: The argument types of the function
-    pub arg_types: PhantomData<*mut Args>,
+    pub arg_types: PhantomData<fn(Args) -> ()>,
 }
 impl<'a, R, Args> FunctionDeclaration<'a, R, Args> {
     /// If the function has a known location at runtime

src/lib.rs

@@ -13,6 +13,7 @@
 #![feature(
     const_fn, // We rely on const eval :(
     const_panic, const_option, // We use Option::unwrap
+    const_fn_fn_ptr_basics, // We use PhantomData<fn() -> T>
     // Used for field_offset macro
     const_raw_ptr_deref,
     const_raw_ptr_to_usize_cast,

src/types.rs

@@ -525,7 +525,7 @@ impl<T: StaticReflect> Clone for TypeId<'_, T> {
 #[derive(Eq, PartialEq, Hash)]
 pub struct TypeId<'a, T: StaticReflect = ()> {
     value: &'a TypeInfo<'a>,
-    marker: PhantomData<*mut T>
+    marker: PhantomData<fn() -> T>
 }
 /*
   * TODO: Fix to use derive