Change register_traits to create_object to not associate wrong v-tables

Signed-off-by: trivernis <trivernis@protonmail.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "multi-trait-object"
-version = "0.1.3"
+version = "0.2.0"
 edition = "2021"
 readme = "README.md"
 license = "Apache-2.0"

src/lib.rs

@@ -20,14 +20,21 @@ pub struct FatPointer {
     pub vptr: *const (),
 }
 
+#[allow(unused)]
+#[doc(hidden)]
+#[inline]
+pub unsafe fn null_ptr<T: Sized>(_value: &T) -> *const T {
+    // creates a typed clone that is actually null
+    std::ptr::null::<T>() as *const T
+}
+
 /// A container to store data with the associated type and trait objects
 /// allowing for casting down to trait_impl or the concrete type
 /// ```rust
 /// use multi_trait_object::*;
 /// use std::fmt::{Debug, Display};
 ///
-/// let mut mto = MultitraitObject::new(String::new());
-/// register_traits!(mto, String, dyn Debug, dyn Display);
+/// let mto = create_object!(String::new(), dyn Debug, dyn Display);
 ///
 /// let debug = mto.downcast_trait::<dyn Debug>().unwrap();
 /// println!("{:?}", debug);
@@ -46,6 +53,7 @@ impl MultitraitObject {
     /// Creates a new multitrait object from the given value
     /// All trait_impl except Any must be registered on this object
     /// in order to access them.
+    #[doc(hidden)]
     pub fn new<T: 'static + Any>(value: T) -> Self {
         let any_vtable = __fat_pointer!(T as dyn Any).vptr;
         let data = Box::into_raw(Box::new(value)) as *mut ();

src/macros.rs

@@ -20,7 +20,7 @@ macro_rules! impl_trait_object {
             fn into_multitrait(self) -> $crate::MultitraitObject {
                 let mut mto = $crate::MultitraitObject::new(self);
                 $(
-                    $crate::register_traits!(mto, $obj, $trt);
+                    mto._register::<$trt>($crate::__fat_pointer!($obj as $trt).vptr);
                 )*
 
                 mto
@@ -29,32 +29,43 @@ macro_rules! impl_trait_object {
     }
 }
 
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __fat_pointer {
+    ($v:ty as $t:ty) => {{
+        let x = ::std::ptr::null::<$v>() as *const $v as *const $t;
+        #[allow(unused_unsafe)]
+        unsafe {
+            std::mem::transmute::<_, $crate::FatPointer>(x)
+        }
+    }}
+}
+
 /// Registers multiple trait_impl on a multitrait object
 /// ```rust
 /// use multi_trait_object::*;
 /// use std::fmt::{Debug, Display};
 ///
-/// let value = String::new();
-/// let mut mto = MultitraitObject::new(value);
-/// register_traits!(mto, String, dyn Debug, dyn Display);
+/// let mto = create_object!(String::new(), dyn Debug, dyn Display);
 /// ```
 #[macro_export]
-macro_rules! register_traits {
-    ($r:expr, $v:ty, $($t:ty), +) => {
+macro_rules! create_object {
+    ($v:expr, $($t:ty), +) => {
+        {
+            let null_ptr = unsafe {
+                // SAFETY: We're never accessing the null value
+                $crate::null_ptr(&$v)
+            };
+            let mut mto = $crate::MultitraitObject::new($v);
             $(
-            $r._register::<$t>($crate::__fat_pointer!($v as $t).vptr);
+                let vptr = unsafe {
+                    // SAFETY: We're never accessing the null value
+                    let ptr = $crate::null_ptr(&*null_ptr) as *const $t;
+                    std::mem::transmute::<_, $crate::FatPointer>(ptr).vptr
+                };
+                mto._register::<$t>(vptr);
             )+
+            mto
         }
-}
-
-#[doc(hidden)]
-#[macro_export]
-macro_rules! __fat_pointer {
-    ($v:ty as $t:ty) => {{
-        let x = ::std::ptr::null::<$v>() as *const $v as *const $t;
-        #[allow(unused_unsafe)]
-        unsafe {
-            std::mem::transmute::<_, $crate::FatPointer>(x)
     }
-    }}
 }

src/tests.rs

@@ -1,5 +1,5 @@
 use std::fmt::Debug;
-use crate::{__fat_pointer, impl_trait_object, IntoMultitrait, TryClone, TryPartialEq, RawClone, PartialEqAny};
+use crate::{__fat_pointer, impl_trait_object, IntoMultitrait, TryClone, TryPartialEq, RawClone, PartialEqAny, create_object};
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 struct TestStruct {
@@ -42,6 +42,8 @@ fn it_creates_fat_pointers() {
 fn it_constructs() {
     TestStruct::default().into_multitrait();
     String::from("test").into_multitrait();
+    let mto = create_object!(String::from("test"), dyn Debug);
+    assert!(mto.is::<String>())
 }
 
 #[test]