vtable_macro/

macro.rs

// Copyright © SixtyFPS GmbH <info@slint.dev>
// SPDX-License-Identifier: MIT OR Apache-2.0

// cSpell: ignore asyncness constness containee defaultness impls qself supertraits vref

/*!
Implementation detail for the vtable crate
*/

extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;
use syn::parse::Parser;
use syn::spanned::Spanned;
use syn::*;

/// Returns true if the type `ty` is  "Container<Containee>"
fn match_generic_type(ty: &Type, container: &str, containee: &Ident) -> bool {
    if let Type::Path(pat) = ty
        && let Some(seg) = pat.path.segments.last()
    {
        if seg.ident != container {
            return false;
        }
        if let PathArguments::AngleBracketed(args) = &seg.arguments
            && let Some(GenericArgument::Type(Type::Path(arg))) = args.args.last()
        {
            return Some(containee) == arg.path.get_ident();
        }
    }
    false
}

/// Returns Some(type) if the type is `Pin<type>`
fn is_pin(ty: &Type) -> Option<&Type> {
    if let Type::Path(pat) = ty
        && let Some(seg) = pat.path.segments.last()
    {
        if seg.ident != "Pin" {
            return None;
        }
        if let PathArguments::AngleBracketed(args) = &seg.arguments
            && let Some(GenericArgument::Type(t)) = args.args.last()
        {
            return Some(t);
        }
    }
    None
}

/**
This macro needs to be applied to a VTable structure

The design choice is that it is applied to a VTable and not to a trait so that cbindgen
can see the actual vtable struct.

This macro needs to be applied to a struct whose name ends with "VTable", and which
contains members which are function pointers.

For example, if it is applied to `struct MyTraitVTable`, it will create:
 - The `MyTrait` trait with all the functions.
 - The `MyTraitConsts` trait for the associated constants, if any
 - `MyTraitVTable_static!` macro.

It will also implement the `VTableMeta` and `VTableMetaDrop` traits so that VRef and so on can work,
allowing to access methods from the trait directly from VRef.

This macro does the following transformation:

For function type fields:
 - `unsafe` is added to the signature, since it is unsafe to call these functions directly from
   the vtable without having a valid pointer to the actual object. But if the original function was
   marked unsafe, the unsafety is forwarded to the trait.
 - If a field is called `drop`, then it is understood that this is the destructor for a VBox.
   It must have the type `fn(VRefMut<MyVTable>)`
 - If two fields called `drop_in_place` and `dealloc` are present, then they are understood to be
   in-place destructors and deallocation functions. `drop_in_place` must have the signature
   `fn(VRefMut<MyVTable> -> Layout`, and `dealloc` must have the signature
   `fn(&MyVTable, ptr: *mut u8, layout: Layout)`.
   `drop_in_place` is responsible for destructing the object and returning the memory layout that
   was used for the initial allocation. It will be passed to `dealloc`, which is responsible for releasing
   the memory. These two functions are used to enable the use of `VRc` and `VWeak`.
 - If the first argument of the function is `VRef<MyVTable>` or `VRefMut<MyVTable>`, then it is
   understood as a `&self` or `&mut self` argument in the trait.
 - Similarly, if it is a `Pin<VRef<MyVTable>>` or `Pin<VRefMut<MyVTable>>`, self is mapped
   to `Pin<&Self>` or `Pin<&mut Self>`

For the other fields:
 - They are considered associated constants of the MyTraitConsts trait.
 - If they are annotated with the `#[field_offset(FieldType)]` attribute, the type of the field must be `usize`,
   and the associated const in the trait will be of type `FieldOffset<Self, FieldType>`, and an accessor to
   the field reference and reference mut will be added to the Target of VRef and VRefMut.

The VRef/VRefMut/VBox structure will dereference to a type which has the following associated items:
 - The functions from the vtable that have a VRef or VRefMut first parameter for self.
 - For each `#[field_offset]` attributes, a corresponding getter returns a reference
   to that field, and mutable accessor that ends with `_mut` returns a mutable reference.
 - `as_ptr` returns a `*mut u8`
 - `get_vtable` Return a reference to the VTable so one can access the associated consts.

The VTable struct gets a `new` associated function that creates a vtable for any type
that implements the generated traits.

## Example


```
use vtable::*;
// we are going to declare a VTable structure for an Animal trait
#[vtable]
#[repr(C)]
struct AnimalVTable {
    /// Pointer to a function that make noise.
    /// `unsafe` will automatically be added
    make_noise: fn(VRef<AnimalVTable>, i32) -> i32,

    /// if there is a 'drop' member, it is considered as the destructor
    drop: fn(VRefMut<AnimalVTable>),

    /// Associated constant.
    LEG_NUMBER: i8,

    /// There exist a `bool` field in the structure and this is an offset
    #[field_offset(bool)]
    IS_HUNGRY: usize,

}

#[repr(C)]
struct Dog{ strength: i32, is_hungry: bool };

// The #[vtable] macro created the Animal Trait
impl Animal for Dog {
    fn make_noise(&self, intensity: i32) -> i32 {
        println!("Wof!");
        return self.strength * intensity;
    }
}

// The #[vtable] macro created the AnimalConsts Trait
impl AnimalConsts for Dog {
    const LEG_NUMBER: i8 = 4;
    const IS_HUNGRY: vtable::FieldOffset<Self, bool> = unsafe { vtable::FieldOffset::new_from_offset(4) };
}


// The #[vtable] macro also exposed a macro to create a vtable
AnimalVTable_static!(static DOG_VT for Dog);

// with that, it is possible to instantiate a vtable::VRefMut
let mut dog = Dog { strength: 100, is_hungry: false };
{
    let mut animal_vref = VRefMut::<AnimalVTable>::new(&mut dog);

    // access to the vtable through the get_vtable() function
    assert_eq!(animal_vref.get_vtable().LEG_NUMBER, 4);
    // functions are also added for the #[field_offset] member
    assert_eq!(*animal_vref.IS_HUNGRY(), false);
    *animal_vref.IS_HUNGRY_mut() = true;
}
assert_eq!(dog.is_hungry, true);
```


*/
#[proc_macro_attribute]
pub fn vtable(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let mut input = parse_macro_input!(item as ItemStruct);

    let fields = if let Fields::Named(fields) = &mut input.fields {
        fields
    } else {
        return Error::new(
            proc_macro2::Span::call_site(),
            "Only supported for structure with named fields",
        )
        .to_compile_error()
        .into();
    };

    let vtable_name = input.ident.to_string();
    if !vtable_name.ends_with("VTable") {
        return Error::new(input.ident.span(), "The structure does not ends in 'VTable'")
            .to_compile_error()
            .into();
    }

    let trait_name = Ident::new(&vtable_name[..vtable_name.len() - 6], input.ident.span());
    let to_name = quote::format_ident!("{}TO", trait_name);
    let module_name = quote::format_ident!("{}_vtable_mod", trait_name);
    let static_vtable_macro_name = quote::format_ident!("{}_static", vtable_name);

    let vtable_name = input.ident.clone();

    let mut drop_impls = Vec::new();

    let mut generated_trait = ItemTrait {
        attrs: input
            .attrs
            .iter()
            .filter(|a| a.path().get_ident().as_ref().map(|i| *i == "doc").unwrap_or(false))
            .cloned()
            .collect(),
        vis: Visibility::Public(Default::default()),
        unsafety: None,
        auto_token: None,
        trait_token: Default::default(),
        ident: trait_name.clone(),
        generics: Generics::default(),
        colon_token: None,
        supertraits: Default::default(),
        brace_token: Default::default(),
        items: Default::default(),
        restriction: Default::default(),
    };

    let additional_doc =
        format!("\nNote: Was generated from the [`#[vtable]`](vtable) macro on [`{vtable_name}`]");
    generated_trait
        .attrs
        .append(&mut Attribute::parse_outer.parse2(quote!(#[doc = #additional_doc])).unwrap());

    let mut generated_trait_assoc_const = None;

    let mut generated_to_fn_trait = Vec::new();
    let mut generated_type_assoc_fn = Vec::new();
    let mut vtable_ctor = Vec::new();

    for field in &mut fields.named {
        // The vtable can only be accessed in unsafe code, so it is ok if all its fields are Public
        field.vis = Visibility::Public(Default::default());

        let ident = field.ident.as_ref().unwrap();
        let mut some = None;

        let func_ty = if let Type::BareFn(f) = &mut field.ty {
            Some(f)
        } else if let Type::Path(pat) = &mut field.ty {
            pat.path.segments.last_mut().and_then(|seg| {
                if seg.ident == "Option" {
                    some = Some(quote!(Some));
                    if let PathArguments::AngleBracketed(args) = &mut seg.arguments {
                        if let Some(GenericArgument::Type(Type::BareFn(f))) = args.args.first_mut()
                        {
                            Some(f)
                        } else {
                            None
                        }
                    } else {
                        None
                    }
                } else {
                    None
                }
            })
        } else {
            None
        };

        if let Some(f) = func_ty {
            let mut sig = Signature {
                constness: None,
                asyncness: None,
                unsafety: f.unsafety,
                abi: None,
                fn_token: f.fn_token,
                ident: ident.clone(),
                generics: Default::default(),
                paren_token: f.paren_token,
                inputs: Default::default(),
                variadic: None,
                output: f.output.clone(),
            };

            let mut sig_extern = sig.clone();
            sig_extern.generics = parse_str(&format!("<T : {trait_name}>")).unwrap();

            // check parameters
            let mut call_code = None;
            let mut self_call = None;
            let mut forward_code = None;

            let mut has_self = false;

            for param in &f.inputs {
                let arg_name = quote::format_ident!("_{}", sig_extern.inputs.len());
                let typed_arg = FnArg::Typed(PatType {
                    attrs: param.attrs.clone(),
                    pat: Box::new(Pat::Path(syn::PatPath {
                        attrs: Default::default(),
                        qself: None,
                        path: arg_name.clone().into(),
                    })),
                    colon_token: Default::default(),
                    ty: Box::new(param.ty.clone()),
                });
                sig_extern.inputs.push(typed_arg.clone());

                // check for the vtable
                if let Type::Ptr(TypePtr { mutability, elem, .. })
                | Type::Reference(TypeReference { mutability, elem, .. }) = &param.ty
                    && let Type::Path(p) = &**elem
                    && let Some(pointer_to) = p.path.get_ident()
                    && pointer_to == &vtable_name
                {
                    if mutability.is_some() {
                        return Error::new(p.span(), "VTable cannot be mutable")
                            .to_compile_error()
                            .into();
                    }
                    if call_code.is_some() || !sig.inputs.is_empty() {
                        return Error::new(p.span(), "VTable pointer need to be the first")
                            .to_compile_error()
                            .into();
                    }
                    call_code = Some(quote!(vtable as _,));
                    continue;
                }

                let (is_pin, self_ty) = match is_pin(&param.ty) {
                    Some(t) => (true, t),
                    None => (false, &param.ty),
                };

                // check for self
                if let (true, mutability) = if match_generic_type(self_ty, "VRef", &vtable_name) {
                    (true, None)
                } else if match_generic_type(self_ty, "VRefMut", &vtable_name) {
                    (true, Some(Default::default()))
                } else {
                    (false, None)
                } {
                    if !sig.inputs.is_empty() {
                        return Error::new(param.span(), "Self pointer need to be the first")
                            .to_compile_error()
                            .into();
                    }

                    let const_or_mut = mutability.map_or_else(|| quote!(const), |x| quote!(#x));
                    has_self = true;
                    if !is_pin {
                        sig.inputs.push(FnArg::Receiver(Receiver {
                            attrs: param.attrs.clone(),
                            reference: Some(Default::default()),
                            mutability,
                            self_token: Default::default(),
                            colon_token: None,
                            ty: Box::new(parse_quote!(& #mutability Self)),
                        }));
                        call_code =
                            Some(quote!(#call_code <#self_ty>::from_raw(self.vtable, self.ptr),));
                        self_call =
                            Some(quote!(&#mutability (*(#arg_name.as_ptr() as *#const_or_mut T)),));
                    } else {
                        // Pinned
                        sig.inputs.push(FnArg::Typed(PatType {
                            attrs: param.attrs.clone(),
                            pat: Box::new(Pat::parse_single.parse2(quote!(self)).unwrap()),
                            colon_token: Default::default(),
                            ty: parse_quote!(core::pin::Pin<& #mutability Self>),
                        }));

                        call_code = Some(
                            quote!(#call_code ::core::pin::Pin::new_unchecked(<#self_ty>::from_raw(self.vtable, self.ptr)),),
                        );
                        self_call = Some(
                            quote!(::core::pin::Pin::new_unchecked(&#mutability (*(#arg_name.as_ptr() as *#const_or_mut T))),),
                        );
                    }
                    continue;
                }
                sig.inputs.push(typed_arg);
                call_code = Some(quote!(#call_code #arg_name,));
                forward_code = Some(quote!(#forward_code #arg_name,));
            }

            // Add unsafe: The function are not safe to call unless the self parameter is of the correct type
            f.unsafety = Some(Default::default());

            sig_extern.abi.clone_from(&f.abi);

            let mut wrap_trait_call = None;
            if !has_self {
                sig.generics = Generics {
                    where_clause: Some(parse_str("where Self : Sized").unwrap()),
                    ..Default::default()
                };

                // Check if this is a constructor functions
                if let ReturnType::Type(_, ret) = &f.output
                    && match_generic_type(ret, "VBox", &vtable_name)
                {
                    // Change VBox<VTable> to Self
                    sig.output = parse_str("-> Self").unwrap();
                    wrap_trait_call = Some(quote! {
                        let wrap_trait_call = |x| unsafe {
                            // Put the object on the heap and get a pointer to it
                            let ptr = ::core::ptr::NonNull::from(Box::leak(Box::new(x)));
                            VBox::<#vtable_name>::from_raw(vtable, ptr.cast())
                        };
                        wrap_trait_call
                    });
                }
            }

            if ident == "drop" {
                vtable_ctor.push(quote!(#ident: {
                    #sig_extern {
                        unsafe {
                            ::core::mem::drop(Box::from_raw((#self_call).0 as *mut _));
                        }
                    }
                    #ident::<T>
                },));

                drop_impls.push(quote! {
                    unsafe impl VTableMetaDrop for #vtable_name {
                        unsafe fn drop(ptr: *mut #to_name) {
                            // Safety: The vtable is valid and inner is a type corresponding to the vtable,
                            // which was allocated such that drop is expected.
                            unsafe {
                                let (vtable, ptr) = ((*ptr).vtable, (*ptr).ptr);
                                (vtable.as_ref().#ident)(VRefMut::from_raw(vtable, ptr)) }
                        }
                        fn new_box<X: HasStaticVTable<#vtable_name>>(value: X) -> VBox<#vtable_name> {
                            // Put the object on the heap and get a pointer to it
                            let ptr = ::core::ptr::NonNull::from(Box::leak(Box::new(value)));
                            unsafe { VBox::from_raw(core::ptr::NonNull::from(X::static_vtable()), ptr.cast()) }
                        }
                    }
                });
                continue;
            }

            if ident == "drop_in_place" {
                vtable_ctor.push(quote!(#ident: {
                    #[allow(unsafe_code)]
                    #sig_extern {
                        #[allow(unused_unsafe)]
                        unsafe { ::core::ptr::drop_in_place((#self_call).0 as *mut T) };
                        ::core::alloc::Layout::new::<T>().into()
                    }
                    #ident::<T>
                },));

                drop_impls.push(quote! {
                    #[allow(unsafe_code)]
                    unsafe impl VTableMetaDropInPlace for #vtable_name {
                        unsafe fn #ident(vtable: &Self::VTable, ptr: *mut u8) -> vtable::Layout {
                            // Safety: The vtable is valid and ptr is a type corresponding to the vtable,
                            (vtable.#ident)(VRefMut::from_raw(core::ptr::NonNull::from(vtable), ::core::ptr::NonNull::new_unchecked(ptr).cast()))
                        }
                        unsafe fn dealloc(vtable: &Self::VTable, ptr: *mut u8, layout: vtable::Layout) {
                            (vtable.dealloc)(vtable, ptr, layout)
                        }
                    }
                });
                continue;
            }
            if ident == "dealloc" {
                let abi = &sig_extern.abi;
                vtable_ctor.push(quote!(#ident: {
                    #[allow(unsafe_code)]
                    unsafe #abi fn #ident(_: &#vtable_name, ptr: *mut u8, layout: vtable::Layout) {
                        use ::core::convert::TryInto;
                        unsafe { vtable::internal::dealloc(ptr, layout.try_into().unwrap()) }
                    }
                    #ident
                },));
                continue;
            }

            generated_trait.items.push(TraitItem::Fn(TraitItemFn {
                attrs: field.attrs.clone(),
                sig: sig.clone(),
                default: None,
                semi_token: Some(Default::default()),
            }));

            generated_to_fn_trait.push(ImplItemFn {
                attrs: field.attrs.clone(),
                vis: Visibility::Public(Default::default()),
                defaultness: None,
                sig: sig.clone(),
                block: if has_self {
                    parse_quote!({
                        // Safety: this rely on the vtable being valid, and the ptr being a valid instance for this vtable
                        #[allow(unsafe_code)]
                        unsafe {
                            let vtable = self.vtable.as_ref();
                            if let #some(func) = vtable.#ident {
                                func (#call_code)
                            } else {
                                panic!("Called a not-implemented method")
                            }
                        }
                    })
                } else {
                    // This should never happen: nobody should be able to access the Trait Object directly.
                    parse_quote!({ panic!("Calling Sized method on a Trait Object") })
                },
            });

            if !has_self {
                sig.inputs.insert(
                    0,
                    FnArg::Receiver(Receiver {
                        attrs: Default::default(),
                        reference: Some(Default::default()),
                        mutability: None,
                        self_token: Default::default(),
                        colon_token: None,
                        ty: Box::new(parse_quote!(&Self)),
                    }),
                );
                sig.output = sig_extern.output.clone();
                generated_type_assoc_fn.push(ImplItemFn {
                    attrs: field.attrs.clone(),
                    vis: generated_trait.vis.clone(),
                    defaultness: None,
                    sig,
                    block: parse_quote!({
                        let vtable = self;
                        // Safety: this rely on the vtable being valid, and the ptr being a valid instance for this vtable
                        #[allow(unsafe_code)]
                        unsafe { (self.#ident)(#call_code) }
                    }),
                });

                vtable_ctor.push(quote!(#ident: {
                    #sig_extern {
                        // This is safe since the self must be a instance of our type
                        #[allow(unused)]
                        #[allow(unsafe_code)]
                        let vtable = unsafe { ::core::ptr::NonNull::from(&*_0) };
                        #wrap_trait_call(T::#ident(#self_call #forward_code))
                    }
                    #some(#ident::<T>)
                },));
            } else {
                let erase_return_type_lifetime = match &sig_extern.output {
                    ReturnType::Default => quote!(),
                    // If the return type contains a implicit lifetime, it is safe to erase it while returning it
                    // because a sound implementation of the trait wouldn't allow unsound things here
                    ReturnType::Type(_, r) => {
                        quote!(#[allow(clippy::useless_transmute)] ::core::mem::transmute::<#r, #r>)
                    }
                };
                vtable_ctor.push(quote!(#ident: {
                    #sig_extern {
                        // This is safe since the self must be a instance of our type
                        #[allow(unsafe_code)]
                        unsafe { #erase_return_type_lifetime(T::#ident(#self_call #forward_code)) }
                    }
                    #ident::<T>
                },));
            }
        } else {
            // associated constant

            let generated_trait_assoc_const =
                generated_trait_assoc_const.get_or_insert_with(|| ItemTrait {
                    attrs: Attribute::parse_outer.parse_str(&format!(
                        "/** Trait containing the associated constant relative to the trait {trait_name}.\n{additional_doc} */",
                    )).unwrap(),
                    ident: quote::format_ident!("{}Consts", trait_name),
                    items: Vec::new(),
                    ..generated_trait.clone()
                });

            let const_type = if let Some(o) = field
                .attrs
                .iter()
                .position(|a| a.path().get_ident().map(|a| a == "field_offset").unwrap_or(false))
            {
                let a = field.attrs.remove(o);
                let member_type = match a.parse_args::<Type>() {
                    Err(e) => return e.to_compile_error().into(),
                    Ok(ty) => ty,
                };

                match &field.ty {
                    Type::Path(p) if p.path.get_ident().map(|i| i == "usize").unwrap_or(false) => {}
                    ty => {
                        return Error::new(
                            ty.span(),
                            "The type of an #[field_offset] member in the vtable must be 'usize'",
                        )
                        .to_compile_error()
                        .into();
                    }
                }

                // add `: Sized` to the trait in case it does not have it
                if generated_trait_assoc_const.supertraits.is_empty() {
                    generated_trait_assoc_const.colon_token = Some(Default::default());
                    generated_trait_assoc_const.supertraits.push(parse_quote!(Sized));
                }

                let offset_type: Type = parse_quote!(vtable::FieldOffset<Self, #member_type>);

                vtable_ctor.push(quote!(#ident: T::#ident.get_byte_offset(),));

                let attrs = &field.attrs;

                let vis = &field.vis;
                generated_to_fn_trait.push(
                    parse_quote! {
                        #(#attrs)*
                        #vis fn #ident(&self) -> &#member_type {
                            unsafe {
                                &*(self.ptr.as_ptr().add(self.vtable.as_ref().#ident) as *const #member_type)
                            }
                        }
                    },
                );
                let ident_mut = quote::format_ident!("{}_mut", ident);
                generated_to_fn_trait.push(
                    parse_quote! {
                        #(#attrs)*
                        #vis fn #ident_mut(&mut self) -> &mut #member_type {
                            unsafe {
                                &mut *(self.ptr.as_ptr().add(self.vtable.as_ref().#ident) as *mut #member_type)
                            }
                        }
                    },
                );

                offset_type
            } else {
                vtable_ctor.push(quote!(#ident: T::#ident,));
                field.ty.clone()
            };

            generated_trait_assoc_const.items.push(TraitItem::Const(TraitItemConst {
                attrs: field.attrs.clone(),
                const_token: Default::default(),
                ident: ident.clone(),
                colon_token: Default::default(),
                ty: const_type,
                default: None,
                semi_token: Default::default(),
                generics: Default::default(),
            }));
        };
    }

    let vis = input.vis;
    input.vis = Visibility::Public(Default::default());

    let new_trait_extra = generated_trait_assoc_const.as_ref().map(|x| {
        let i = &x.ident;
        quote!(+ #i)
    });

    let static_vtable_macro_doc = format!(
        r"Instantiate a static {vtable} for a given type and implements `vtable::HasStaticVTable<{vtable}>` for it.

```ignore
// The preview above is misleading because of rust-lang/rust#45939, so it is reproduced below
macro_rules! {macro} {{
    ($(#[$meta:meta])* $vis:vis static $ident:ident for $ty:ty) => {{ ... }}
}}
```

Given a type `MyType` that implements the trait `{trait} {trait_extra}`,
create a static variable of type {vtable},
and implements HasStaticVTable for it.

```ignore
    struct Foo {{ ... }}
    impl {trait} for Foo {{ ... }}
    {macro}!(static FOO_VTABLE for Foo);
    // now VBox::new can be called
    let vbox = VBox::new(Foo{{ ... }});
```

        {extra}",
        vtable = vtable_name,
        trait = trait_name,
        trait_extra = new_trait_extra.as_ref().map(|x| x.to_string()).unwrap_or_default(),
        macro = static_vtable_macro_name,
        extra = additional_doc,
    );

    let result = quote!(
        #[allow(non_snake_case)]
        #[macro_use]
        /// This private module is generated by the `vtable` macro
        mod #module_name {
            #![allow(unused_parens)]
            #[allow(unused)]
            use super::*;
            use ::vtable::*;
            use ::vtable::internal::*;
            #input

            impl #vtable_name {
                // unfortunately cannot be const in stable rust because of the bounds (depends on rfc 2632)
                /// Create a vtable suitable for a given type implementing the trait.
                pub /*const*/ fn new<T: #trait_name #new_trait_extra>() -> Self {
                    Self {
                        #(#vtable_ctor)*
                    }
                }
                #(#generated_type_assoc_fn)*
            }

            #generated_trait
            #generated_trait_assoc_const

            /// Invariant, same as vtable::Inner: vtable and ptr has to be valid and ptr an instance matching the vtable
            #[doc(hidden)]
            #[repr(C)]
            pub struct #to_name {
                vtable: ::core::ptr::NonNull<#vtable_name>,
                ptr: ::core::ptr::NonNull<u8>,
            }
            impl #to_name {
                #(#generated_to_fn_trait)*

                /// Returns a reference to the VTable
                pub fn get_vtable(&self) -> &#vtable_name {
                    unsafe { self.vtable.as_ref() }
                }

                /// Return a raw pointer to the object
                pub fn as_ptr(&self) -> *const u8 {
                    self.ptr.as_ptr()
                }
            }

            unsafe impl VTableMeta for #vtable_name {
                type VTable = #vtable_name;
                type Target = #to_name;
            }

            #(#drop_impls)*

            #[macro_export]
            #[doc = #static_vtable_macro_doc]
            macro_rules! #static_vtable_macro_name {
                ($(#[$meta:meta])* $vis:vis static $ident:ident for $ty:ty) => {
                    $(#[$meta])* $vis static $ident : #vtable_name = {
                        use vtable::*;
                        type T = $ty;
                        #vtable_name {
                            #(#vtable_ctor)*
                        }
                    };
                    #[allow(unsafe_code)]
                    unsafe impl vtable::HasStaticVTable<#vtable_name> for $ty {
                        fn static_vtable() -> &'static #vtable_name {
                            &$ident
                        }
                    }
                }
            }
        }
        #[doc(inline)]
        #[macro_use]
        #vis use #module_name::*;
    );
    //println!("{}", result);
    result.into()
}