servo/components/dom_struct/domobject.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

use quote::{TokenStreamExt, quote};

/// First field of DomObject must be either reflector or another dom_struct,
/// all other fields must not implement DomObject
pub(crate) fn expand_dom_object(
    input: syn::ItemStruct,
    associated_memory: bool,
) -> proc_macro2::TokenStream {
    let fields = input.fields.iter().collect::<Vec<&syn::Field>>();
    let (first_field, fields) = fields
        .split_first()
        .expect("#[dom_struct] should not be applied on empty structs");

    let first_field_name = first_field.ident.as_ref().unwrap();
    let reflector_type = if associated_memory {
        quote! { crate::AssociatedMemory }
    } else {
        quote! { () }
    };
    let mut field_types_and_cfgs = vec![];
    for field in fields {
        if field_types_and_cfgs.contains(&(&field.ty, vec![])) {
            continue;
        }
        let cfgs = field
            .attrs
            .iter()
            .filter(|a| a.path().is_ident("cfg"))
            .collect::<Vec<_>>();
        field_types_and_cfgs.push((&field.ty, cfgs));
    }

    let name = &input.ident;
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
    let items = quote! {
        impl #impl_generics ::js::conversions::ToJSValConvertible for #name #ty_generics #where_clause {
            #[expect(unsafe_code)]
            unsafe fn to_jsval(&self,
                                cx: *mut js::jsapi::JSContext,
                                rval: js::rust::MutableHandleValue) {
                let object = crate::DomObject::reflector(self).get_jsobject();
                object.to_jsval(cx, rval)
            }
        }

        impl #impl_generics crate::DomObject for #name #ty_generics #where_clause {
            type ReflectorType = #reflector_type;

            #[inline]
            fn reflector(&self) -> &crate::Reflector<Self::ReflectorType> {
                self.#first_field_name.reflector()
            }
        }

        impl #impl_generics crate::MutDomObject for #name #ty_generics #where_clause {
            unsafe fn init_reflector<Actual>(&self, obj: *mut js::jsapi::JSObject) {
                self.#first_field_name.init_reflector::<Actual>(obj);
            }
        }

        impl #impl_generics Eq for #name #ty_generics #where_clause {}

        impl #impl_generics PartialEq for #name #ty_generics #where_clause {
            fn eq(&self, other: &Self) -> bool {
                crate::DomObject::reflector(self) == crate::DomObject::reflector(other)
            }
        }
    };

    let mut params = proc_macro2::TokenStream::new();
    params.append_separated(
        input.generics.type_params().map(|param| &param.ident),
        quote! {,},
    );

    let mut dummy_items = quote! {
        // Generic trait with a blanket impl over `()` for all types.
        // becomes ambiguous if impl
        trait NoDomObjectInDomObject<A> {
            // Required for actually being able to reference the trait.
            fn some_item() {}
        }

        impl<T: ?Sized> NoDomObjectInDomObject<()> for T {}

        // Used for the specialized impl when DomObject is implemented.
        #[expect(dead_code)]
        struct Invalid;
        // forbids DomObject
        impl<T> NoDomObjectInDomObject<Invalid> for T where T: ?Sized + crate::DomObject {}
    };

    dummy_items.append_all(
        field_types_and_cfgs
            .iter()
            .enumerate()
            .map(|(i, (ty, cfgs))| {
                let s = syn::Ident::new(&format!("S{i}"), proc_macro2::Span::call_site());
                quote! {
                    struct #s<#params>(#params);

                    impl #impl_generics #s<#params> #where_clause {
                        #(#cfgs)*
                        fn f() {
                            // If there is only one specialized trait impl, type inference with
                            // `_` can be resolved and this can compile. Fails to compile if
                            // ty implements `NoDomObjectInDomObject<Invalid>`.
                            let _ = <#ty as NoDomObjectInDomObject<_>>::some_item;
                        }
                    }
                }
            }),
    );

    let dummy_const = syn::Ident::new(
        &format!("_IMPL_DOMOBJECT_FOR_{}", name),
        proc_macro2::Span::call_site(),
    );
    let tokens = quote! {
        #[expect(non_upper_case_globals)]
        const #dummy_const: () = { #dummy_items };
        #items
    };

    tokens
}