macro/src/expand.rs

use crate::syntax::atom::Atom::{self, *};
use crate::syntax::namespace::Namespace;
use crate::syntax::report::Errors;
use crate::syntax::symbol::Symbol;
use crate::syntax::{
    self, check, mangle, Api, Enum, ExternFn, ExternType, Signature, Struct, Type, Types,
};
use proc_macro2::{Ident, Span, TokenStream};
use quote::{format_ident, quote, quote_spanned, ToTokens};
use std::collections::HashSet;
use syn::{parse_quote, Error, ItemMod, Result, Token};

pub fn bridge(namespace: &Namespace, mut ffi: ItemMod) -> Result<TokenStream> {
    let ref mut errors = Errors::new();
    let content = ffi.content.take().ok_or(Error::new(
        Span::call_site(),
        "#[cxx::bridge] module must have inline contents",
    ))?;
    let ref apis = syntax::parse_items(errors, content.1);
    let ref types = Types::collect(errors, apis);
    errors.propagate()?;
    check::typecheck(errors, namespace, apis, types);
    errors.propagate()?;

    Ok(expand(namespace, ffi, apis, types))
}

fn expand(namespace: &Namespace, ffi: ItemMod, apis: &[Api], types: &Types) -> TokenStream {
    let mut expanded = TokenStream::new();
    let mut hidden = TokenStream::new();

    for api in apis {
        if let Api::RustType(ety) = api {
            expanded.extend(expand_rust_type(ety));
            let ident = &ety.ident;
            let span = ident.span();
            hidden.extend(quote_spanned! {span=>
                let _ = ::std::ptr::read::<#ident>;
            });
        }
    }

    let mut enums = HashSet::new();
    for api in apis {
        if let Api::Enum(enm) = api {
            enums.insert(&enm.ident);
        }
    }
    for api in apis {
        match api {
            Api::Include(_) | Api::RustType(_) => {}
            Api::Struct(strct) => expanded.extend(expand_struct(strct)),
            Api::Enum(enm) => expanded.extend(expand_enum(enm)),
            Api::CxxType(ety) => {
                if !enums.contains(&ety.ident) {
                    expanded.extend(expand_cxx_type(ety));
                }
            }
            Api::CxxFunction(efn) => {
                expanded.extend(expand_cxx_function_shim(namespace, efn, types));
            }
            Api::RustFunction(efn) => {
                hidden.extend(expand_rust_function_shim(namespace, efn, types))
            }
        }
    }

    for ty in types {
        if let Type::RustBox(ty) = ty {
            if let Type::Ident(ident) = &ty.inner {
                if Atom::from(ident).is_none() {
                    hidden.extend(expand_rust_box(namespace, ident));
                }
            }
        } else if let Type::RustVec(ty) = ty {
            if let Type::Ident(ident) = &ty.inner {
                if Atom::from(ident).is_none() {
                    hidden.extend(expand_rust_vec(namespace, ident));
                }
            }
        } else if let Type::UniquePtr(ptr) = ty {
            if let Type::Ident(ident) = &ptr.inner {
                if Atom::from(ident).is_none() {
                    expanded.extend(expand_unique_ptr(namespace, ident, types));
                }
            }
        } else if let Type::CxxVector(ptr) = ty {
            if let Type::Ident(ident) = &ptr.inner {
                if Atom::from(ident).is_none() {
                    // Generate impl for CxxVector<T> if T is a struct or opaque
                    // C++ type. Impl for primitives is already provided by cxx
                    // crate.
                    expanded.extend(expand_cxx_vector(namespace, ident));
                }
            }
        }
    }

    // Work around https://github.com/rust-lang/rust/issues/67851.
    if !hidden.is_empty() {
        expanded.extend(quote! {
            #[doc(hidden)]
            const _: () = {
                #hidden
            };
        });
    }

    let attrs = ffi
        .attrs
        .into_iter()
        .filter(|attr| attr.path.is_ident("doc"));
    let vis = &ffi.vis;
    let ident = &ffi.ident;

    quote! {
        #(#attrs)*
        #[deny(improper_ctypes)]
        #[allow(non_snake_case)]
        #vis mod #ident {
            #expanded
        }
    }
}

fn expand_struct(strct: &Struct) -> TokenStream {
    let ident = &strct.ident;
    let doc = &strct.doc;
    let derives = &strct.derives;
    let fields = strct.fields.iter().map(|field| {
        // This span on the pub makes "private type in public interface" errors
        // appear in the right place.
        let vis = Token![pub](field.ident.span());
        quote!(#vis #field)
    });
    quote! {
        #doc
        #[derive(#(#derives),*)]
        #[repr(C)]
        pub struct #ident {
            #(#fields,)*
        }
    }
}

fn expand_enum(enm: &Enum) -> TokenStream {
    let ident = &enm.ident;
    let doc = &enm.doc;
    let variants = enm
        .variants
        .iter()
        .scan(None, |prev_discriminant, variant| {
            let variant_ident = &variant.ident;
            let discriminant = variant
                .discriminant
                .unwrap_or_else(|| prev_discriminant.map_or(0, |n| n + 1));
            *prev_discriminant = Some(discriminant);
            Some(quote! {
                pub const #variant_ident: Self = #ident { repr: #discriminant };
            })
        });
    quote! {
        #doc
        #[derive(Copy, Clone, PartialEq, Eq)]
        #[repr(transparent)]
        pub struct #ident {
            pub repr: u32,
        }

        #[allow(non_upper_case_globals)]
        impl #ident {
            #(#variants)*
        }
    }
}

fn expand_cxx_type(ety: &ExternType) -> TokenStream {
    let ident = &ety.ident;
    let doc = &ety.doc;
    quote! {
        #doc
        #[repr(C)]
        pub struct #ident {
            _private: ::cxx::private::Opaque,
        }
    }
}

fn expand_cxx_function_decl(namespace: &Namespace, efn: &ExternFn, types: &Types) -> TokenStream {
    let ident = &efn.ident;
    let receiver = efn.receiver.iter().map(|receiver| {
        let receiver_type = receiver.ty();
        quote!(_: #receiver_type)
    });
    let args = efn.args.iter().map(|arg| {
        let ident = &arg.ident;
        let ty = expand_extern_type(&arg.ty);
        if arg.ty == RustString {
            quote!(#ident: *const #ty)
        } else if let Type::RustVec(_) = arg.ty {
            quote!(#ident: *const #ty)
        } else if let Type::Fn(_) = arg.ty {
            quote!(#ident: ::cxx::private::FatFunction)
        } else if types.needs_indirect_abi(&arg.ty) {
            quote!(#ident: *mut #ty)
        } else {
            quote!(#ident: #ty)
        }
    });
    let all_args = receiver.chain(args);
    let ret = if efn.throws {
        quote!(-> ::cxx::private::Result)
    } else {
        expand_extern_return_type(&efn.ret, types)
    };
    let mut outparam = None;
    if indirect_return(efn, types) {
        let ret = expand_extern_type(efn.ret.as_ref().unwrap());
        outparam = Some(quote!(__return: *mut #ret));
    }
    let link_name = mangle::extern_fn(namespace, efn);
    let local_name = format_ident!("__{}", ident);
    quote! {
        #[link_name = #link_name]
        fn #local_name(#(#all_args,)* #outparam) #ret;
    }
}

fn expand_cxx_function_shim(namespace: &Namespace, efn: &ExternFn, types: &Types) -> TokenStream {
    let ident = &efn.ident;
    let doc = &efn.doc;
    let decl = expand_cxx_function_decl(namespace, efn, types);
    let receiver = efn.receiver.iter().map(|receiver| {
        let ampersand = receiver.ampersand;
        let mutability = receiver.mutability;
        let var = receiver.var;
        quote!(#ampersand #mutability #var)
    });
    let args = efn.args.iter().map(|arg| quote!(#arg));
    let all_args = receiver.chain(args);
    let ret = if efn.throws {
        let ok = match &efn.ret {
            Some(ret) => quote!(#ret),
            None => quote!(()),
        };
        quote!(-> ::std::result::Result<#ok, ::cxx::Exception>)
    } else {
        expand_return_type(&efn.ret)
    };
    let indirect_return = indirect_return(efn, types);
    let receiver_var = efn
        .receiver
        .iter()
        .map(|receiver| receiver.var.to_token_stream());
    let arg_vars = efn.args.iter().map(|arg| {
        let var = &arg.ident;
        match &arg.ty {
            Type::Ident(ident) if ident == RustString => {
                quote!(#var.as_mut_ptr() as *const ::cxx::private::RustString)
            }
            Type::RustBox(_) => quote!(::std::boxed::Box::into_raw(#var)),
            Type::UniquePtr(_) => quote!(::cxx::UniquePtr::into_raw(#var)),
            Type::RustVec(_) => quote!(#var.as_mut_ptr() as *const ::cxx::private::RustVec<_>),
            Type::Ref(ty) => match &ty.inner {
                Type::Ident(ident) if ident == RustString => {
                    quote!(::cxx::private::RustString::from_ref(#var))
                }
                Type::RustVec(_) => quote!(::cxx::private::RustVec::from_ref(#var)),
                _ => quote!(#var),
            },
            Type::Str(_) => quote!(::cxx::private::RustStr::from(#var)),
            Type::SliceRefU8(_) => quote!(::cxx::private::RustSliceU8::from(#var)),
            ty if types.needs_indirect_abi(ty) => quote!(#var.as_mut_ptr()),
            _ => quote!(#var),
        }
    });
    let vars = receiver_var.chain(arg_vars);
    let trampolines = efn
        .args
        .iter()
        .filter_map(|arg| {
            if let Type::Fn(f) = &arg.ty {
                let var = &arg.ident;
                Some(expand_function_pointer_trampoline(
                    namespace, efn, var, f, types,
                ))
            } else {
                None
            }
        })
        .collect::<TokenStream>();
    let mut setup = efn
        .args
        .iter()
        .filter(|arg| types.needs_indirect_abi(&arg.ty))
        .map(|arg| {
            let var = &arg.ident;
            // These are arguments for which C++ has taken ownership of the data
            // behind the mut reference it received.
            quote! {
                let mut #var = ::std::mem::MaybeUninit::new(#var);
            }
        })
        .collect::<TokenStream>();
    let local_name = format_ident!("__{}", ident);
    let call = if indirect_return {
        let ret = expand_extern_type(efn.ret.as_ref().unwrap());
        setup.extend(quote! {
            let mut __return = ::std::mem::MaybeUninit::<#ret>::uninit();
        });
        if efn.throws {
            setup.extend(quote! {
                #local_name(#(#vars,)* __return.as_mut_ptr()).exception()?;
            });
            quote!(::std::result::Result::Ok(__return.assume_init()))
        } else {
            setup.extend(quote! {
                #local_name(#(#vars,)* __return.as_mut_ptr());
            });
            quote!(__return.assume_init())
        }
    } else if efn.throws {
        quote! {
            #local_name(#(#vars),*).exception()
        }
    } else {
        quote! {
            #local_name(#(#vars),*)
        }
    };
    let expr = if efn.throws {
        efn.ret.as_ref().and_then(|ret| match ret {
            Type::Ident(ident) if ident == RustString => {
                Some(quote!(#call.map(|r| r.into_string())))
            }
            Type::RustBox(_) => Some(quote!(#call.map(|r| ::std::boxed::Box::from_raw(r)))),
            Type::RustVec(_) => Some(quote!(#call.map(|r| r.into_vec()))),
            Type::UniquePtr(_) => Some(quote!(#call.map(|r| ::cxx::UniquePtr::from_raw(r)))),
            Type::Ref(ty) => match &ty.inner {
                Type::Ident(ident) if ident == RustString => {
                    Some(quote!(#call.map(|r| r.as_string())))
                }
                Type::RustVec(_) => Some(quote!(#call.map(|r| r.as_vec()))),
                _ => None,
            },
            Type::Str(_) => Some(quote!(#call.map(|r| r.as_str()))),
            Type::SliceRefU8(_) => Some(quote!(#call.map(|r| r.as_slice()))),
            _ => None,
        })
    } else {
        efn.ret.as_ref().and_then(|ret| match ret {
            Type::Ident(ident) if ident == RustString => Some(quote!(#call.into_string())),
            Type::RustBox(_) => Some(quote!(::std::boxed::Box::from_raw(#call))),
            Type::RustVec(_) => Some(quote!(#call.into_vec())),
            Type::UniquePtr(_) => Some(quote!(::cxx::UniquePtr::from_raw(#call))),
            Type::Ref(ty) => match &ty.inner {
                Type::Ident(ident) if ident == RustString => Some(quote!(#call.as_string())),
                Type::RustVec(_) => Some(quote!(#call.as_vec())),
                _ => None,
            },
            Type::Str(_) => Some(quote!(#call.as_str())),
            Type::SliceRefU8(_) => Some(quote!(#call.as_slice())),
            _ => None,
        })
    }
    .unwrap_or(call);
    let function_shim = quote! {
        #doc
        pub fn #ident(#(#all_args,)*) #ret {
            extern "C" {
                #decl
            }
            #trampolines
            unsafe {
                #setup
                #expr
            }
        }
    };
    match &efn.receiver {
        None => function_shim,
        Some(receiver) => {
            let receiver_type = &receiver.ty;
            quote!(impl #receiver_type { #function_shim })
        }
    }
}

fn expand_function_pointer_trampoline(
    namespace: &Namespace,
    efn: &ExternFn,
    var: &Ident,
    sig: &Signature,
    types: &Types,
) -> TokenStream {
    let c_trampoline = mangle::c_trampoline(namespace, efn, var);
    let r_trampoline = mangle::r_trampoline(namespace, efn, var);
    let local_name = parse_quote!(__);
    let catch_unwind_label = format!("::{}::{}", efn.ident, var);
    let shim = expand_rust_function_shim_impl(
        sig,
        types,
        &r_trampoline,
        local_name,
        catch_unwind_label,
        None,
    );

    quote! {
        let #var = ::cxx::private::FatFunction {
            trampoline: {
                extern "C" {
                    #[link_name = #c_trampoline]
                    fn trampoline();
                }
                #shim
                trampoline as usize as *const ()
            },
            ptr: #var as usize as *const (),
        };
    }
}

fn expand_rust_type(ety: &ExternType) -> TokenStream {
    let ident = &ety.ident;
    quote! {
        use super::#ident;
    }
}

fn expand_rust_function_shim(namespace: &Namespace, efn: &ExternFn, types: &Types) -> TokenStream {
    let ident = &efn.ident;
    let link_name = mangle::extern_fn(namespace, efn);
    let local_name = format_ident!("__{}", ident);
    let catch_unwind_label = format!("::{}", ident);
    let invoke = Some(ident);
    expand_rust_function_shim_impl(
        efn,
        types,
        &link_name,
        local_name,
        catch_unwind_label,
        invoke,
    )
}

fn expand_rust_function_shim_impl(
    sig: &Signature,
    types: &Types,
    link_name: &Symbol,
    local_name: Ident,
    catch_unwind_label: String,
    invoke: Option<&Ident>,
) -> TokenStream {
    let receiver_var = sig
        .receiver
        .as_ref()
        .map(|receiver| quote_spanned!(receiver.var.span=> __self));
    let receiver = sig.receiver.as_ref().map(|receiver| {
        let receiver_type = receiver.ty();
        quote!(#receiver_var: #receiver_type)
    });
    let args = sig.args.iter().map(|arg| {
        let ident = &arg.ident;
        let ty = expand_extern_type(&arg.ty);
        if types.needs_indirect_abi(&arg.ty) {
            quote!(#ident: *mut #ty)
        } else {
            quote!(#ident: #ty)
        }
    });
    let all_args = receiver.into_iter().chain(args);

    let arg_vars = sig.args.iter().map(|arg| {
        let ident = &arg.ident;
        match &arg.ty {
            Type::Ident(i) if i == RustString => {
                quote!(::std::mem::take((*#ident).as_mut_string()))
            }
            Type::RustBox(_) => quote!(::std::boxed::Box::from_raw(#ident)),
            Type::RustVec(_) => quote!(::std::mem::take((*#ident).as_mut_vec())),
            Type::UniquePtr(_) => quote!(::cxx::UniquePtr::from_raw(#ident)),
            Type::Ref(ty) => match &ty.inner {
                Type::Ident(i) if i == RustString => quote!(#ident.as_string()),
                Type::RustVec(_) => quote!(#ident.as_vec()),
                _ => quote!(#ident),
            },
            Type::Str(_) => quote!(#ident.as_str()),
            Type::SliceRefU8(_) => quote!(#ident.as_slice()),
            ty if types.needs_indirect_abi(ty) => quote!(::std::ptr::read(#ident)),
            _ => quote!(#ident),
        }
    });
    let vars = receiver_var.into_iter().chain(arg_vars);

    let mut call = match invoke {
        Some(ident) => match &sig.receiver {
            None => quote!(super::#ident),
            Some(receiver) => {
                let receiver_type = &receiver.ty;
                quote!(#receiver_type::#ident)
            }
        },
        None => quote!(__extern),
    };
    call.extend(quote! { (#(#vars),*) });

    let mut expr = sig
        .ret
        .as_ref()
        .and_then(|ret| match ret {
            Type::Ident(ident) if ident == RustString => {
                Some(quote!(::cxx::private::RustString::from(#call)))
            }
            Type::RustBox(_) => Some(quote!(::std::boxed::Box::into_raw(#call))),
            Type::RustVec(_) => Some(quote!(::cxx::private::RustVec::from(#call))),
            Type::UniquePtr(_) => Some(quote!(::cxx::UniquePtr::into_raw(#call))),
            Type::Ref(ty) => match &ty.inner {
                Type::Ident(ident) if ident == RustString => {
                    Some(quote!(::cxx::private::RustString::from_ref(#call)))
                }
                Type::RustVec(_) => Some(quote!(::cxx::private::RustVec::from_ref(#call))),
                _ => None,
            },
            Type::Str(_) => Some(quote!(::cxx::private::RustStr::from(#call))),
            Type::SliceRefU8(_) => Some(quote!(::cxx::private::RustSliceU8::from(#call))),
            _ => None,
        })
        .unwrap_or(call);

    let mut outparam = None;
    let indirect_return = indirect_return(sig, types);
    if indirect_return {
        let ret = expand_extern_type(sig.ret.as_ref().unwrap());
        outparam = Some(quote!(__return: *mut #ret,));
    }
    if sig.throws {
        let out = match sig.ret {
            Some(_) => quote!(__return),
            None => quote!(&mut ()),
        };
        expr = quote!(::cxx::private::r#try(#out, #expr));
    } else if indirect_return {
        expr = quote!(::std::ptr::write(__return, #expr));
    }

    expr = quote!(::cxx::private::catch_unwind(__fn, move || #expr));

    let ret = if sig.throws {
        quote!(-> ::cxx::private::Result)
    } else {
        expand_extern_return_type(&sig.ret, types)
    };

    let pointer = match invoke {
        None => Some(quote!(__extern: #sig)),
        Some(_) => None,
    };

    quote! {
        #[doc(hidden)]
        #[export_name = #link_name]
        unsafe extern "C" fn #local_name(#(#all_args,)* #outparam #pointer) #ret {
            let __fn = concat!(module_path!(), #catch_unwind_label);
            #expr
        }
    }
}

fn expand_rust_box(namespace: &Namespace, ident: &Ident) -> TokenStream {
    let link_prefix = format!("cxxbridge03$box${}{}$", namespace, ident);
    let link_uninit = format!("{}uninit", link_prefix);
    let link_drop = format!("{}drop", link_prefix);

    let local_prefix = format_ident!("{}__box_", ident);
    let local_uninit = format_ident!("{}uninit", local_prefix);
    let local_drop = format_ident!("{}drop", local_prefix);

    let span = ident.span();
    quote_spanned! {span=>
        #[doc(hidden)]
        #[export_name = #link_uninit]
        unsafe extern "C" fn #local_uninit(
            this: *mut ::std::boxed::Box<::std::mem::MaybeUninit<#ident>>,
        ) {
            ::std::ptr::write(
                this,
                ::std::boxed::Box::new(::std::mem::MaybeUninit::uninit()),
            );
        }
        #[doc(hidden)]
        #[export_name = #link_drop]
        unsafe extern "C" fn #local_drop(this: *mut ::std::boxed::Box<#ident>) {
            ::std::ptr::drop_in_place(this);
        }
    }
}

fn expand_rust_vec(namespace: &Namespace, elem: &Ident) -> TokenStream {
    let link_prefix = format!("cxxbridge03$rust_vec${}{}$", namespace, elem);
    let link_new = format!("{}new", link_prefix);
    let link_drop = format!("{}drop", link_prefix);
    let link_len = format!("{}len", link_prefix);
    let link_data = format!("{}data", link_prefix);
    let link_stride = format!("{}stride", link_prefix);

    let local_prefix = format_ident!("{}__vec_", elem);
    let local_new = format_ident!("{}new", local_prefix);
    let local_drop = format_ident!("{}drop", local_prefix);
    let local_len = format_ident!("{}len", local_prefix);
    let local_data = format_ident!("{}data", local_prefix);
    let local_stride = format_ident!("{}stride", local_prefix);

    let span = elem.span();
    quote_spanned! {span=>
        #[doc(hidden)]
        #[export_name = #link_new]
        unsafe extern "C" fn #local_new(this: *mut ::cxx::private::RustVec<#elem>) {
            ::std::ptr::write(this, ::cxx::private::RustVec::new());
        }
        #[doc(hidden)]
        #[export_name = #link_drop]
        unsafe extern "C" fn #local_drop(this: *mut ::cxx::private::RustVec<#elem>) {
            ::std::ptr::drop_in_place(this);
        }
        #[doc(hidden)]
        #[export_name = #link_len]
        unsafe extern "C" fn #local_len(this: *const ::cxx::private::RustVec<#elem>) -> usize {
            (*this).len()
        }
        #[doc(hidden)]
        #[export_name = #link_data]
        unsafe extern "C" fn #local_data(this: *const ::cxx::private::RustVec<#elem>) -> *const #elem {
            (*this).as_ptr()
        }
        #[doc(hidden)]
        #[export_name = #link_stride]
        unsafe extern "C" fn #local_stride() -> usize {
            ::std::mem::size_of::<#elem>()
        }
    }
}

fn expand_unique_ptr(namespace: &Namespace, ident: &Ident, types: &Types) -> TokenStream {
    let name = ident.to_string();
    let prefix = format!("cxxbridge03$unique_ptr${}{}$", namespace, ident);
    let link_null = format!("{}null", prefix);
    let link_new = format!("{}new", prefix);
    let link_raw = format!("{}raw", prefix);
    let link_get = format!("{}get", prefix);
    let link_release = format!("{}release", prefix);
    let link_drop = format!("{}drop", prefix);

    let new_method = if types.structs.contains_key(ident) {
        Some(quote! {
            fn __new(mut value: Self) -> *mut ::std::ffi::c_void {
                extern "C" {
                    #[link_name = #link_new]
                    fn __new(this: *mut *mut ::std::ffi::c_void, value: *mut #ident);
                }
                let mut repr = ::std::ptr::null_mut::<::std::ffi::c_void>();
                unsafe { __new(&mut repr, &mut value) }
                repr
            }
        })
    } else {
        None
    };

    quote! {
        unsafe impl ::cxx::private::UniquePtrTarget for #ident {
            const __NAME: &'static dyn ::std::fmt::Display = &#name;
            fn __null() -> *mut ::std::ffi::c_void {
                extern "C" {
                    #[link_name = #link_null]
                    fn __null(this: *mut *mut ::std::ffi::c_void);
                }
                let mut repr = ::std::ptr::null_mut::<::std::ffi::c_void>();
                unsafe { __null(&mut repr) }
                repr
            }
            #new_method
            unsafe fn __raw(raw: *mut Self) -> *mut ::std::ffi::c_void {
                extern "C" {
                    #[link_name = #link_raw]
                    fn __raw(this: *mut *mut ::std::ffi::c_void, raw: *mut #ident);
                }
                let mut repr = ::std::ptr::null_mut::<::std::ffi::c_void>();
                __raw(&mut repr, raw);
                repr
            }
            unsafe fn __get(repr: *mut ::std::ffi::c_void) -> *const Self {
                extern "C" {
                    #[link_name = #link_get]
                    fn __get(this: *const *mut ::std::ffi::c_void) -> *const #ident;
                }
                __get(&repr)
            }
            unsafe fn __release(mut repr: *mut ::std::ffi::c_void) -> *mut Self {
                extern "C" {
                    #[link_name = #link_release]
                    fn __release(this: *mut *mut ::std::ffi::c_void) -> *mut #ident;
                }
                __release(&mut repr)
            }
            unsafe fn __drop(mut repr: *mut ::std::ffi::c_void) {
                extern "C" {
                    #[link_name = #link_drop]
                    fn __drop(this: *mut *mut ::std::ffi::c_void);
                }
                __drop(&mut repr);
            }
        }
    }
}

fn expand_cxx_vector(namespace: &Namespace, elem: &Ident) -> TokenStream {
    let name = elem.to_string();
    let prefix = format!("cxxbridge03$std$vector${}{}$", namespace, elem);
    let link_size = format!("{}size", prefix);
    let link_get_unchecked = format!("{}get_unchecked", prefix);
    let unique_ptr_prefix = format!("cxxbridge03$unique_ptr$std$vector${}{}$", namespace, elem);
    let link_unique_ptr_null = format!("{}null", unique_ptr_prefix);
    let link_unique_ptr_raw = format!("{}raw", unique_ptr_prefix);
    let link_unique_ptr_get = format!("{}get", unique_ptr_prefix);
    let link_unique_ptr_release = format!("{}release", unique_ptr_prefix);
    let link_unique_ptr_drop = format!("{}drop", unique_ptr_prefix);

    quote! {
        unsafe impl ::cxx::private::VectorElement for #elem {
            const __NAME: &'static dyn ::std::fmt::Display = &#name;
            fn __vector_size(v: &::cxx::CxxVector<Self>) -> usize {
                extern "C" {
                    #[link_name = #link_size]
                    fn __vector_size(_: &::cxx::CxxVector<#elem>) -> usize;
                }
                unsafe { __vector_size(v) }
            }
            unsafe fn __get_unchecked(v: &::cxx::CxxVector<Self>, pos: usize) -> &Self {
                extern "C" {
                    #[link_name = #link_get_unchecked]
                    fn __get_unchecked(_: &::cxx::CxxVector<#elem>, _: usize) -> *const #elem;
                }
                &*__get_unchecked(v, pos)
            }
            fn __unique_ptr_null() -> *mut ::std::ffi::c_void {
                extern "C" {
                    #[link_name = #link_unique_ptr_null]
                    fn __unique_ptr_null(this: *mut *mut ::std::ffi::c_void);
                }
                let mut repr = ::std::ptr::null_mut::<::std::ffi::c_void>();
                unsafe { __unique_ptr_null(&mut repr) }
                repr
            }
            unsafe fn __unique_ptr_raw(raw: *mut ::cxx::CxxVector<Self>) -> *mut ::std::ffi::c_void {
                extern "C" {
                    #[link_name = #link_unique_ptr_raw]
                    fn __unique_ptr_raw(this: *mut *mut ::std::ffi::c_void, raw: *mut ::cxx::CxxVector<#elem>);
                }
                let mut repr = ::std::ptr::null_mut::<::std::ffi::c_void>();
                __unique_ptr_raw(&mut repr, raw);
                repr
            }
            unsafe fn __unique_ptr_get(repr: *mut ::std::ffi::c_void) -> *const ::cxx::CxxVector<Self> {
                extern "C" {
                    #[link_name = #link_unique_ptr_get]
                    fn __unique_ptr_get(this: *const *mut ::std::ffi::c_void) -> *const ::cxx::CxxVector<#elem>;
                }
                __unique_ptr_get(&repr)
            }
            unsafe fn __unique_ptr_release(mut repr: *mut ::std::ffi::c_void) -> *mut ::cxx::CxxVector<Self> {
                extern "C" {
                    #[link_name = #link_unique_ptr_release]
                    fn __unique_ptr_release(this: *mut *mut ::std::ffi::c_void) -> *mut ::cxx::CxxVector<#elem>;
                }
                __unique_ptr_release(&mut repr)
            }
            unsafe fn __unique_ptr_drop(mut repr: *mut ::std::ffi::c_void) {
                extern "C" {
                    #[link_name = #link_unique_ptr_drop]
                    fn __unique_ptr_drop(this: *mut *mut ::std::ffi::c_void);
                }
                __unique_ptr_drop(&mut repr);
            }
        }
    }
}

fn expand_return_type(ret: &Option<Type>) -> TokenStream {
    match ret {
        Some(ret) => quote!(-> #ret),
        None => TokenStream::new(),
    }
}

fn indirect_return(sig: &Signature, types: &Types) -> bool {
    sig.ret
        .as_ref()
        .map_or(false, |ret| sig.throws || types.needs_indirect_abi(ret))
}

fn expand_extern_type(ty: &Type) -> TokenStream {
    match ty {
        Type::Ident(ident) if ident == RustString => quote!(::cxx::private::RustString),
        Type::RustBox(ty) | Type::UniquePtr(ty) => {
            let inner = expand_extern_type(&ty.inner);
            quote!(*mut #inner)
        }
        Type::RustVec(ty) => {
            let elem = expand_extern_type(&ty.inner);
            quote!(::cxx::private::RustVec<#elem>)
        }
        Type::Ref(ty) => match &ty.inner {
            Type::Ident(ident) if ident == RustString => quote!(&::cxx::private::RustString),
            Type::RustVec(ty) => {
                let inner = expand_extern_type(&ty.inner);
                quote!(&::cxx::private::RustVec<#inner>)
            }
            _ => quote!(#ty),
        },
        Type::Str(_) => quote!(::cxx::private::RustStr),
        Type::SliceRefU8(_) => quote!(::cxx::private::RustSliceU8),
        _ => quote!(#ty),
    }
}

fn expand_extern_return_type(ret: &Option<Type>, types: &Types) -> TokenStream {
    let ret = match ret {
        Some(ret) if !types.needs_indirect_abi(ret) => ret,
        _ => return TokenStream::new(),
    };
    let ty = expand_extern_type(ret);
    quote!(-> #ty)
}