Minimal support for Rust type aliases

book/src/extern-rust.md

@@ -163,3 +163,48 @@ mod ffi {
 
 Bounds on a lifetime (like `<'a, 'b: 'a>`) are not currently supported. Nor are
 type parameters or where-clauses.
+
+## Type equivalence across bridges
+
+Similar to type aliases for C++ types, it is possible to create type aliases for
+previously-exported types via the `extern "Rust"` block in another bridge.
+However, current support is very limited:
+
+- The type name must be the same as that of the target type.
+- If the target is in a different C++ namespace, then the namespace must be
+  explicitly specified on the alias, otherwise C++ won't consider the types as
+  equivalent.
+
+Basically, this is enough to import the type from another bridge and nothing more.
+
+In the first module `crate::mod1`, you export the type:
+
+```rust,noplayground
+pub struct MyType {
+    ...
+}
+
+#[cxx::bridge(namespace = "mod1")]
+mod ffi {
+    extern "Rust" {
+        type MyType;
+    }
+}
+```
+
+And in another crate/module `mod2`, you can now import the type and use it as
+a parameter or a return value in C++ and Rust functions:
+
+```rust,noplayground
+#[cxx::bridge(namespace = "mod2")]
+mod ffi {
+    extern "Rust" {
+        #[namespace = "mod1"]
+        type MyType = crate::mod1::MyType;
+    }
+
+    extern "C++" {
+        fn c_func(param: &MyType);
+    }
+}
+```

gen/src/namespace.rs

@@ -6,9 +6,10 @@ impl Api {
         match self {
             Api::CxxFunction(efn) | Api::RustFunction(efn) => &efn.name.namespace,
             Api::CxxType(ety) | Api::RustType(ety) => &ety.name.namespace,
+            Api::TypeAlias(ety) => &ety.name.namespace,
             Api::Enum(enm) => &enm.name.namespace,
             Api::Struct(strct) => &strct.name.namespace,
-            Api::Impl(_) | Api::Include(_) | Api::TypeAlias(_) => Default::default(),
+            Api::Impl(_) | Api::Include(_) => Default::default(),
         }
     }
 }

gen/src/write.rs

@@ -9,8 +9,8 @@ use crate::syntax::set::UnorderedSet;
 use crate::syntax::symbol::{self, Symbol};
 use crate::syntax::trivial::{self, TrivialReason};
 use crate::syntax::{
-    derive, mangle, Api, Doc, Enum, EnumRepr, ExternFn, ExternType, Pair, Signature, Struct, Trait,
-    Type, TypeAlias, Types, Var,
+    derive, mangle, Api, Doc, Enum, EnumRepr, ExternFn, ExternType, Lang, Pair, Signature, Struct,
+    Trait, Type, TypeAlias, Types, Var,
 };
 use proc_macro2::Ident;
 
@@ -35,6 +35,7 @@ pub(super) fn gen(apis: &[Api], types: &Types, opt: &Opt, header: bool) -> Vec<u
 fn write_forward_declarations(out: &mut OutFile, apis: &[Api]) {
     let needs_forward_declaration = |api: &&Api| match api {
         Api::Struct(_) | Api::CxxType(_) | Api::RustType(_) => true,
+        Api::TypeAlias(ety) => ety.lang == Lang::Rust,
         Api::Enum(enm) => !out.types.cxx.contains(&enm.name.rust),
         _ => false,
     };
@@ -54,6 +55,7 @@ fn write_forward_declarations(out: &mut OutFile, apis: &[Api]) {
                 Api::Enum(enm) => write_enum_decl(out, enm),
                 Api::CxxType(ety) => write_struct_using(out, &ety.name),
                 Api::RustType(ety) => write_struct_decl(out, &ety.name),
+                Api::TypeAlias(ety) => write_struct_decl(out, &ety.name),
                 _ => unreachable!(),
             }
         }
@@ -128,8 +130,17 @@ fn write_data_structures<'a>(out: &mut OutFile<'a>, apis: &'a [Api]) {
     out.next_section();
     for api in apis {
         if let Api::TypeAlias(ety) = api {
-            if let Some(reasons) = out.types.required_trivial.get(&ety.name.rust) {
-                check_trivial_extern_type(out, ety, reasons)
+            match ety.lang {
+                Lang::Cxx => {
+                    if let Some(reasons) = out.types.required_trivial.get(&ety.name.rust) {
+                        check_trivial_extern_type(out, ety, reasons)
+                    }
+                }
+                Lang::Rust => {
+                    // nothing to write here, the alias is only used to generate
+                    // forward declaration in C++ (so C++ shims for Rust functions
+                    // using the type compile correctly).
+                }
             }
         }
     }

macro/src/expand.rs

@@ -15,6 +15,7 @@ use crate::{derive, generics};
 use proc_macro2::{Ident, Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::mem;
+use syn::spanned::Spanned;
 use syn::{parse_quote, punctuated, Generics, Lifetime, Result, Token};
 
 pub fn bridge(mut ffi: Module) -> Result<TokenStream> {
@@ -84,10 +85,16 @@ fn expand(ffi: Module, doc: Doc, attrs: OtherAttrs, apis: &[Api], types: &Types)
                 hidden.extend(expand_rust_type_layout(ety, types));
             }
             Api::RustFunction(efn) => hidden.extend(expand_rust_function_shim(efn, types)),
-            Api::TypeAlias(alias) => {
-                expanded.extend(expand_type_alias(alias));
-                hidden.extend(expand_type_alias_verify(alias, types));
-            }
+            Api::TypeAlias(alias) => match alias.lang {
+                syntax::Lang::Cxx => {
+                    expanded.extend(expand_type_alias(alias));
+                    hidden.extend(expand_type_alias_verify(alias, types));
+                }
+                syntax::Lang::Rust => {
+                    expanded.extend(expand_type_alias_rust(alias));
+                    hidden.extend(expand_type_alias_verify_rust(alias));
+                }
+            },
         }
     }
 
@@ -1217,6 +1224,24 @@ fn expand_type_alias(alias: &TypeAlias) -> TokenStream {
     }
 }
 
+fn expand_type_alias_rust(alias: &TypeAlias) -> TokenStream {
+    let doc = &alias.doc;
+    let attrs = &alias.attrs;
+    let visibility = alias.visibility;
+    let _type_token = alias.type_token;
+    let ident = &alias.name.rust;
+    let _generics = &alias.generics;
+    let _eq_token = alias.eq_token;
+    let ty = &alias.ty;
+    let semi_token = alias.semi_token;
+
+    quote! {
+        #doc
+        #attrs
+        #visibility use #ty as #ident #semi_token
+    }
+}
+
 fn expand_type_alias_verify(alias: &TypeAlias, types: &Types) -> TokenStream {
     let ident = &alias.name.rust;
     let type_id = type_id(&alias.name);
@@ -1239,6 +1264,15 @@ fn expand_type_alias_verify(alias: &TypeAlias, types: &Types) -> TokenStream {
     verify
 }
 
+fn expand_type_alias_verify_rust(alias: &TypeAlias) -> TokenStream {
+    let mut ident = alias.name.rust.clone();
+    let span = alias.ty.span();
+    ident.set_span(span);
+    quote_spanned! {span=>
+        const _: fn() = ::cxx::private::verify_rust_type::< #ident >;
+    }
+}
+
 fn type_id(name: &Pair) -> TokenStream {
     let namespace_segments = name.namespace.iter();
     let mut segments = Vec::with_capacity(namespace_segments.len() + 1);

src/lib.rs

@@ -506,7 +506,7 @@ pub mod private {
     pub use crate::rust_str::RustStr;
     #[cfg(feature = "alloc")]
     pub use crate::rust_string::RustString;
-    pub use crate::rust_type::{ImplBox, ImplVec, RustType};
+    pub use crate::rust_type::{ImplBox, ImplVec, RustType, verify_rust_type};
     #[cfg(feature = "alloc")]
     pub use crate::rust_vec::RustVec;
     pub use crate::shared_ptr::SharedPtrTarget;

src/rust_type.rs

@@ -3,3 +3,6 @@
 pub unsafe trait RustType {}
 pub unsafe trait ImplBox {}
 pub unsafe trait ImplVec {}
+
+#[doc(hidden)]
+pub fn verify_rust_type<T: RustType>() {}

syntax/check.rs

@@ -3,7 +3,8 @@ use crate::syntax::report::Errors;
 use crate::syntax::visit::{self, Visit};
 use crate::syntax::{
     error, ident, trivial, Api, Array, Enum, ExternFn, ExternType, Impl, Lang, Lifetimes,
-    NamedType, Ptr, Receiver, Ref, Signature, SliceRef, Struct, Trait, Ty1, Type, TypeAlias, Types,
+    NamedType, Ptr, Receiver, Ref, RustType, Signature, SliceRef, Struct, Trait, Ty1, Type,
+    TypeAlias, Types,
 };
 use proc_macro2::{Delimiter, Group, Ident, TokenStream};
 use quote::{quote, ToTokens};
@@ -500,6 +501,25 @@ fn check_api_type_alias(cx: &mut Check, alias: &TypeAlias) {
         let msg = format!("derive({}) on extern type alias is not supported", derive);
         cx.error(derive, msg);
     }
+
+    if alias.lang == Lang::Rust {
+        let ty = &alias.ty;
+        if let RustType::Path(path) = &ty {
+            // OK, we support path
+            if let Some(last) = path.path.segments.last() {
+                if last.ident != alias.name.rust {
+                    cx.error(
+                        &alias.name.rust,
+                        "`extern \"Rust\"` alias must have the same name as the target type",
+                    );
+                }
+            } else {
+                cx.error(ty, "unsupported `extern \"Rust\"` alias target type");
+            }
+        } else {
+            cx.error(ty, "unsupported `extern \"Rust\"` alias target");
+        }
+    }
 }
 
 fn check_api_impl(cx: &mut Check, imp: &Impl) {

syntax/mod.rs

@@ -154,6 +154,7 @@ pub struct TypeAlias {
     pub visibility: Token![pub],
     pub type_token: Token![type],
     pub name: Pair,
+    pub lang: Lang,
     pub generics: Lifetimes,
     pub eq_token: Token![=],
     pub ty: RustType,

syntax/parse.rs

@@ -874,12 +874,6 @@ fn parse_type_alias(
         },
     ));
 
-    if lang == Lang::Rust {
-        let span = quote!(#type_token #semi_token);
-        let msg = "type alias in extern \"Rust\" block is not supported";
-        return Err(Error::new_spanned(span, msg));
-    }
-
     let visibility = visibility_pub(&visibility, type_token.span);
     let name = pair(namespace, &ident, cxx_name, rust_name);
 
@@ -891,6 +885,7 @@ fn parse_type_alias(
         visibility,
         type_token,
         name,
+        lang,
         generics,
         eq_token,
         ty,

tests/ffi/module.rs

@@ -17,6 +17,10 @@ pub mod ffi {
 
 #[cxx::bridge(namespace = "tests")]
 pub mod ffi2 {
+    extern "Rust" {
+        type R = crate::R;
+    }
+
     unsafe extern "C++" {
         include!("tests/ffi/tests.h");
 
@@ -69,6 +73,8 @@ pub mod ffi2 {
 
         #[namespace = "I"]
         fn ns_c_return_unique_ptr_ns() -> UniquePtr<I>;
+
+        fn c_return_box_from_aliased_rust_type() -> Box<R>;
     }
 
     impl UniquePtr<D> {}

tests/ffi/tests.cc

@@ -72,6 +72,8 @@ rust::Box<R> c_return_box() {
   return rust::Box<R>::from_raw(cxx_test_suite_get_box());
 }
 
+rust::Box<R> c_return_box_from_aliased_rust_type() { return r_return_box(); }
+
 std::unique_ptr<C> c_return_unique_ptr() {
   return std::unique_ptr<C>(new C{2020});
 }

tests/ffi/tests.h

@@ -91,6 +91,7 @@ Shared c_return_shared();
 ::A::AShared c_return_ns_shared();
 ::A::B::ABShared c_return_nested_ns_shared();
 rust::Box<R> c_return_box();
+rust::Box<R> c_return_box_from_aliased_rust_type();
 std::unique_ptr<C> c_return_unique_ptr();
 std::shared_ptr<C> c_return_shared_ptr();
 std::unique_ptr<::H::H> c_return_ns_unique_ptr();

tests/test.rs

@@ -41,6 +41,7 @@ fn test_c_return() {
     assert_eq!(2020, ffi::c_return_primitive());
     assert_eq!(2020, ffi::c_return_shared().z);
     assert_eq!(2020, ffi::c_return_box().0);
+    assert_eq!(2020, ffi2::c_return_box_from_aliased_rust_type().0);
     ffi::c_return_unique_ptr();
     ffi2::c_return_ns_unique_ptr();
     assert_eq!(2020, *ffi::c_return_ref(&shared));

tests/ui/type_alias_rust.stderr

@@ -1,5 +1,5 @@
-error: type alias in extern "Rust" block is not supported
- --> tests/ui/type_alias_rust.rs:5:9
+error: `extern "Rust"` alias must have the same name as the target type
+ --> tests/ui/type_alias_rust.rs:5:14
   |
 5 |         type Alias = crate::Type;
-  |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |              ^^^^^

tests/ui/type_alias_rust2.rs

@@ -0,0 +1,15 @@
+pub mod other_module {
+    pub struct Source {
+        member: u32,
+    }
+}
+
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        // Not allowed - the target is not `extern "Rust"`.
+        type Source = crate::other_module::Source;
+    }
+}
+
+fn main() {}

tests/ui/type_alias_rust2.stderr

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `other_module::Source: RustType` is not satisfied
+  --> tests/ui/type_alias_rust2.rs:11:23
+   |
+11 |         type Source = crate::other_module::Source;
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `RustType` is not implemented for `other_module::Source`
+   |
+note: required by a bound in `verify_rust_type`
+  --> src/rust_type.rs
+   |
+   | pub fn verify_rust_type<T: RustType>() {}
+   |                            ^^^^^^^^ required by this bound in `verify_rust_type`

tests/ui/type_alias_rust3.rs

@@ -0,0 +1,11 @@
+struct Type;
+
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        // Not allowed - the target is not a path.
+        type Source = &crate::Type;
+    }
+}
+
+fn main() {}

tests/ui/type_alias_rust3.stderr

@@ -0,0 +1,5 @@
+error: unsupported `extern "Rust"` alias target
+ --> tests/ui/type_alias_rust3.rs:7:23
+  |
+7 |         type Source = &crate::Type;
+  |                       ^^^^^^^^^^^^