Add support for renamed type aliases

book/src/extern-c++.md

@@ -286,6 +286,33 @@ value, and include it in `struct`s that you have declared to `cxx::bridge`. Your
 claim about the triviality of the C++ type will be checked by a `static_assert`
 in the generated C++ side of the binding.
 
+### Handling renamed or aliased types
+
+By default, the name of an Extern C++ type must match the `Id` of the Rust
+type's `ExternType` impl. For example, a `i32` in Rust may only be used as a
+`std::int32_t` in C++.
+
+In the case where a single Rust type maps to multiple C++ types, or a different
+type than in its `ExternType` impl, then you can use the `#[renamed]` attribute
+in the `extern "C++"` block to indicate that the type is intentionally renamed.
+
+For example, if we wanted to call an extern C++ function called `may_fail` that
+returns a Windows-style `HRESULT` and we wanted to use the name `HRESULT` in the
+Rust definition to note that the return value is a result and not a general
+number, then we can add `#[renamed]` to the type alias:
+
+```rust,noplayground
+#[cxx::bridge]
+mod ffi {
+    extern "C++" {
+        #[renamed]
+        type HRESULT = i32;
+
+        fn may_fail() -> HRESULT;
+    }
+}
+```
+
 ## Explicit shim trait impls
 
 This is a somewhat niche feature, but important when you need it.

macro/src/expand.rs

@@ -1222,11 +1222,18 @@ fn expand_type_alias_verify(alias: &TypeAlias, types: &Types) -> TokenStream {
     let type_id = type_id(&alias.name);
     let begin_span = alias.type_token.span;
     let end_span = alias.semi_token.span;
-    let begin = quote_spanned!(begin_span=> ::cxx::private::verify_extern_type::<);
     let end = quote_spanned!(end_span=> >);
 
-    let mut verify = quote! {
-        const _: fn() = #begin #ident, #type_id #end;
+    let mut verify = if alias.is_renamed {
+        let begin = quote_spanned!(begin_span=> ::cxx::private::verify_extern_type_renamed::<);
+        quote! {
+            const _: fn() = #begin #ident #end;
+        }
+    } else {
+        let begin = quote_spanned!(begin_span=> ::cxx::private::verify_extern_type::<);
+        quote! {
+            const _: fn() = #begin #ident, #type_id #end;
+        }
     };
 
     if types.required_trivial.contains_key(&alias.name.rust) {

src/extern_type.rs

@@ -183,6 +183,9 @@ mod private {
 #[doc(hidden)]
 pub fn verify_extern_type<T: ExternType<Id = Id>, Id>() {}
 
+#[doc(hidden)]
+pub fn verify_extern_type_renamed<T: ExternType>() {}
+
 #[doc(hidden)]
 pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind: self::Kind>() {}
 

src/lib.rs

@@ -496,7 +496,9 @@ pub type Vector<T> = CxxVector<T>;
 pub mod private {
     pub use crate::c_char::c_char;
     pub use crate::cxx_vector::VectorElement;
-    pub use crate::extern_type::{verify_extern_kind, verify_extern_type};
+    pub use crate::extern_type::{
+        verify_extern_kind, verify_extern_type, verify_extern_type_renamed,
+    };
     pub use crate::function::FatFunction;
     pub use crate::hash::hash;
     pub use crate::opaque::Opaque;

syntax/attrs.rs

@@ -36,6 +36,7 @@ pub struct Parser<'a> {
     pub cxx_name: Option<&'a mut Option<ForeignName>>,
     pub rust_name: Option<&'a mut Option<Ident>>,
     pub variants_from_header: Option<&'a mut Option<Attribute>>,
+    pub is_renamed: Option<&'a mut bool>,
     pub ignore_unrecognized: bool,
 
     // Suppress clippy needless_update lint ("struct update has no effect, all
@@ -152,6 +153,14 @@ pub fn parse(cx: &mut Errors, attrs: Vec<Attribute>, mut parser: Parser) -> Othe
                 **variants_from_header = Some(attr);
                 continue;
             }
+        } else if attr.path.is_ident("renamed") {
+            if let Err(err) = Nothing::parse.parse2(attr.tokens.clone()) {
+                cx.push(err);
+            }
+            if let Some(is_renamed) = &mut parser.is_renamed {
+                **is_renamed = true;
+                continue;
+            }
         } else if attr.path.is_ident("allow")
             || attr.path.is_ident("warn")
             || attr.path.is_ident("deny")

syntax/mod.rs

@@ -158,6 +158,7 @@ pub struct TypeAlias {
     pub eq_token: Token![=],
     pub ty: RustType,
     pub semi_token: Token![;],
+    pub is_renamed: bool,
 }
 
 pub struct Impl {

syntax/parse.rs

@@ -860,6 +860,7 @@ fn parse_type_alias(
     let mut cxx_name = None;
     let mut rust_name = None;
     let mut attrs = attrs.clone();
+    let mut is_renamed = false;
     attrs.extend(attrs::parse(
         cx,
         unparsed_attrs,
@@ -870,6 +871,7 @@ fn parse_type_alias(
             namespace: Some(&mut namespace),
             cxx_name: Some(&mut cxx_name),
             rust_name: Some(&mut rust_name),
+            is_renamed: Some(&mut is_renamed),
             ..Default::default()
         },
     ));
@@ -895,6 +897,7 @@ fn parse_type_alias(
         eq_token,
         ty,
         semi_token,
+        is_renamed,
     }))
 }
 

tests/ffi/lib.rs

@@ -347,6 +347,7 @@ mod other {
     pub struct D {
         pub d: u64,
     }
+    pub type DRenamed = D;
 
     #[repr(C)]
     pub struct E {

tests/ffi/module.rs

@@ -21,6 +21,8 @@ pub mod ffi2 {
         include!("tests/ffi/tests.h");
 
         type D = crate::other::D;
+        #[renamed]
+        type DRenamed = crate::other::DRenamed;
         type E = crate::other::E;
         #[namespace = "F"]
         type F = crate::other::f::F;
@@ -30,6 +32,9 @@ pub mod ffi2 {
         #[namespace = "H"]
         type H;
 
+        #[renamed]
+        type Int64Alias = i64;
+
         fn c_take_trivial_ptr(d: UniquePtr<D>);
         fn c_take_trivial_ref(d: &D);
         fn c_take_trivial_mut_ref(d: &mut D);
@@ -56,6 +61,7 @@ pub mod ffi2 {
         fn c_return_ns_opaque_ptr() -> UniquePtr<F>;
         fn c_return_ns_unique_ptr() -> UniquePtr<H>;
         fn c_take_ref_ns_c(h: &H);
+        fn c_take_renamed(d: DRenamed, val: Int64Alias);
 
         #[namespace = "other"]
         fn ns_c_take_trivial(d: D);

tests/ffi/tests.cc

@@ -717,6 +717,12 @@ void c_take_opaque_ns_ref(const ::F::F &f) {
   }
 }
 
+void c_take_renamed(DRenamed d, Int64Alias val) {
+  if (d.d == val) {
+    cxx_test_suite_set_correct();
+  }
+}
+
 std::unique_ptr<D> c_return_trivial_ptr() {
   auto d = std::unique_ptr<D>(new D());
   d->d = 30;

tests/ffi/tests.h

@@ -64,6 +64,8 @@ struct D {
   void c_take_trivial_ref_method() const;
   void c_take_trivial_mut_ref_method();
 };
+using DRenamed = D;
+using Int64Alias = uint64_t;
 
 struct E {
   uint64_t e;
@@ -197,6 +199,8 @@ void c_take_trivial_pin_ref(const D &d);
 void c_take_trivial_pin_mut_ref(D &d);
 void c_take_trivial(D d);
 
+void c_take_renamed(D d, Int64Alias val);
+
 void c_take_trivial_ns_ptr(std::unique_ptr<::G::G> g);
 void c_take_trivial_ns_ref(const ::G::G &g);
 void c_take_trivial_ns(::G::G g);

tests/test.rs

@@ -378,3 +378,9 @@ fn test_raw_ptr() {
     assert_eq!(2025, unsafe { ffi::c_take_const_ptr(c3) });
     assert_eq!(2025, unsafe { ffi::c_take_mut_ptr(c3 as *mut ffi::C) }); // deletes c3
 }
+
+#[test]
+fn test_renamed() {
+    let d = ffi2::DRenamed { d: 42 };
+    check!(ffi2::c_take_renamed(d, 42));
+}