Order sensitivity across namespaces

[dtolnay]

The namespace sorting introduced in #370 does not handle alphabetically lesser namespaces referencing items from alphabetically greater namespaces.

#[cxx::bridge]
mod ffi {
    #[namespace = "first"]
    struct First {
        second: Box<Second>,
    }

    #[namespace = "second"]
    struct Second {
        i: i32,
    }
}

target/debug/build/demo-d74fe843f72e57b6/out/cxxbridge/sources/demo/src/main.rs.cc:154:17: error: ‘::second’ has not been declared
  154 |   ::rust::Box<::second::Second> second;
      |                 ^~~~~~
target/debug/build/demo-d74fe843f72e57b6/out/cxxbridge/sources/demo/src/main.rs.cc:154:31: error: template argument 1 is invalid
  154 |   ::rust::Box<::second::Second> second;
      |                               ^

[adetaylor]

Good spot.

I am assuming that the correct fix is to write out forward declarations across all namespaces, and then iterate all the namespaces again to write everything else. I shall do so when I get a moment.

[adetaylor]

Actually that is nonsense and won't work at all. I need to first look into how sensitive cxx is to definition order irrespective of namespaces.

[adetaylor]

There is order sensitivity without namespaces too:

    struct A {
        b: B
    }

    struct B {
        c: i32
    }

works in normal Rust code, but does not work in a #[cxx::bridge] mod.

cargo:warning=/Users/adetaylor/dev/cxx/target/debug/build/demo-0110b4ddc5238254/out/cxxbridge/sources/demo/src/main.rs.cc:205:21: error: field has incomplete type '::org::example::B'
  cargo:warning=  ::org::example::B b;
  cargo:warning=                    ^
  cargo:warning=/Users/adetaylor/dev/cxx/target/debug/build/demo-0110b4ddc5238254/out/cxxbridge/sources/demo/src/main.rs.cc:189:8: note: forward declaration of 'org::example::B'
  cargo:warning=struct B;
  cargo:warning=       ^
  cargo:warning=1 error generated.

I assume this is a known limitation of what can be put in cxx::bridge mods, but I think it probably makes sense to fix this at the same time by doing a topological sort of all the structs then outputting them in that order. This sounds fun. However, it would be nice to use an algorithm which, instead of sorting into a list of types, sorts into a list of lists of types. Then within each inner list we can continue to sort by namespace to avoid outputting too many zillions of namespace declarations.

[dtolnay]

The "field has incomplete type" issue is #292. I should have used second: Box<Second> for this one; fixed the repro at the top. Topological sort could solve both but I would also accept just forward declarations to close this one.

[adetaylor]

OK cool. I will probably just do the forward declaration thing then. It may not be for a few days; it's proving to be a busy weekend.