Cannot instantiate C++ std::vec of a simple C++ type due to lack of cxx VectorElement trait implementation.

[johnoneil]

Description

Returning a C++ Vector back to rust only seems to work for basic types (int, float) as they have the trivial VectorElement trait defined.

For more complex types, the VectorElement trait is not defined, despite the fact that (in my reading) cxx describes it as trivial. See here..

We can't define an implementation for VectorElement in external code, as I believe it has to be defined in generated bindings.

I guess this boils down to emitting impl CxxVector<MyType> {} in generated code for bound types. I don't know if this would be trivial or not.

Reproduce

We have a C++ class called TextRun which is exposed to Rust via autocxx:

generate!("TextRun")

I'd like to return a C++ vector of this type to Rust (maybe to manipulate via other bound C++ functions), so we have a helper like what follows:

inline std::vector<TextRun> MakeTextRunList() {
  return {};
}

We also provide a binding to this simple function:

generate!("MakeTextRunList")

The above approach works for basic types, but for this example I get the error:

error[E0277]: the trait bound `root::impeller::TextRun: VectorElement` is not satisfied
  --> /home/johnoneil/code/display-safety/target/debug/build/impeller-rs-ca3ef0ac788db410/out/autocxx-build-dir/rs/autocxx-ffi-default-gen.rs:1:249233
   |
1  | ...94ac7 () -> UniquePtr < CxxVector < TextRun > > ; # [doc = "Synthesized move constructor."] pub unsafe fn impeller_Radians_new_synthet...
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `VectorElement` is not implemented for `root::impeller::TextRun`
   |
   = help: the following other types implement trait `VectorElement`:
             isize
             i8
             i16
             i32
             i64
             usize
             u8
             u16
           and 38 others
   = note: required for `CxxVector<root::impeller::TextRun>` to implement `UniquePtrTarget`
note: required by a bound in `UniquePtr`
  --> /home/johnoneil/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cxx-1.0.107/src/unique_ptr.rs:17:8
   |
15 | pub struct UniquePtr<T>
   |            --------- required by a bound in this struct
16 | where
17 |     T: UniquePtrTarget,
   |        ^^^^^^^^^^^^^^^ required by this bound in `UniquePtr`

Expected Behavior

For simple types i would believe the test code above should build and be useful out of the box. Not sure what the limiting factors would be, but I think for at least simple C++ structs, std::vector should be easily supported.

Additional Context

I can try to provide a test case for this if necessary.

I believe emitting the correct trait might be relatively simple, but I haven't yet delved into autocxx code, so I'm unsure if I could deliver a fix as yet. It might take me some time.

[BabaBert]

It depends on what type of class TextRun is. It is possible that autocxx is unable to generate this for virtual or templated classes. I don't know the exact restrictions, but you might want to write a simple wrapper for TextRun instead.

[johnoneil]

Thanks for your advice @BabaBert . I'll try that approach when possible.

If I have success I'll update this bug and close.

[BabaBert]

Hey @johnoneil, my initial response wasn't quite correct. Your C++ type needs ta Move Constructor. Otherwise, autocxxcan't safely implement VectorElement for you as described here: #1170.
If you can't do that, then a simple wrapper may suffice.
If you ignore this, you will most likely get a segfault down the line.