dtolnay/cxx

Duplicate const generated on cxx::bridge file in C++

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#830 opened on Apr 13, 2021

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 (2 comments) (0 reactions) (0 assignees)Rust (4,472 stars) (253 forks)batch import
bughelp wanted

Description

Thanks for your work on this useful library! Right now I'm using cxx::bridge to provide Elixir bindings for PyTorch via Rustler (https://github.com/andfoy/extorch). I'm trying to use a rust::Slice with std::shared_ptr<CrossTensor>, where CrossTensor is a C++ opaque type as follows:

/// Index a tensor using a list of tensors.
fn index(
    tensor: &SharedPtr<CrossTensor>,
    indices: &[&SharedPtr<CrossTensor>]
) -> Result<SharedPtr<CrossTensor>>;

Where index has the following signature:

std::shared_ptr<CrossTensor> index(
    const std::shared_ptr<CrossTensor> &tensor,
    rust::Slice<const std::shared_ptr<CrossTensor>&> indices);

When I try to compile the library, it seems that the code generator is producing a bridge signature with duplicate const keywords, which causes compilation errors:

::rust::repr::PtrLen cxxbridge1$index(const ::std::shared_ptr<::CrossTensor> &tensor, ::rust::Slice<const const ::std::shared_ptr<::CrossTensor> &> indices, ::std::shared_ptr<::CrossTensor> *return$) noexcept

I don't know if the &[&SharedPtr<CrossTensor>] declaration is malformed or if I should not use a slice in this case. I would appreciate any help with this issue.

Contributor guide

Tech stack
rustcpp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
RustC++cxx basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Investigate the code generation logic in the cxx crate, specifically how function signatures with rust::Slice are handled. Look at the file that generates the C++ bridge code (e.g., src/gen/write.rs or similar). The issue shows that using &[&SharedPtr<T>] leads to duplicate const; the generator needs to avoid adding const when the inner type is already a reference. Check if there is existing handling for nested references and fix the condition that adds const.