Provide Rust interface for libResult by eclipse-impl · Pull Request #403 · eclipse-score/baselibs

eclipse-impl · 2026-07-22T15:30:47Z

With this interface, Rust code can convert C++ score::Result objects to Rust's own std::Result<T, E> transparently.

Note: The PR is on draft state because it's overriding @score_crates until it's officially released. Due to this overriding, also @score_tooling had to be overridden.

github-actions · 2026-07-23T14:40:28Z

The created documentation from the pull request is available at: docu-html

Copilot

Pull request overview

This PR introduces Rust-side bindings to interoperate with the C++ score::Result<T> / score::result::Error API across the FFI boundary, including cxx integration helpers and supporting “ABI-aware” Rust representations of selected C++ standard library types.

Changes:

Add a new result_rs Rust crate implementing an FFI-compatible Expected<T, Error> / Result<T> wrapper plus cxx import macros.
Add a new libcpp + libcpp_derive Rust crates to model ABI-dependent C++ stdlib types (currently std::string_view and std::variant) and provide layout tests.
Wire everything into Bazel and the Cargo workspace, including C++ test helpers and a cxx bridge example.

Reviewed changes

Copilot reviewed 25 out of 27 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
score/result/rust/result.rs	Core Rust FFI types for `score::Result` + cxx integration macros + tests.
score/result/rust/result_example.rs	cxx-bridge example demonstrating passing `score::Result<T>`-like values into Rust.
score/result/rust/result_example_cpp.h	C++ declarations for the cxx example.
score/result/rust/result_example_cpp.cpp	C++ implementation for the cxx example.
score/result/rust/result_cxx.h	cxx relocatability traits for `score::Result` with smart pointers.
score/result/rust/README.md	Documentation for the Rust result bindings and how to build/test them.
score/result/rust/plantuml/sequence.puml	Sequence diagram describing the ABI flow for returning `Result` across FFI.
score/result/rust/ffi_test_helpers.cpp	C++ helpers used by Rust tests to validate layout/behavior across FFI.
score/result/rust/Cargo.toml	Cargo crate definition for `result_rs` with ABI-layout feature flags.
score/result/rust/BUILD	Bazel targets for `result_rs`, docs, unit tests, and examples.
score/language/rust/libcpp/string_view.rs	ABI-dependent `std::string_view` representation + layout tests.
score/language/rust/libcpp/README.md	Rationale and strategy notes for ABI-dependent bindings.
score/language/rust/libcpp/libcpp_test.rs	Simple Rust-side usage tests for imported `std::variant`.
score/language/rust/libcpp/lib.rs	`libcpp` crate root exporting `CStringView` and the import macro.
score/language/rust/libcpp/ffi_test_helpers.cpp	C++ helpers for `std::string_view` layout validation tests.
score/language/rust/libcpp/Cargo.toml	Cargo crate definition for `libcpp`.
score/language/rust/libcpp/BUILD	Bazel targets for `libcpp` + C++ helpers + tests/docs.
score/language/rust/libcpp_derive/unit_test.rs	Rust tests validating `libcpp_derive::import` against C++ variants.
score/language/rust/libcpp_derive/unit_test_helpers.cpp	C++ helpers for `libcpp_derive` variant tests.
score/language/rust/libcpp_derive/lib.rs	Procedural macro generating Rust layouts/APIs for imported `std::variant`.
score/language/rust/libcpp_derive/Cargo.toml	Cargo crate definition for `libcpp_derive`.
score/language/rust/libcpp_derive/BUILD	Bazel build rules for proc-macro crate and tests.
MODULE.bazel	Adds/updates Bazel module deps needed for the Rust/cxx integration.
Cargo.toml	Adds new crates to the workspace members/default-members and deps.
Cargo.lock	Locks new Rust dependencies introduced by the new crates.
.bazelrc	Adjusts test config filters (notably around Rust doc tests).

Suppressed comments (2)

score/result/rust/result.rs:482

Same issue as above: fixed-size array + transmute for the error case is size/layout fragile. Copy bytes into MaybeUninit<Expected<...>> instead.

                let mut error_result_array = [0u8; MAX_SIZE];
                error_result_array
                    .copy_from_slice(&cpp_error_result_slice[..std::mem::size_of::<Expected<i32, Error>>()]);
                let rust_error_result: Expected<i32, Error> = std::mem::transmute(error_result_array);

score/result/rust/result.rs:759

In main_module_tests, the helper functions are also missing #[test], so none of these conversions/round-trip checks are executed in CI.

    #[cfg(all(test, not(miri)))]
    fn test_cpp_result_to_std_result_success() {
        unsafe {

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eclipse-impl · 2026-08-06T13:42:45Z

+std::size_t create_result_int32_error_for_layout_test(int32_t error_code,
+                                                      score::Result<std::int32_t>* out_result,
+                                                      std::size_t bufsize)
+{


eclipse-impl · 2026-08-05T12:02:36Z

+    try
+    {
+        if (should_have_value)
+        {
+            if (!cpp_result->has_value())
+            {
+                return false;
+            }
+            return cpp_result->value() == expected_value_or_error_code;
+        }
+        else
+        {
+            if (cpp_result->has_value())
+            {
+                return false;
+            }
+            // For error case, just check if we can access the error and it matches
+            return *cpp_result->error() == expected_value_or_error_code;
+        }
+    }
+    catch (...)
+    {
+        // If any exception occurs during C++ access, the layout is incompatible
+        return false;
+    }


Although the current repository does not use exceptions, the library can be included on other modules. We cannot control their environment, so it's foreseeable that some of them might have exceptions active. In that case, the lack of the try/catch would be a problem. So, although it's not extrictly necessary for this repository itself, it does not cause any harm for it, while also protecting potential future use-cases. Therefore, we choose to keep it.

eclipse-impl · 2026-08-05T13:23:38Z

+                        pub fn #fn_get(&self) -> &#generic {
+                            assert_eq!(self.discriminant, #discriminant);
+                            unsafe { &self.storage.#value_name }
+                        }
+
+                        pub fn #fn_get_mut(&mut self) ->&mut std::mem::ManuallyDrop<#generic> {
+                            assert_eq!(self.discriminant, #discriminant);
+                            unsafe { &mut self.storage.#value_name }
+                        }


The code does compile. The gist is that C++ does the clean-up, therefore it's not a problem to have a ManuallyDrop<T> here. False positive.

eclipse-impl · 2026-08-05T13:24:31Z

+                    fn get_index(&self) -> usize {
+                        self.discriminant as usize
+                    }


Clean-up is performed on C++ side. False positive.

eclipse-impl · 2026-08-06T13:48:44Z

+//! This module provides FFI-compatible types and functions to interoperate with bmw::Result and
+//! associated types.
+//!
+//! It also offers integration with cxx, allowing type aliases of bmw::Result to be used as types
+//! of arguments or return values of C++ functions and methods. See the documentation of


eclipse-impl · 2026-08-06T13:36:18Z

+        #[cfg(all(test, not(miri)))]
+        fn test_binary_compatibility_sizes() {
+            unsafe {


False positive, it's marked as test, but not under miri.

eclipse-impl · 2026-08-06T13:50:19Z

+This directory contains Rust FFI bindings that enable seamless interoperability
+between C++ and Rust components using BMW's result types.
+
+## Overview
+
+The Rust bindings provide type-safe, zero-cost abstractions for working with:
+
+- **`bmw::Result<T>`** - C++ result type with error handling
+- **`bmw::result::Error`** - Error objects with domain and error codes
+


eclipse-impl · 2026-08-06T14:03:11Z

+A detailed sequence diagram illustrating the complete FFI flow is available in [sequence.puml](plantuml/sequence.puml), and depicted bellow:
+
+![Result C++ to Rust Integration](plantuml/Result_CppToRust_Integration.svg)


eclipse-impl · 2026-08-06T13:53:48Z

+```bash
+# Basic Result example
+bazel run --config=spp_host_gcc //platform/aas/lib/result/rust:result_rs_example
+
+# SafeResult integration
+bazel run --config=spp_host_gcc //platform/aas/lib/result/rust:safe_result_integration_example
+
+# Type mismatch detection
+bazel run --config=spp_host_gcc //platform/aas/lib/result/rust:safe_result_type_mismatch_example
+```
+
+### Run Tests
+
+```bash
+bazel test --config=spp_host_gcc //platform/aas/lib/result/rust:...
+```


eclipse-impl · 2026-08-06T13:57:53Z

+score::result::Error MakeError(ConversionErrorCode code, std::string_view user_message = {}) noexcept
+{
+    return {static_cast<score::result::ErrorCode>(code), conversion_error_domain, user_message};
+}


False positive. This is needed by result.h. The reason why it's being provided on an example is that client code using he lib should provide it, as does the example.

With this interface, Rust code can convert C++ `score::Result` objects to Rust's own `std::Result<T, E>` transparently.

This type of target creates a shell script, intended to run on host. However, the target configuration tries to run the script on target, which fails to run. As agreed with component management, it's fine to excluse these tests from the target runs.

github-project-automation Bot added this to BAS - Baselibs FT Jul 22, 2026

github-project-automation Bot moved this to In Progress in BAS - Baselibs FT Jul 22, 2026

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eclipse-impl requested review from 4og and antonkri as code owners July 30, 2026 07:55

github-actions Bot added comp-result Related to score/result component c++ C++ code rust Rust code bazel Bazel and Starlark build files labels Aug 3, 2026

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eclipse-impl added 4 commits August 5, 2026 16:02

Provide Rust interface for libResult

4b7af74

With this interface, Rust code can convert C++ `score::Result` objects to Rust's own `std::Result<T, E>` transparently.

Mark tests incompatible with Miri as such

da2e31f

Spelling fixes and other small corrections

a292dfa

darkwisebear approved these changes Aug 7, 2026

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		A detailed sequence diagram illustrating the complete FFI flow is available in [sequence.puml](plantuml/sequence.puml), and depicted bellow:

		![Result C++ to Rust Integration](plantuml/Result_CppToRust_Integration.svg)

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