This code makes use of some Rust features that are not supported in Rust 1.85 (our currently supported version), so we cannot enable the codegen for this feature yet.
This CL introduces support for [Protobuf Extensions](https://engdoc.corp.google.com/eng/doc/devguide/proto/programming-guides/proto2.html#extensions) to the Rust Protobuf library, a key feature for full Proto2 API parity.
Key changes include:
1. **Code Generation:**
* Added `compiler/rust/extension.cc` and `extension.h` to handle the generation of Rust code for extension definitions.
* Updated `generator.cc` and `message.cc` to invoke extension generation for both top-level and nested extensions.
* Generates `ExtensionId` constants for each extension.
2. **Runtime API (`rust/extension.rs`):**
* Introduced the generic `ExtensionId<Extendee, T>` type to identify and type-check extensions.
* Defined traits for extension operations: `ExtHas`, `ExtGet`, `ExtSet`, `ExtClear`, and `ExtGetMut`.
* Implemented methods on `ExtensionId` that dispatch to the trait implementations provided by the active kernel.
3. **Kernel Implementations:**
* **C++ Kernel (`rust/cpp_kernel/extension.rs`):** Implemented extension traits by calling into new C++ FFI functions (defined in `rust/cpp_kernel/extension.cc`) that interact with the C++ `ExtensionSet`.
* **UPB Kernel (`rust/upb_kernel/extension.rs`):** Implemented extension traits using UPB's MiniTable-based extension API. Extension MiniTables are registered at static initialization time using the `linkme` crate.
4. **Build System:**
* Updated `protobuf/rust/BUILD` and `protobuf/compiler/rust/BUILD` to include new files.
* Added a dependency on `//third_party/rust/linkme/v0_3` for the UPB kernel's extension registration.
5. **Testing:**
* Added `test/extensions.proto` and `test/extensions_separate_file.proto` to define various extension types.
* Added `test/shared/extensions_test.rs` with comprehensive tests covering presence, getters, setters, defaults, nested extensions, submessage extensions, repeated extensions, serialization, and `MergeFrom`.
This change enables Rust users to define and interact with Protobuf extensions in a type-safe manner, compatible with both the C++ and UPB runtimes.
PiperOrigin-RevId: 896506594
This CL contains no functional change, it is a pure splitting of the existing files.
I elected to move the C++ kernel files to `cpp_kernel/` which already contained the C++ API for the C++ kernel. Unfortunately this required removing the existing `third_party/protobuf/rust/cpp_kernel/BUILD` file, because, the Rust parts of the cpp kernel have circular dependencies with the shared code in `third_party/protobuf/rust`.
PiperOrigin-RevId: 886884124
