https://hsivonen.fi/modern-cpp-in-rust/

Since version 56, Firefox has had a new character encoding conversion library called encoding_rs. It is written in Rust and replaced the old C++ character encoding conversion library called uconv that dated from early 1999. Initially, all the callers of the character encoding conversion library were C++ code, so the new library, despite being written in Rust, needed to feel usable when used from C++ code. In fact, the library appears to C++ callers as a modern C++ library. Here are the patterns that I used to accomplish that.

(There is another write-up about encoding_rs itself. I presented most of the content in this write-up in my talk at RustFest Paris: video, slides.)

Modern C++ in What Way?

By “modern” C++ I mean that the interface that C++ callers see conforms to the C++ Core Guidelines and uses certain new features:

• Heap allocations are managed by returning pointers to heap-allocated objects within std::unique_ptr / mozilla::UniquePtr.
• Caller-allocated buffers are represented using gsl::span / mozilla::Span instead of plain pointer and length.
• Multiple return values are represented using std::tuple / mozilla::Tuple instead of out params.
• Non-null plain pointers are annotated using gsl::not_null / mozilla::NotNull.

gsl:: above refers to the Guidelines Support Library, which provides things that the Core Guidelines expect to have available but that are not (yet) in the C++ standard library.

C++ Library in Rust?

By writing a C++ library “in Rust” I mean that the bulk of the library is actually a library written in Rust, but the interface provided to C++ callers makes it look and feel like a real C++ library as far as the C++ callers can tell.

Both C++ and Rust Have C Interop

C++ has a very complex ABI, and the Rust ABI is not frozen. However, both C++ and Rust support functions that use the C ABI. Therefore, interoperability between C++ and Rust involves writing things in such a way that C++ sees Rust code as C code and Rust sees C++ code as C code.

Simplifying Factors

This write-up should not be considered a comprehensive guide to exposing Rust code to C++. The interface to encoding_rs is simple enough that it lacks some complexities that one could expect from the general case of interoperability between the two languages. However, the factors that simplify the C++ exposure of encoding_rs can be taken as a guide to simplifications that one should seek to achieve in the interest of easy cross-language interoperability when designing libraries. Specifically:

• encoding_rs never calls out to C++: The cross-language calls are unidirectional.
• encoding_rs does not hold references to C++ objects after a call returns: There is no need for Rust code to manage C++ memory.
• encoding_rs does not present an inheritance hierarchy either in Rust or in C++: There are no vtables on either side.
• The datatypes that encoding_rs operates on are very simple: Contiguous buffers of primitives (buffers of u8/uint8_t and u16/char16_t).
• Only the panic=abort configuration (i.e. a Rust panic terminates the program instead of unwinding the stack) is supported and the code presented here is only correct if that option is used. The code presented here does not try to prevent Rust panics from unwinding across the FFI, and letting a panic unwind across the FFI is Undefined Behavior. (Update: The default behavior is on track to being changed.)

A Very Quick Look at the API

To get an idea about the Rust API under discussion, let’s take a high-level look. The library has three public structs: Encoding, Decoder and Encoder. From the point of view of the library user, these structs are used like traits, superclasses or interfaces in the sense that they provide a uniform interface to various concrete encodings, but technically they are indeed structs. Instances of Encoding are statically allocated. Decoder and Encoder encapsulate the state of a streaming conversion and are allocated at run-time.

A reference to an Encoding, that is &'static Encoding, can be obtained either from label (textual identification extracted from protocol text) or by a named static. The Encoding can then be used as a factor for a Decoder, which is stack-allocated.

let encoding: &'static Encoding =
    Encoding::for_label( // by label
        byte_slice_from_protocol
    ).unwrap_or(
        WINDOWS_1252     // by named static
    );

let decoder: Decoder =
    encoding.new_decoder();

In the streaming case, a method for decoding from a caller-allocated slice into another caller-allocate slice is available on the Decoder. The decoder performs no heap allocations.

pub enum DecoderResult {
    InputEmpty,
    OutputFull,
    Malformed(u8, u8),
}

impl Decoder {
    pub fn decode_to_utf16_without_replacement(
        &mut self,
        src: &[u8],
        dst: &mut [u16],
        last: bool
    ) -> (DecoderResult, usize, usize)
}

In the non-streaming case, the caller does not need to deal with Decoder and Encoder at all. Instead, methods for handling an entire logical input stream in one buffer are provided on Encoding.

impl Encoding {
    pub fn decode_without_bom_handling_and_without_replacement<'a>(
        &'static self,
        bytes: &'a [u8],
    ) -> Option<Cow<'a, str>>
}

The Process

0. Designing for FFI-friendliness

Some of the simplifying factors arise from the problem domain itself. Others are a matter of choice.

A character encoding library could reasonably present traits (similar to abstract superclasses with no fields in C++) for each of the concepts of an encoding, a decoder and an encoder. Instead, encoding_rs has structs for these that internally match on an enum for dispatch instead of relying on a vtable.

pub struct Decoder { // no vtable
   variant: VariantDecoder,
   // ...
}

enum VariantDecoder { // no extensibility
    SingleByte(SingleByteDecoder),
    Utf8(Utf8Decoder),
    Gb18030(Gb18030Decoder),
    // ...
}

The primary motivation for this wasn’t as much eliminating vtables per se but to make the hierarchy intentionally unextensible. This reflects a philosophy that adding character encodings is not something that programmers should do. Instead, programs should use UTF-8 for interchange, and programs should support legacy encodings only to the extent necessary for compatibility with existing content. The non-extensibility of the hierarchy provides stronger type-safety. If you have an Encoding from encoding_rs, you can trust that it doesn’t exhibit characteristics that aren’t exhibited by the encodings defined in the Encoding Standard. That is, you can trust that it won’t behave like UTF-7 or EBCDIC.

Additionally, by dispatching on an enum, a decoder for one encoding can internally morph into a decoder for another encoding in response to BOM sniffing.

One might argue that the Rustic way to provide encoding converters would be making them into iterator adaptors that consume an iterator of bytes and yield Unicode scalar values or vice versa. In addition to iterators being more complex to expose across the FFI, iterators make it harder to perform tricks to accelerate ASCII processing. Taking a slice to read from and a slice to write to not only makes it easier to represent things in a C API (in C terms, a Rust slice decomposes to an aligned non-null pointer and a length) but also enables ASCII acceleration by processing more than one code unit at a time making use of the observation that multiple code units fit in a single register (either an ALU register or a SIMD register).

If the Rust-native API deals only with primitives, slices and (non-trait object) structs, it is easier to map to a C API than a Rust API that deals with fancier Rust features. (In Rust, you have a trait object when type erasure happens. That is, you have a trait-typed reference that does not say the concrete struct type of the referent that implements the trait.)

1. Creating the C API

When the types involved are simple enough, the main mismatches between C and Rust are the lack of methods and multiple return values in C and the inability to transfer non-C-like structs by value.

• Methods are wrapped by functions whose first argument is a pointer to the struct whose method is being wrapped.
• Slice arguments become two arguments: the pointer to the start of the slice and the length of the slice.
• One primitive value is returned as a function return value and the rest become out params. When the output params clearly relate to inputs of the same type, it makes sense to use in/out params.
• When a Rust method returns the struct by value, the wrapper function boxes it and returns a pointer such that the Rust side forgets about the struct. Additionally, a function for freeing a given struct type by pointer is added. Such a method simply turns pointer back into a box and drops the box. The struct is opaque from the C point of view.
• As a special case, the method for getting the name of an encoding, which in Rust would return &'static str is wrapped by a function that takes a pointer to writable buffer whose length must be at least the length of the longest name.
• enums signaling the exhaustion of the input buffer, the output buffer becoming full or errors with detail about the error became uint32_t with constants for “input empty” and “output full” and rules for how to interpret the other error details. This isn’t ideal but works pretty well in this case.
• Overflow-checking length computations are presented as saturating instead. That is, the caller has to treat SIZE_MAX as a value signaling overflow.

2. Re-Creating the Rust API in C++ over the C API

Even an idiomatic C API doesn’t make for a modern C++ API. Fortunately, Rustic concepts like multiple return values and slices can be represented in C++, and by reinterpreting pointers returned by the C API as pointers to C++ objects, it’s possible to present the ergonomics of C++ methods.

Most of the examples are from a version of the API that uses C++17 standard library types. In Gecko, we generally avoid the C++ standard library and use a version of the C++ API to encoding_rs that uses Gecko-specific types. I assume that the standard-library-type examples make more sense to a broader audience.

Method Ergonomics

For each opaque struct pointer type in C, a class is defined in C++ and the C header is tweaked such that the pointer types become pointers to instances of the C++ classes from the point of view of the C++ compiler. This amounts to a reinterpret_cast of the pointers without actually writing out the reinterpret_cast.

Since the pointers don’t truly point to instances of the classes that they appear to point to but point to instances of Rust structs instead, it’s a good idea to take some precautions. No fields are declared for the classes. The default no-argument and copy constructors are deleted as is the default operator=. Additionally, there must be no virtual methods. (This last point is an important limitation that will come back to later.)

class Encoding final {
// ...
private:
    Encoding() = delete;
    Encoding(const Encoding&) = delete;
    Encoding& operator=(const Encoding&) = delete;
    ~Encoding() = delete;
};

In the case of Encoding whose all instances are static, the destructor is deleted as well. In the case of the dynamically-allocated Decoder and Encoder both an empty destructor and a static void operator delete is added. (An example follows a bit later.) This enables the destruction of the fake C++ class to be routed to the right type-specific freeing function in the C API.

With that foundation in place to materialize pointers that look like pointers to C++ class instances, it’s possible to make method calls on this pointers work. (An example follows after introducing the next concept, too.)

Returning Dynamically-Allocated Objects

As noted earlier, the cases where the Rust API would return an Encoder or a Decoder by value so that the caller can place them on the stack is replaced by the FFI wrapper boxing the objects so that the C API exposes only heap-allocated objects by pointer. Also, the reinterpretation of these pointers as deleteable C++ object pointers was already covered.

That still leaves making sure that delete is actually used at an appropriate time. In modern C++, when an object can have only one legitimate owner of the time, this is accomplished by wrapping the object pointer in std::unique_ptr or mozilla::UniquePtr. The old uconv converters supported reference counting, but all the actual uses in the Gecko code base involved only one owner for each converter. Since the usage patterns of encoders and decoders are such that there is only one legitimate owner of the time, using std::unique_ptr and mozilla::UniquePtr is what the two C++ wrappers for encoding_rs do.

Let’s take a look at a factory method on Encoding that returns a Decoder. In Rust, we have a method that takes a reference to self and returns Decoder by value.

impl Encoding {
    pub fn new_decoder(&'static self) -> Decoder {
        // ...
    }
}

On the FFI layer, we have an explicit pointer-typed first argument that corresponds to Rust &self and C++ this (specifically, the const version of this). We allocate memory on the heap (Box::new()) and place the Decoder into the allocated memory. We then forget about the allocation (Box::into_raw) so that we can return the pointer to C without deallocating at the end of the scope. In order to be able to free the memory, we introduce a new function that puts the Box back together and assigns it into a variable that immediately goes out of scope causing the heap allocation to be freed.

#[no_mangle]
pub unsafe extern "C" fn encoding_new_decoder(
    encoding: *const Encoding) -> *mut Decoder
{
    Box::into_raw(Box::new((*encoding).new_decoder()))
}

#[no_mangle]
pub unsafe extern "C" fn decoder_free(decoder: *mut Decoder) {
    let _ = Box::from_raw(decoder);
}

In the C header, they look like this:

ENCODING_RS_DECODER*
encoding_new_decoder(ENCODING_RS_ENCODING const* encoding);

void
decoder_free(ENCODING_RS_DECODER* decoder);

ENCODING_RS_DECODER is a macro that is used for substituting the right C++ type when the C header is used in the C++ context instead of being used as a plain C API.

On the C++ side, then, we use std::unique_ptr, which is the C++ analog of Rust’s Box. They are indeed very similar:

let ptr: Box<Foo>

std::unique_ptr<Foo> ptr

Box::new(Foo::new(a, b, c))

make_unique<Foo>(a, b, c)

Box::into_raw(ptr)

ptr.release()

let ptr = Box::from_raw(raw_ptr);

std::unique_ptr<Foo> ptr(raw_ptr);

We wrap the pointer obtained from the C API in a std::unique_ptr:

class Encoding final {
public:
    inline std::unique_ptr<Decoder> new_decoder() const
    {
        return std::unique_ptr<Decoder>(
            encoding_new_decoder(this));
    }
};

When the std::unique_ptr<Decoder> goes out of scope, the deletion is routed back to Rust via FFI thanks to declarations like this:

class Decoder final {
public:
    ~Decoder() {}
    static inline void operator delete(void* decoder)
    {
        decoder_free(reinterpret_cast<Decoder*>(decoder));
    }
private:
    Decoder() = delete;
    Decoder(const Decoder&) = delete;
    Decoder& operator=(const Decoder&) = delete;
};

How Can it Work?

In Rust, non-trait methods are just syntactic sugar:

impl Foo {
    pub fn get_val(&self) -> usize {
        self.val
    }
}

fn test(bar: Foo) {
    assert_eq!(bar.get_val(), Foo::get_val(&bar));
}

A method call on non-trait-typed reference is just a plain function call with the reference to self as the first argument. On the C++ side, non-virtual method calls work the same way: A non-virtual C++ method call is really just a function call whose first argument is the this pointer.

On the FFI/C layer, we can pass the same pointer as an explicit pointer-typed first argument.

When calling ptr->Foo() where ptr is of type T*, the type of this is T* if the method is declared as void Foo() (which maps to &mut self in Rust) and const T* if the method is declared as void Foo() const (which maps to &self in Rust), so const-correctness is handled, too.

fn foo(&self, bar: usize) -> usize

size_t foo(size_t bar) const

fn foo(&mut self, bar: usize) -> usize

size_t foo(size_t bar)

The qualifications about “non-trait-typed” and “non-virtual” are important. For the above to work, we can’t have vtables on either side. This means no Rust trait objects and no C++ inheritance. In Rust, trait objects, i.e. trait-typed references to any struct that implements the trait, are implemented as two pointers: one to the struct instance and another to the vtable appropriate for the concrete type of the data. We need to be able to pass reference to self across the FFI as a single pointer, so there’s no place for the vtable pointer when crossing the FFI. In order to keep pointers to C++ objects as C-compatible plain pointers, C++ puts the vtable pointer on the objects themselves. Since the pointers don’t really point to C++ objects carrying vtable pointers but point to Rust objects, we must make sure not to make the C++ implementation expect to find a vtable pointer on the pointee.

As a consequence, the C++ reflector classes for the Rust structs cannot inherit from a common baseclass of a C++ framework. In the Gecko case, the reflector classes cannot inherit from nsISupports. E.g. in the context of Qt, the reflector classes wouldn’t be able to inherit from QObject.

Non-Nullable Pointers

There are methods in the Rust API that return &'static Encoding. Rust references can never be null, and it would be nice to relay this piece of information in the C++ API. It turns out that there is a C++ idiom for this: gsl::not_null and mozilla::NotNull.

Since gsl::not_null and mozilla::NotNull are just type system-level annotations that don’t change the machine representation of the underlying pointer and since from the guarantees Rust we know which pointers that we get from the FFI really never are null, it is tempting to apply the same reinterpretation trick of lying to the C++ compiler about types that we use to reinterpret pointers returned by the FFI as pointers to fieldless C++ objects with no virtual methods and to claim in a header file that the pointers that we know not to be null in the FFI return values are of the type mozilla::NotNull<const Encoding*>. Unfortunately, this doesn’t actually work because types involving templates are not allowed in the declarations of extern "C" functions in C++, so the C++ code ends up executing a branch for the null check when wrapping pointers received from the C API with gsl::not_null or mozilla::NotNull.

However, there are also declarations of static pointers to the constant encoding objects (where the pointees are defined in Rust) and it happens that C++ does allow declaring those as gsl::not_null<const Encoding*>, so that is what is done. (Thanks to Masatoshi Kimura for pointing out that this is possible.)

The statically-allocated instances of Encoding are declared in Rust like this:

pub static UTF_8_INIT: Encoding = Encoding {
    name: "UTF-8",
    variant: VariantEncoding::Utf8,
};

pub static UTF_8: &'static Encoding = &UTF_8_INIT;

In Rust, the general rule is that you use static for an unchanging memory location and const for an unchanging value. Therefore, UTF_8_INIT should be static and UTF_8 should be const: the value of the reference to the static instance is unchanging, but statically allocating a memory location for the reference is not logically necessary. Unfortunately, Rust has a rule that says that the right-hand side of const may not contain anything static and this is applied so heavily as to prohibit even references to static, in order to ensure that the right-hand side of a const declaration can be statically checked to be suitable for use within any imaginable const declaration—even one that tried to dereference the reference at compile time.

For FFI, though, we need to allocate an unchanging memory location to a pointer to UTF_8_INIT, because such memory locations work in C linkage and allow us provide a pointer-typed named thing to C. The representation of UTF_8 above is already what we need, but for Rust ergonomics, we want UTF_8 to participate in Rust’s crate namespacing. This means that from the C perspective the name gets mangled. We waste some space by statically allocating pointers again without name mangling for C usage:

pub struct ConstEncoding(*const Encoding);

unsafe impl Sync for ConstEncoding {}

#[no_mangle]
pub static UTF_8_ENCODING: ConstEncoding =
    ConstEncoding(&UTF_8_INIT);

A pointer type is used to make in clear that C is supposed to see a pointer (even if a Rust reference type would have the same representation). However, the Rust compiler refuses to compile a program with globally-visible pointer. Since globals are reachable from different threads, multiple threads accessing the pointee might be problem. In this case, the pointee cannot be mutated, so global visibility is fine. To tell the compiler that this is fine, we need to implement the Sync marker trait for the pointer. However, traits cannot be implemented on pointer types. As a workaround, we create a newtype for *const Encoding. A newtype has the same representation as the type it wraps, but we can implement traits on the newtype. Implementing Sync is unsafe, because we are asserting to the compiler that something is OK when the compiler does not figure it out on its own.

In C++, we can then say (what via macros expands to):

extern "C" {
    extern gsl::not_null<const encoding_rs::Encoding*> const UTF_8_ENCODING;
}

The pointers to the encoders and decoders are also known not to be null, since allocation failure would terminate the program, but std::unique_ptr / mozilla::UniquePtr and gsl::not_null / mozilla::NotNull cannot be combined.

Optional Values

In Rust, it’s idiomatic to use Option<T> to represent return values might either have a value or might not have a value. C++ these days provides the same thing as std::optional<T>. In Gecko, we instead have mozilla::Maybe<T>.

Rust’s Option<T> and C++’s std::optional<T> indeed are basically the same thing:

return None;

return std::nullopt;

return Some(foo);

return foo;

is_some()

operator bool()

has_value()

unwrap()

value()

unwrap_or(bar)

value_or(bar)

Unfortunately, though, C++ reverses the safety ergonomics. The most ergonomic way to extract the wrapped value from a std::optional<T> is via operator*(), which is unchecked and, therefore, unsafe.