Callbacks: Fn, FnMut, FnOnce

Crubit supports exporting Rust functions that take closures (Fn, FnMut, FnOnce) to C++ via cpp_api_from_rust.

In C++, callable parameters are represented using rs::FnRef (for borrowed callbacks) and rs::Fn (for owned callbacks). C++ callers can satisfy these parameters using C++ lambdas, function pointers, std::function, or absl::AnyInvocable.

For the reverse direction (calling C++ APIs taking absl::AnyInvocable from Rust), see Rust bindings for C++ callbacks.

BUILD configuration

When generating C++ bindings for a Rust library via cpp_api_from_rust:

rust_library(
    name = "rust_api",
    srcs = ["rust_api.rs"],
)

cpp_api_from_rust(
    name = "rust_api_cc",
    crate = ":rust_api",
)

cc_test(
    name = "user_of_rust_api",
    srcs = ["user_of_rust_api.cc"],
    deps = [
        ":rust_api_cc",
        "//testing/base/public:gunit_main",
    ],
)

The complete, tested version is in examples/types/callbacks/BUILD.

Type mapping

Rust functions can take callable parameters as borrowed references, owned boxes, or generic impl Trait parameters:

The generated headers include the necessary support headers automatically:

• rs::FnRef: #include "support/rs_std/fn_ref.h"
• rs::Fn: #include "support/rs_std/fn.h"

Passing callbacks from C++ to Rust

Given a Rust library defining functions that accept closures:

#![allow(unused)]
fn main() {
// In rust_api.rs:

// Borrowed reference: zero allocation
pub fn apply_fn(f: impl Fn(i32) -> i32, x: i32) -> i32 {
    f(x)
}

// Mutable borrowed reference: zero allocation
pub fn apply_fn_mut(mut f: impl FnMut(i32) -> i32, x: i32) -> i32 {
    f(x)
}

// Owned callback: invoked once
pub fn apply_fn_once(f: Box<dyn FnOnce(i32) -> i32>, x: i32) -> i32 {
    f(x)
}
}

C++ callers can pass C++ lambdas, function pointers, std::function, or absl::AnyInvocable:

#include "examples/types/callbacks/rust_api.h"

// 1. Pass a C++ lambda to an `impl Fn` parameter:
int32_t res1 = rust_api::apply_fn([](int32_t x) { return x * 2; }, 21);
// res1 == 42

// 2. Pass an absl::AnyInvocable:
absl::AnyInvocable<int32_t(int32_t) const> cb = [](int32_t x) { return x * 3; };
int32_t res2 = rust_api::apply_fn(cb, 14);
// res2 == 42

// 3. Pass a mutable lambda to `impl FnMut`:
int32_t total = 0;
auto mutable_lambda = [&total](int32_t x) mutable {
  total += x;
  return total;
};
rust_api::apply_fn_mut(mutable_lambda, 10);
rust_api::apply_fn_mut(mutable_lambda, 32);
// total == 42

// 4. Pass a move-only lambda to `Box<dyn FnOnce>`:
auto ptr = std::make_unique<int32_t>(10);
int32_t res3 = rust_api::apply_fn_once(
    [p = std::move(ptr)](int32_t x) { return x + *p; }, 32);
// res3 == 42

rs::FnRef vs. rs::Fn

• rs::FnRef (borrowed): Non-owning view of a callable, similar to absl::FunctionRef. It stores a pointer to the callable and an invoker trampoline. It performs no heap allocation and must not outlive the function call.
• rs::Fn (owned): Owning callable wrapper. It heap-allocates the provided callable and manages its destruction, making it suitable for storing in Rust structs or transferring ownership across threads. rs::Fn<Sig &&> indicates a callable that can only be invoked once; call it using std::move(f)(...).

Converting Rust callbacks to absl::AnyInvocable (rs::DynCallable)

When C++ code receives an owned Rust callback (for example, stored in a Rust struct field exposed to C++), the C++ type is rs::DynCallable<Sig> (support/rs_std/dyn_callable.h).

rs::DynCallable is a move-only functional wrapper around Rust’s Box<dyn Trait>. It is implicitly convertible to absl::AnyInvocable:

rs::DynCallable<void() &&> rust_callback = ...;
absl::AnyInvocable<void() &&> any_invocable = std::move(rust_callback);

Because rs::DynCallable uses an internal representation compatible with absl::AnyInvocable, this conversion is efficient and avoids re-wrapping or additional heap allocations.