Rust-Powered Kotlin Multiplatform: Migrating CPU-Intensive Modules to Rust for Safer, Faster Mobile Apps ‣ 2026-01-03

The move to Rust-Powered Kotlin Multiplatform unlocks a path to safer, faster mobile apps by migrating CPU-intensive modules from Kotlin to Rust; this article lays out a step-by-step strategy that covers tooling (cargo, cbindgen, KMP), FFI patterns, testing, and CI integration so teams can modernize performance-sensitive code with confidence.

Why migrate CPU-heavy code to Rust in a KMP project?

Kotlin Multiplatform (KMP) makes sharing logic between Android and iOS simple, but compute-bound algorithms (signal processing, crypto, ML inference) benefit from Rust’s low-level performance, zero-cost abstractions, and memory-safety guarantees. Migrating these hotspots to Rust reduces latencies and keeps business logic readable in Kotlin while letting Rust own performance-critical implementations.

High-level migration strategy

Follow a phased approach to minimize risk and keep the app runnable during migration:

Identify hotspots with profiling (Systrace, Android Profiler, Instruments).
Extract a minimal, well-specified module interface in Kotlin that will become the Rust FFI boundary.
Implement the core logic in Rust and expose a C-compatible API.
Integrate Rust artifacts into KMP using Gradle / XCFrameworks.
Test end-to-end and validate performance regressions/improvements.
Automate builds and tests in CI for both mobile targets.

Essential tooling and project layout

Tooling makes or breaks the migration. Use these standard tools and layouts:

cargo — Rust package manager; builds libraries for multiple targets.
cbindgen — generates C headers from Rust public types and functions.
Kotlin Multiplatform — integrates native libraries into shared modules (use K/N C interop for iOS and JNI for Android).
bindgen or kotlin-native cinterop — generate Kotlin bindings for the C headers (K/N cinterop uses .def).

Suggested repo layout

/rust-core — Rust crate producing a cdylib/staticlib per target.
/kotlin-shared — KMP module that calls the native functions via C interop/JNI.
/android-app and /ios-app — platform shells for verification and profiling.

Designing the FFI boundary

Design the API to be small, stable, and C-friendly:

Prefer functions that operate on buffers (pointer + length) over complex generics.
Use opaque pointers for complex Rust-managed objects: return a pointer handle and expose explicit free functions to avoid Kotlin attempting to manage Rust memory.
Return results via status codes or out-parameters; for richer errors, design an error struct and an accessor to fetch messages.
Avoid panics crossing the FFI boundary — catch them in Rust and convert to error codes.

FFI pattern examples

Common safe patterns include:

create()/process_buffer()/destroy() using opaque handles
async callbacks: queue results on the Rust side and call a lightweight C callback on completion (mind thread-safety)
zero-copy buffers for large data: share a pointer and size, with ownership rules documented

Generating headers with cbindgen

Use cbindgen to keep C headers in sync with Rust exports:

# Cargo.toml: crate-type = ["cdylib","staticlib"]
# Build and emit header
cbindgen --config cbindgen.toml --crate rust_core --output include/rust_core.h

Keep an automated cbindgen step in your build so Kotlin’s cinterop or JNI layer always uses the latest header.

Integrating Rust into Kotlin Multiplatform

Two mainstream integration flows:

Android (JNI) — build an .so for each ABI and package via Gradle (jniLibs). Use the Kotlin/Java JNI bridge to wrap C signatures in Kotlin-friendly APIs.
iOS (Kotlin/Native) — build an XCFramework (or staticlib + header). Use K/N cinterop: create a .def referencing the generated header and link the library in the KMP iOS target.

KMP Gradle tips

Set up a Gradle task to run cargo build –release –target for each platform and copy outputs into the expected KMP directories.
Version native artifacts and checksum them to avoid accidental mismatches between header and binary.

Testing strategy

Test at multiple levels:

Rust unit tests — run cargo test during local development and CI for algorithmic correctness.
FFI smoke tests — small Kotlin tests that call the C API directly, validating pointer semantics and error handling.
Integration tests — full KMP shared module tests that exercise business logic through the Rust-backed implementation.
Performance benchmarks — microbenchmarks in Rust (criterion) and profiling runs on device to compare before/after.

CI integration: example workflow

Automate reproducible builds and cross-platform tests with GitHub Actions (or your CI of choice):

Step 1: Checkout, set up Rust toolchain (rustup + targets) and install cbindgen.
Step 2: cargo build –release for Android targets (armv7, arm64) and iOS targets (aarch64-apple-ios, x86_64-apple-ios-simulator).
Step 3: Run cargo test and criterion benchmarks.
Step 4: Generate headers via cbindgen and run K/N cinterop generation.
Step 5: Build Kotlin Multiplatform tests (Gradle) and run unit and instrumented tests on emulators or connect to device farms.
Step 6: Archive artifacts (XCFrameworks, .so) and publish to internal artifact repository or GitHub Releases.

Sample GitHub Actions snippet (conceptual)

- name: Build Rust for Android
  run: |
    rustup target add aarch64-linux-android armv7-linux-androideabi
    cargo build --release --target aarch64-linux-android

Keep CI steps idempotent: pin Rust toolchain and Gradle wrapper versions, cache cargo registry, and fail fast on mismatched headers vs binaries.

Debugging, safety, and performance tips

Enable LTO and opt-level = “z” or “s” cautiously; test size vs performance trade-offs.
Use addr2line/backtrace on native crashes; include symbol files in CI artifacts for postmortem analysis.
Sanitize (ASAN) and fuzz (cargo-fuzz) critical Rust modules during development to catch memory/logic errors before mobile integration.
Document ownership and threading rules at the FFI boundary—misunderstandings there cause the hardest bugs.

Practical migration checklist

Profile and select module(s).
Create Rust crate and define C-friendly API surface.
Set up cbindgen and generate headers automatically.
Integrate with KMP via JNI (Android) and cinterop/XCFramework (iOS).
Write Rust unit tests + Kotlin integration tests; run both locally.
Add CI pipeline with cross-target builds and artifact publishing.
Measure before/after performance and iterate on bottlenecks.

Migrating CPU-intensive modules to Rust within a Kotlin Multiplatform project delivers measurable performance and safety improvements when approached methodically: small stable FFI boundaries, automated header generation with cbindgen, rigorous testing in Rust and Kotlin, and robust CI pipelines are the pillars of a successful transition.

Ready to accelerate your KMP app with Rust? Start by profiling a single hotspot, scaffold a minimal Rust crate, and wire up a smoke test — then iterate from there.