Rust Cross-Compilation for Mobile

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: rust-cross-compile or cargo-ndk.

Targets Cheat Sheet

rustup target list                              # all
rustup target list --installed
rustup target add aarch64-apple-ios aarch64-apple-ios-sim x86_64-apple-ios
rustup target add aarch64-linux-android armv7-linux-androideabi x86_64-linux-android

Cargo.toml — Library for FFI

[package]
name = "bhodl-ffi"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib", "staticlib"]            # both for mobile bundling
name = "bhodl_ffi"

[dependencies]
uniffi = { version = "0.28", features = ["cli"] }
tokio = { version = "1", features = ["rt-multi-thread"] }

[profile.release]
opt-level = 3
lto = true
codegen-units = 1
strip = true                                     # strip symbols for smaller binary
panic = "abort"                                  # smaller; use "unwind" if you catch panics

cdylib → .so (Android), .dylib (macOS). staticlib → .a (iOS, where dynamic libs are restricted).

Android — cargo-ndk

Wraps cargo with the Android NDK toolchain. Replaces older cargo build --target=... + manual env var dance.

# Install
cargo install cargo-ndk

# Set NDK path (or use ANDROID_NDK_HOME env)
export ANDROID_NDK_HOME=$HOME/Android/Sdk/ndk/27.1.12297006

# Build for multiple ABIs
cargo ndk \
    -t arm64-v8a \
    -t armeabi-v7a \
    -t x86_64 \
    -t x86 \
    -o ./jniLibs \                              # output dir
    build --release

# Output structure (matches Android's expected layout):
# jniLibs/
# ├── arm64-v8a/libbhodl_ffi.so
# ├── armeabi-v7a/libbhodl_ffi.so
# ├── x86_64/libbhodl_ffi.so
# └── x86/libbhodl_ffi.so

In Gradle (Android module):

android {
    sourceSets["main"].jniLibs.srcDirs("../rust-ffi/jniLibs")
}

NDK Version Pinning

Specify NDK version in local.properties or via env:

# local.properties
ndk.dir=/Users/me/Library/Android/sdk/ndk/27.1.12297006

Or in app/build.gradle.kts:

android {
    ndkVersion = "27.1.12297006"
}

Android API Level

Set min API level for generated .so:

cargo ndk -t arm64-v8a --platform 26 -o ./jniLibs build --release
# Compatible with Android API 26+ (Android 8.0)

Match this to your Android minSdk.

iOS — Native Cargo

iOS uses staticlib only (Apple disallows dynamic libs in apps).

# Device
cargo build --release --target aarch64-apple-ios

# Simulator (Apple Silicon Mac)
cargo build --release --target aarch64-apple-ios-sim

# Simulator (Intel Mac, legacy)
cargo build --release --target x86_64-apple-ios

# Output:
# target/aarch64-apple-ios/release/libbhodl_ffi.a
# target/aarch64-apple-ios-sim/release/libbhodl_ffi.a
# target/x86_64-apple-ios/release/libbhodl_ffi.a

Universal Simulator Library (lipo)

Combine sim arm64 + sim x86_64 into single .a for unified simulator support:

mkdir -p target/universal/release
lipo -create \
    target/aarch64-apple-ios-sim/release/libbhodl_ffi.a \
    target/x86_64-apple-ios/release/libbhodl_ffi.a \
    -output target/universal/release/libbhodl_ffi.a

# Verify
lipo -info target/universal/release/libbhodl_ffi.a
# → arm64 x86_64

XCFramework (Recommended Distribution)

Bundle device + simulator binaries with headers into single artifact:

mkdir -p Bhodl.xcframework
xcodebuild -create-xcframework \
    -library target/aarch64-apple-ios/release/libbhodl_ffi.a \
        -headers ./bindings/ios/include \
    -library target/universal/release/libbhodl_ffi.a \
        -headers ./bindings/ios/include \
    -output Bhodl.xcframework

Drop Bhodl.xcframework into Xcode project or vendor via Swift Package Manager.

For UniFFI: see languages/uniffi/quick-ref/kmp-bindings.md for the full pipeline.

cross — General-Purpose Cross-Compilation

For Linux/Windows/etc. targets without manual toolchain setup. Uses Docker to provide pre-built sysroots.

# Install
cargo install cross --git https://github.com/cross-rs/cross

# Build for ARM Linux
cross build --target aarch64-unknown-linux-gnu --release

# Build for musl (static, Alpine-friendly)
cross build --target x86_64-unknown-linux-musl --release

# Build for Windows from Linux
cross build --target x86_64-pc-windows-gnu --release

Cross.toml for custom config:

[target.aarch64-unknown-linux-gnu]
image = "ghcr.io/cross-rs/aarch64-unknown-linux-gnu:main"
pre-build = [
    "apt-get update",
    "apt-get install -y libssl-dev:arm64",
]

[build.env]
passthrough = ["CARGO_TERM_COLOR"]

cross requires Docker/Podman. Doesn't work for Apple targets (those need Xcode toolchain on macOS).

C Dependency Pitfalls

Crates depending on C libraries (openssl, sqlite, libsodium) often need extra config when cross-compiling.

OpenSSL

# Avoid system openssl on iOS / Android — use vendored
[dependencies]
openssl = { version = "0.10", features = ["vendored"] }

Or switch to rustls (pure Rust):

reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }

SQLCipher (rusqlite)

[dependencies]
rusqlite = { version = "0.32", features = ["bundled-sqlcipher"] }

bundled-sqlcipher compiles SQLCipher from source, statically linked — no system dep.

libsodium

[dependencies]
dryoc = "0.7"            # pure Rust, no C dependency, easiest to cross-compile

If you must use libsodium-sys:

[dependencies]
libsodium-sys-stable = { version = "1.20", features = ["fetch-latest"] }

The fetch-latest feature builds libsodium from source (no system dep).

Build Profiles for Mobile

[profile.release]
opt-level = "z"          # optimize for size (vs "3" for speed)
lto = true               # link-time optimization
codegen-units = 1        # better optimization, slower build
strip = true             # remove debug symbols
panic = "abort"          # smaller binary; no unwinding stacks

[profile.release-with-debug]
inherits = "release"
debug = true             # for profiling release builds
strip = false

For smallest mobile binary:

[profile.release]
opt-level = "z"
lto = "fat"
codegen-units = 1
strip = "symbols"
panic = "abort"
overflow-checks = false  # don't include checks (default in release)

Trade-off: slower compile, smaller binary, less debugging info.

Linker Configuration

.cargo/config.toml (per-project):

[target.aarch64-linux-android]
linker = "aarch64-linux-android26-clang"        # set by cargo-ndk

[target.aarch64-apple-ios]
rustflags = ["-C", "link-arg=-Wl,-application_extension"]   # for app extensions

[target.x86_64-unknown-linux-musl]
linker = "x86_64-linux-musl-gcc"

cargo-ndk sets the right linker automatically when building Android targets.

Build Caching

sccache (incremental cross-target cache)

cargo install sccache
export RUSTC_WRAPPER=sccache

Speeds up cross-compile when targets share intermediate artifacts.

Target-specific build dirs

# Avoid clobbering host-target cache
cargo ndk ... build --target-dir target/android
cargo build --target aarch64-apple-ios --target-dir target/ios

CI Patterns

GitHub Actions — Multi-Target Build

# .github/workflows/rust-cross.yml
name: Rust cross-compile

on: [push, pull_request]

jobs:
  android:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Setup Rust
        uses: dtolnay/rust-toolchain@stable
        with:
          targets: aarch64-linux-android,armv7-linux-androideabi,x86_64-linux-android

      - name: Setup Android NDK
        uses: nttld/setup-ndk@v1
        with:
          ndk-version: r27c

      - name: Install cargo-ndk
        run: cargo install cargo-ndk

      - name: Cache cargo
        uses: Swatinem/rust-cache@v2

      - name: Build Android libs
        env:
          ANDROID_NDK_HOME: ${{ steps.setup-ndk.outputs.ndk-path }}
        run: |
          cargo ndk -t arm64-v8a -t armeabi-v7a -t x86_64 -o jniLibs build --release

      - uses: actions/upload-artifact@v4
        with:
          name: android-libs
          path: jniLibs/

  ios:
    runs-on: macos-14
    steps:
      - uses: actions/checkout@v4

      - name: Setup Rust
        uses: dtolnay/rust-toolchain@stable
        with:
          targets: aarch64-apple-ios,aarch64-apple-ios-sim,x86_64-apple-ios

      - name: Cache cargo
        uses: Swatinem/rust-cache@v2

      - name: Build iOS libs
        run: |
          cargo build --release --target aarch64-apple-ios
          cargo build --release --target aarch64-apple-ios-sim
          cargo build --release --target x86_64-apple-ios

      - name: Build XCFramework
        run: |
          mkdir -p target/universal/release
          lipo -create \
              target/aarch64-apple-ios-sim/release/libbhodl_ffi.a \
              target/x86_64-apple-ios/release/libbhodl_ffi.a \
              -output target/universal/release/libbhodl_ffi.a

          xcodebuild -create-xcframework \
              -library target/aarch64-apple-ios/release/libbhodl_ffi.a \
                -headers bindings/ios/include \
              -library target/universal/release/libbhodl_ffi.a \
                -headers bindings/ios/include \
              -output Bhodl.xcframework

      - uses: actions/upload-artifact@v4
        with:
          name: ios-xcframework
          path: Bhodl.xcframework/

Build Matrix

strategy:
  matrix:
    include:
      - target: aarch64-linux-android
        runner: ubuntu-latest
        ndk: true
      - target: x86_64-linux-android
        runner: ubuntu-latest
        ndk: true
      - target: aarch64-apple-ios
        runner: macos-14
      - target: x86_64-unknown-linux-musl
        runner: ubuntu-latest
        cross: true

Inspecting Output

# Check archive contents
ar -t libbhodl_ffi.a | head

# Check symbols
nm -gU libbhodl_ffi.a | head             # exported globals
nm -gU libbhodl_ffi.a | grep wallet

# iOS — check architecture
lipo -info libbhodl_ffi.a
# → Architectures in the fat file: libbhodl_ffi.a are: arm64

# Android — check ABI
file libbhodl_ffi.so
# → ELF 64-bit LSB shared object, ARM aarch64

Anti-Patterns

Troubleshooting

When NOT to Use This Skill