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Rust Android Gradle Plugin

Cross compile Rust Cargo projects for Android targets.

Usage

Add the plugin to your root build.gradle, like:

buildscript {
    repositories {
        maven {
            url "https://plugins.gradle.org/m2/"
        }
    }
    dependencies {
        classpath 'org.mozilla.rust-android-gradle:plugin:0.9.5'
    }
}

or

buildscript {
    //...
}

plugins {
    id "org.mozilla.rust-android-gradle.rust-android" version "0.9.5"
}

In your project's build.gradle, apply plugin and add the cargo configuration:

android { ... }

apply plugin: 'org.mozilla.rust-android-gradle.rust-android'

cargo {
    module  = "../rust"       // Or whatever directory contains your Cargo.toml
    libname = "rust"          // Or whatever matches Cargo.toml's [package] name.
    targets = ["arm", "x86"]  // See bellow for a longer list of options
}

Install the rust toolchains for your target platforms:

rustup target add armv7-linux-androideabi   # for arm
rustup target add i686-linux-android        # for x86
rustup target add aarch64-linux-android     # for arm64
rustup target add x86_64-linux-android      # for x86_64
rustup target add x86_64-unknown-linux-gnu  # for linux-x86-64
rustup target add x86_64-apple-darwin       # for darwin x86_64 (if you have an Intel MacOS)
rustup target add aarch64-apple-darwin      # for darwin arm64 (if you have a M1 MacOS)
rustup target add x86_64-pc-windows-gnu     # for win32-x86-64-gnu
rustup target add x86_64-pc-windows-msvc    # for win32-x86-64-msvc
...

Finally, run the cargoBuild task to cross compile:

./gradlew cargoBuild

Or add it as a dependency to one of your other build tasks, to build your rust code when you normally build your project:

tasks.whenTaskAdded { task ->
    if ((task.name == 'javaPreCompileDebug' || task.name == 'javaPreCompileRelease')) {
        task.dependsOn 'cargoBuild'
    }
}

Configuration

The cargo Gradle configuration accepts many options.

Linking Java code to native libraries

Generated libraries will be added to the Android jniLibs source-sets, when correctly referenced in the cargo configuration through the libname and/or targetIncludes options. The latter defaults to ["lib${libname}.so", "lib${libname}.dylib", "{$libname}.dll"], so the following configuration will include all libbackend libraries generated in the Rust project in ../rust:

cargo {
    module = "../rust"
    libname = "backend"
}

Now, Java code can reference the native library using, e.g.,

static {
    System.loadLibrary("backend");
}

Native apiLevel

The Android NDK also fixes an API level, which can be specified using the apiLevel option. This option defaults to the minimum SDK API level. As of API level 21, 64-bit builds are possible; and conversely, the arm64 and x86_64 targets require apiLevel >= 21.

Cargo release profile

The profile option selects between the --debug and --release profiles in cargo. Defaults to debug!

Extension reference

module

The path to the Rust library to build with Cargo; required. module can be absolute; if it is not, it is interpreted as a path relative to the Gradle projectDir.

cargo {
    // Note: path is either absolute, or relative to the gradle project's `projectDir`.
    module = '../rust'
}

libname

The library name produced by Cargo; required.

libname is used to determine which native libraries to include in the produced AARs and/or APKs. See also targetIncludes.

libname is also used to determine the ELF SONAME to declare in the Android libraries produced by Cargo. Different versions of the Android system linker depend on the ELF SONAME.

In Cargo.toml:

[lib]
name = "test"

In build.gradle:

cargo {
    libname = 'test'
}

targets

A list of Android targets to build with Cargo; required.

Valid targets for Android are:

'arm',
'arm64',
'x86',
'x86_64'

Valid targets for Desktop are:

'linux-x86-64',
'darwin-x86-64',
'darwin-aarch64',
'win32-x86-64-gnu',
'win32-x86-64-msvc'

The desktop targets are useful for testing native code in Android unit tests that run on the host, not on the target device. Better support for this feature is planned.

cargo {
    targets = ['arm', 'x86', 'linux-x86-64']
}

prebuiltToolchains

When set to true (which requires NDK version 19+), use the prebuilt toolchains bundled with the NDK. When set to false, generate per-target architecture standalone NDK toolchains using make_standalone_toolchain.py. When unset, use the prebuilt toolchains if the NDK version is 19+, and fall back to generated toolchains for older NDK versions.

Defaults to null.

cargo {
    prebuiltToolchains = true
}

verbose

When set, execute cargo build with or without the --verbose flag. When unset, respect the Gradle log level: execute cargo build with or without the --verbose flag according to whether the log level is at least INFO. In practice, this makes ./gradlew ... --info (and ./gradlew ... --debug) execute cargo build --verbose ....

Defaults to null.

cargo {
    verbose = true
}

profile

The Cargo release profile to build.

Defaults to "debug".

cargo {
    profile = 'release'
}

features

Set the Cargo features.

Defaults to passing no flags to cargo.

To pass --all-features, use

cargo {
    features {
        all()
    }
}

To pass an optional list of --features, use

cargo {
    features {
        defaultAnd("x")
        defaultAnd("x", "y")
    }
}

To pass --no-default-features, and an optional list of replacement --features, use

cargo {
    features {
        noDefaultBut()
        noDefaultBut("x")
        noDefaultBut "x", "y"
    }
}

targetDirectory

The target directory into which Cargo writes built outputs. You will likely need to specify this if you are using a cargo virtual workspace, as our default will likely fail to locate the correct target directory.

Defaults to ${module}/target. targetDirectory can be absolute; if it is not, it is interpreted as a path relative to the Gradle projectDir.

Note that if CARGO_TARGET_DIR (see https://doc.rust-lang.org/cargo/reference/environment-variables.html) is specified in the environment, it takes precedence over targetDirectory, as cargo will output all build artifacts to it, regardless of what is being built, or where it was invoked.

You may also override CARGO_TARGET_DIR variable by setting rust.cargoTargetDir in local.properties, however it seems very unlikely that this will be useful, as we don't pass this information to cargo itself. That said, it can be used to control where we search for the built library on a per-machine basis.

cargo {
    // Note: path is either absolute, or relative to the gradle project's `projectDir`.
    targetDirectory = 'path/to/workspace/root/target'
}

targetIncludes

Which Cargo outputs to consider JNI libraries.

Defaults to ["lib${libname}.so", "lib${libname}.dylib", "{$libname}.dll"].

cargo {
    targetIncludes = ['libnotlibname.so']
}

apiLevel

The Android NDK API level to target. NDK API levels are not the same as SDK API versions; they are updated less frequently. For example, SDK API versions 18, 19, and 20 all target NDK API level 18.

Defaults to the minimum SDK version of the Android project's default configuration.

cargo {
    apiLevel = 21
}

You may specify the API level per target in targets using the apiLevels option. At most one of apiLevel and apiLevels may be specified. apiLevels must have an entry for each target in targets.

cargo {
    targets = ["arm", "x86_64"]
    apiLevels = [
        "arm": 16,
        "x86_64": 21,
    ]
}

extraCargoBuildArguments

Sometimes, you need to do things that the plugin doesn't anticipate. Use extraCargoBuildArguments to append a list of additional arguments to each cargo build invocation.

cargo {
    extraCargoBuildArguments = ['a', 'list', 'of', 'strings']
}

exec

This is a callback taking the ExecSpec we're going to use to invoke cargo build, and the relevant toolchain. It's called for each invocation of cargo build. This generally is useful for the following scenarios:

  1. Specifying target-specific environment variables.
  2. Adding target-specific flags to the command line.
  3. Removing/modifying environment variables or command line options the rust-android-gradle plugin would provide by default.
cargo {
    exec { spec, toolchain ->
        if (toolchain.target != "x86_64-apple-darwin") {
            // Don't statically link on macOS desktop builds, for some
            // entirely hypothetical reason.
            spec.environment("EXAMPLELIB_STATIC", "1")
        }
    }
}

Specifying NDK toolchains

The plugin can either use prebuilt NDK toolchain binaries, or search for (and if missing, build) NDK toolchains as generated by make_standalone_toolchain.py.

A prebuilt NDK toolchain will be used if:

  1. rust.prebuiltToolchain=true in the per-(multi-)project ${rootDir}/local.properties
  2. prebuiltToolchain=true in the cargo { ... } block (if not overridden by local.properties)
  3. The discovered NDK is version 19 or higher (if not overridden per above)

The toolchains are rooted in a single Android NDK toolchain directory. In order of preference, the toolchain root directory is determined by:

  1. rust.androidNdkToolchainDir in the per-(multi-)project ${rootDir}/local.properties
  2. the environment variable ANDROID_NDK_TOOLCHAIN_DIR
  3. ${System.getProperty(java.io.tmpdir)}/rust-android-ndk-toolchains

Note that the Java system property java.io.tmpdir is not necessarily /tmp, including on macOS hosts.

Each target architecture toolchain is named like $arch-$apiLevel: for example, arm-16 or arm64-21.

Specifying local targets

When developing a project that consumes rust-android-gradle locally, it's often convenient to temporarily change the set of Rust target architectures. In order of preference, the plugin determines the per-project targets by:

  1. rust.targets.${project.Name} for each project in ${rootDir}/local.properties
  2. rust.targets in ${rootDir}/local.properties
  3. the cargo { targets ... } block in the per-project build.gradle

The targets are split on ','. For example:

rust.targets.library=linux-x86-64
rust.targets=arm,linux-x86-64,darwin

Specifying paths to sub-commands (Python, Cargo, and Rustc)

The plugin invokes Python, Cargo and Rustc. In order of preference, the plugin determines what command to invoke for Python by:

  1. the value of cargo { pythonCommand = "..." }, if non-empty
  2. rust.pythonCommand in ${rootDir}/local.properties
  3. the environment variable RUST_ANDROID_GRADLE_PYTHON_COMMAND
  4. the default, python

In order of preference, the plugin determines what command to invoke for Cargo by:

  1. the value of cargo { cargoCommand = "..." }, if non-empty
  2. rust.cargoCommand in ${rootDir}/local.properties
  3. the environment variable RUST_ANDROID_GRADLE_CARGO_COMMAND
  4. the default, cargo

In order of preference, the plugin determines what command to invoke for rustc by:

  1. the value of cargo { rustcCommand = "..." }, if non-empty
  2. rust.rustcCommand in ${rootDir}/local.properties
  3. the environment variable RUST_ANDROID_GRADLE_RUSTC_COMMAND
  4. the default, rustc

(Note that failure to locate rustc is not fatal, however it may result in rebuilding the code more often than is necessary).

Paths must be host operating system specific. For example, on Windows:

rust.pythonCommand=c:\Python27\bin\python

On Linux,

env RUST_ANDROID_GRADLE_CARGO_COMMAND=$HOME/.cargo/bin/cargo ./gradlew ...

Specifying Rust channel

Rust is released to three different "channels": stable, beta, and nightly (see https://rust-lang.github.io/rustup/concepts/channels.html). The rustup tool, which is how most people install Rust, allows multiple channels to be installed simultaneously and to specify which channel to use by invoking cargo +channel ....

In order of preference, the plugin determines what channel to invoke cargo with by:

  1. the value of cargo { rustupChannel = "..." }, if non-empty
  2. rust.rustupChannel in ${rootDir}/local.properties
  3. the environment variable RUST_ANDROID_GRADLE_RUSTUP_CHANNEL
  4. the default, no channel specified (which cargo installed via rustup generally defaults to the stable channel)

The channel should be recognized by cargo installed via rustup, i.e.:

  • "stable"
  • "beta"
  • "nightly"

A single leading '+' will be stripped, if present.

(Note that Cargo installed by a method other than rustup will generally not understand +channel and builds will likely fail.)

Passing arguments to cargo

The plugin passes project properties named like RUST_ANDROID_GRADLE_target_..._KEY=VALUE through to the Cargo invocation for the given Rust target as KEY=VALUE. Target should be upper-case with "-" replaced by "_". (See the links from this Cargo issue.) So, for example,

project.RUST_ANDROID_GRADLE_I686_LINUX_ANDROID_FOO=BAR

and

./gradlew -PRUST_ANDROID_GRADLE_ARMV7_LINUX_ANDROIDEABI_FOO=BAR ...

and

env ORG_GRADLE_PROJECT_RUST_ANDROID_GRADLE_ARMV7_LINUX_ANDROIDEABI_FOO=BAR ./gradlew ...

all set FOO=BAR in the cargo execution environment (for the "armv7-linux-androideabi` Rust target, corresponding to the "x86" target in the plugin).

Development

At top-level, the publish Gradle task updates the Maven repository under build/local-repo:

$ ./gradlew publish
...
$ ls -al build/local-repo/org/mozilla/rust-android-gradle/org.mozilla.rust-android-gradle.gradle.plugin/0.4.0/org.mozilla.rust-android-gradle.gradle.plugin-0.4.0.pom
-rw-r--r--  1 nalexander  staff  670 18 Sep 10:09
build/local-repo/org/mozilla/rust-android-gradle/org.mozilla.rust-android-gradle.gradle.plugin/0.4.0/org.mozilla.rust-android-gradle.gradle.plugin-0.4.0.pom

Sample projects

The easiest way to get started is to run the sample projects. The sample projects have dependency substitutions configured so that changes made to plugin/ are reflected in the sample projects immediately.

$ ./gradlew -p samples/library :assembleDebug
...
$ file samples/library/build/outputs/aar/library-debug.aar
samples/library/build/outputs/aar/library-debug.aar: Zip archive data, at least v1.0 to extract

$ ./gradlew -p samples/app :assembleDebug
...
$ file samples/app/build/outputs/apk/debug/app-debug.apk
samples/app/build/outputs/apk/debug/app-debug.apk: Zip archive data, at least v?[0] to extract

Testing Local changes

An easy way to locally test changes made in this plugin is to simply add this to your project's settings.gradle:

// Switch this to point to your local plugin dir
includeBuild('../rust-android-gradle') {
    dependencySubstitution {
        // As required.
        substitute module('gradle.plugin.org.mozilla.rust-android-gradle:plugin') with project(':plugin')
    }
}

Publishing

Automatically via the Bump version Github Actions workflow

You will need to be a collaborator. First, manually invoke the Bump version Github Actions workflow. Specify a version (like "x.y.z", without quotes) and a single line changelog entry. (This entry will have a dash prepended, so that it would look normal in a list. This is working around the lack of a multi-line input in Github Actions.) This will push a preparatory commit updating version numbers and the changelog like this one, and make a draft Github Release with a name like vx.y.z. After verifying that tests pass, navigate to the releases panel and edit the release, finally pressing "Publish release". The release Github workflow will build and publish the plugin, although it may take some days for it to be reflected on the Gradle plugin portal.

By hand

You will need credentials to publish to the Gradle plugin portal in the appropriate place for the plugin-publish to find them. Usually, that's in ~/.gradle/gradle.properties.

At top-level, the publishPlugins Gradle task publishes the plugin for consumption:

$ ./gradlew publishPlugins
...
Publishing plugin org.mozilla.rust-android-gradle.rust-android version 0.8.1
Publishing artifact build/libs/plugin-0.8.1.jar
Publishing artifact build/libs/plugin-0.8.1-sources.jar
Publishing artifact build/libs/plugin-0.8.1-javadoc.jar
Publishing artifact build/publish-generated-resources/pom.xml
Activating plugin org.mozilla.rust-android-gradle.rust-android version 0.8.1

Real projects

To test in a real project, use the local Maven repository in your build.gradle, like:

buildscript {
    repositories {
        maven {
            url "file:///Users/nalexander/Mozilla/rust-android-gradle/build/local-repo"
        }
    }

    dependencies {
        classpath 'org.mozilla.rust-android-gradle:plugin:0.9.0'
    }
}