One more plugin to build Rust for Android

There are already several good articles on how to develop on Rust for Android: Building and Deploying a Rust library on Android by Mozilla, and Rust once and share it with Android, iOS, and Flutter by Roberto Huertas.

However, they describe how to build Rust manually (from the command line). That might be not the most convenient way to do it, since it makes the whole compilation and installing a several step process. I would prefer if Rust compilation was done automatically by gradle.

As it happens there is already such plugin: Rust Android Gradle by Mozilla. It is very good, but it misses one thing - it doesn't allow you to configure different compilation options for different gradle buildTypes. For me it is crucial, so I decided to write my own plugin.

Here it is Cargo NDK for Android projects.

Let's see how you can use it. I won't spend much time talking about details that were already covered in the article above. I will only cover the main plugin use cases in detail.

Installing dependencies

I assume that the Rust compiler itself is installed.

To develop for Android you will need Android Studio. It can be downloaded from its official site: Android Studio.

Also, I would suggest installing the Rust plugin: IntelliJ Rust, Toml. Currently (to my opinion) it works better than rls + vscode.

Once Android Studio is installed you also need NDK. Here are official instructions on how to install NDK: Install NDK.

Next step is to add android targets for rust:

rustup target add aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android

And to install cargo-ndk:

cargo install cargo-ndk

cargo-ndk automatically finds and sets all appropriate environment variables to build rust libraries for android. So we won't need to specify them manually in ~/.cargo/config or somewhere else.

We also need to set the NDK_HOME environment variable, as cargo ndk describes it. On my Linux, it is: $HOME/Android/Sdk/ndk-bundle.

Gradle Build Config

I'll assume that you already have a Rust code to build, so here I'll only cover the gradle config.

The plugin is published on the gradle repo, so its usage is more or less straight forward. First in build.gradle we add:

buildscript {
    repositories {
        ...
        maven {
            url "https://plugins.gradle.org/m2/"
        }
    }
    dependencies {
        ...
        classpath "gradle.plugin.com.github.willir.rust:plugin:0.1.1"
    }
}

And then in app/build.gradle:

apply plugin: "com.github.willir.rust.cargo-ndk-android"

And that's it, now the rust library should compile along with the rest Android project.

By default buildDebug gradle task will build dev (debug) cargo profile, and buildRelease will build release cargo profile. It can be changed:

cargoNdk {
    buildTypes {
        release {
            buildType = "release"
        }
        debug {
            buildType = "release"
        }
    }
}

Now, for both buildTypes release profile is going to be built.

The full list of options can be found on the GitHub page: All Options. Any of those options can be overridden for a specific buildType. Let's talk about some features, that might be helpful during development.

Specify target via gradle property

Sometimes, during development, you don't need to build for all targets (arm, arm64), since you only test on one specific phone, so you don't want to spend time waiting for unused targets to be built. In this case, you can specify gradle property rust-target to the target you're testing right now. E.g.: gradle buildDebug -Prust-target=arm64. If rust-target is present plugin will only build for that target.

apiLevel

By default apiLevel equals the android.defaultConfig.minSdkVersion, as far as I can tell, it is the most sensible option. It can be overridden.

However, there is one catch - android supports 64-bit targets only since API 21. So, for example, for API 19 you cannot build arm64 and x86_64. It still makes sense to build arm64 and x86_64, even if your minSdkVersion is 19, they won't work on Android 4.4, but on Android 5.0+ they will work, and that's a lot of phones. So if your apiLevel is 19, and you build arm64, x86_64 targets, the plugin automatically increases apiLevel to 21 for those targets.

That is basically it, I would like if somebody finds this plugin helpful. If you find any bugs, please report them, I'll try to fix them as soon as possible.