rust: Add a README explaining how to build the bindings

rust-sdk/README.md

@@ -0,0 +1,96 @@
+# Kotlin bindings for the Rust SDK crypto layer.
+
+## Prerequisites
+
+### Rust
+
+To build the bindings [Rust] will be needed it can be either installed using an
+OS specific package manager or directly with the provided [installer](https://rustup.rs/).
+
+
+### Android NDK
+
+The Android NDK will be required as well, it can be installed either through
+Android Studio or directly using an [installer](https://developer.android.com/ndk/downloads).
+
+### Uniffi
+
+The bindings are using [uniffi] to generate the C translation layer between Rust
+and Kotlin. Uniffi is a Rust project and can be installed with our freshly
+installed Rust setup using:
+
+```
+$ cargo install uniffi_bindgen
+```
+
+### Configuring Rust for cross compilation
+
+First we'll need to install the Rust target for our desired Android architecture,
+for example:
+
+```
+# rustup target add aarch64-linux-android
+```
+
+This will add support to cross-compile for the aarch64-linux-android target,
+Rust supports many different [targets], you'll have to make sure to pick the
+right one for your device or emulator.
+
+After this is done, we'll have to configure [Cargo] to use the correct linker
+for our target. Cargo is configured using a TOML file that will be found in
+`%USERPROFILE%\.cargo\config.toml` on Windows or `$HOME/.cargo/config` on Unix
+platforms. More details and configuration options for Cargo can be found in the
+official docs over [here](https://doc.rust-lang.org/cargo/reference/config.html).
+
+```
+[target.aarch64-linux-android]
+ar = "NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin/ar"
+linker = "NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android30-clang"
+```
+
+## Building
+
+A `Makefile` is provided that builds and installs the dynamic library into the
+appropriate target specific `jniLibs` directory. But before we can get started
+we'll need to tweak our Rust setup to allow cross compilation.
+
+To enable cross compilation fro `olm-sys` which builds our libolm C library
+we'll need to set the `ANDROID_NDK` environment variable to the location of our
+Android NDK installation.
+
+```
+$ export ANDROID_NDK=$HOME/Android/Sdk/ndk/22.0.7026061/
+```
+
+### Makefile build
+
+After the prerequisites have been installed and the environment variable has
+been set a build for the `aarch64` target can be build using:
+
+```
+make aarch64
+```
+
+### Manual build
+
+If the `Makefile` doesn't work on your system, the bindings can built for the `aarch64`
+target with:
+
+```
+$ cargo build --target aarch64-linux-android
+```
+
+After that, a dynamic library can be found in the `target/aarch64-linux-android/debug` directory.
+The library will be called `libmatrix_crypto.so` and needs to be renamed and
+copied into the `jniLibs` directory:
+
+```
+$ cp target/aarch64-linux-android/debug/libmatrix_crypto.so \
+     ../matrix-sdk-android/src/main/jniLibs/aarch64/libuniffi_olm.so
+```
+
+[Rust]: https://www.rust-lang.org/
+[installer]: https://rustup.rs/
+[targets]: https://doc.rust-lang.org/nightly/rustc/platform-support.html
+[Cargo]: https://doc.rust-lang.org/cargo/
+[uniffi]: https://github.com/mozilla/uniffi-rs/