[Doc] Improve instructions for building Rust SDK locally (#4015)

* Improve instructions for building Rust SDK locally

* Update _developer_onboarding.md

README.md

@@ -83,8 +83,11 @@ You can also come chat with the community in the Matrix [room](https://matrix.to
 
 ## Build instructions
 
-Just clone the project and open it in Android Studio.
+Just clone the project and open it in Android Studio. Make sure to select the
-Makes sure to select the `app` configuration when building (as we also have sample apps in the project).
+`app` configuration when building (as we also have sample apps in the project).
+
+To build against a local copy of the Rust SDK, see the [Developer
+onboarding](docs/_developer_onboarding.md#build-the-sdk-locally) instructions.
 
 ## Support
 

docs/_developer_onboarding.md

@@ -117,41 +117,43 @@ You can also have access to the aars through the [release](https://github.com/ma
 
 #### Build the SDK locally
 
-Easiest way: run the script [../tools/sdk/build_rust_sdk.sh](../tools/sdk/build_rust_sdk.sh) and just answer the questions.
+Prerequisites:
+* Install the Android NDK (Native Development Kit). To do this from within
+  Android Studio:
+    1. **Tools > SDK Manager**
+    2. Click the **SDK Tools** tab.
+    3. Select the **NDK (Side by side)** checkbox
+    4. Click **OK**.
+    5. Click **OK**.
+    6. When the installation is complete, click **Finish**.
+* Install `cargo-ndk`:
+  ```
+  cargo install cargo-ndk
+  ```
+* Install the Android Rust toolchain:
+  ```
+  rustup target add aarch64-linux-android
+  ```
+* Depending on the location of the Android SDK, you may need to set
+  `ANDROID_HOME`:
+  ```
+  export ANDROID_HOME=$HOME/android/sdk
+  ```
 
-Legacy way:
+You can then build the Rust SDK by running the script
+[`tools/sdk/build_rust_sdk.sh`](../tools/sdk/build_rust_sdk.sh) and just answering
+the questions.
 
-If you need to locally build the sdk-android you can use
+This will prompt you for the path to the Rust SDK, then build it and
-the [build](https://github.com/matrix-org/matrix-rust-components-kotlin/blob/main/scripts/build.sh) script.
+`matrix-rust-components-kotlin`, eventually producing an aar file at
-
+`./libraries/rustsdk/matrix-rust-sdk.aar`, which will be picked up
-For this please check the [prerequisites](https://github.com/matrix-org/matrix-rust-components-kotlin/blob/main/README.md#prerequisites) from the repo.
+automatically by the Element X Android build.
-
-Checkout both [matrix-rust-sdk](https://github.com/matrix-org/matrix-rust-sdk) and [matrix-rust-components-kotlin](https://github.com/matrix-org/matrix-rust-components-kotlin) repositories
-```shell
-git clone git@github.com:matrix-org/matrix-rust-sdk.git
-git clone git@github.com:matrix-org/matrix-rust-components-kotlin.git
-```
-
-Then you can launch the build script from the matrix-rust-components-kotlin repository with the following params:
-
-- `-p` Local path to the rust-sdk repository
-- `-o` Optional output path with the expected name of the aar file. By default the aar will be located in the corresponding build/outputs/aar directory.
-- `-r` Flag to build in release mode
-- `-m` Option to select the gradle module to build. Default is sdk.
-- `-t` Option to to select an android target to build against. Default will build for all targets.
-
-So for example to build the sdk against aarch64-linux-android target and copy the generated aar to Element X project:
-
-```shell
-./scripts/build.sh -p [YOUR MATRIX RUST SDK PATH] -t aarch64-linux-android -o [YOUR element-x-android PATH]/libraries/rustsdk/matrix-rust-sdk.aar
-```
 
 Troubleshooting:
  - You may need to set `ANDROID_NDK_HOME` e.g `export ANDROID_NDK_HOME=~/Library/Android/sdk/ndk`.
  - If you get the error `thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', .cargo/registry/src/index.crates.io-6f17d22bba15001f/cargo-ndk-2.11.0/src/cli.rs:345:18` try updating your Cargo NDK version. In this case, 2.11.0 is too old so `cargo install cargo-ndk` to install a newer version.
- - If you get the error `Unsupported class file major version 64` try changing your JVM version. In this case, Java 20 is not supported in Gradle yet, so downgrade to an earlier version (Java 17 worked in this case).
+ - If you get the error `Unsupported class file major version <n>`, try changing your JVM version by setting
-
+   `JAVA_HOME` and, if building via Android Studio, "File | Settings | Build, Execution, Deployment | Build Tools | Gradle | Gradle JDK".
-You are good to test your local rust development now!
 
 ### The Android project