Merge pull request #41 from yageek/features/custom_build

Add option in `RustConfig` to customize the build.

Author: emk

README.md

@@ -98,6 +98,17 @@ But if you only need Rust to build a particular Ruby gem, and you have no top-le
 RUST_SKIP_BUILD=1
 ```
 
+## Customizing build flags
+
+If you want to change the cargo build command, you can set the `RUST_CARGO_BUILD_FLAGS` variable inside the `RustConfig` file.
+
+```sh
+RUST_CARGO_BUILD_FLAGS="--release -p some_package --bin some_exe --bin some_bin_2"
+```
+
+The default value of `RUST_CARGO_BUILD_FLAGS` is `--release`.
+If the variable is not set in `RustConfig`, the default value will be used to build the project.
+
 ## Using the edge version of the buildpack
 
 The `emk/rust` buildpack from the [Heroku Registry](https://devcenter.heroku.com/articles/buildpack-registry) contains the latest stable version of the buildpack. If you'd like to use the latest buildpack code from this Github repository, you can set your buildpack to the Github URL:

bin/compile

@@ -32,6 +32,8 @@ BUILD_PATH=""
 RUST_INSTALL_DIESEL=0
 # These flags are passed to `cargo install diesel`, e.g. '--no-default-features --features postgres'
 DIESEL_FLAGS=""
+# Default build flags to pass to `cargo build`.
+RUST_CARGO_BUILD_FLAGS="--release"
 
 # Load our toolchain configuration, if any was specified.
 if [ -f "$BUILD_DIR/rust-toolchain" ]; then
@@ -135,7 +137,7 @@ if [ $RUST_SKIP_BUILD -ne 1 ]; then
   echo "-----> Building application using Cargo"
   cd "$BUILD_DIR/$BUILD_PATH"
   rm -rf target/
-  cargo build --release
+  cargo build $RUST_CARGO_BUILD_FLAGS
   mkdir -p target/release
   find "$CARGO_TARGET_DIR/release" -maxdepth 1 -type f -executable -exec cp -a -t target/release {} \;
 else