Fix default target triple for cargo libcnb package (#922)

* Allow tests to target a specific platform

Also picks better defaults for the test runner and `libcnb package`.

Fixes #843
Fixes #869

* Update CHANGELOG.md

* Revert libcnb-test changes, limit scope to just determining a default target triple.

Fixes #843
Fixes #869

* Update CHANGELOG.md

Signed-off-by: Colin Casey <[email protected]>

* Update CHANGELOG.md

Signed-off-by: Colin Casey <[email protected]>

* Update CHANGELOG.md

Signed-off-by: Colin Casey <[email protected]>

---------

Signed-off-by: Colin Casey <[email protected]>

Author: colincasey

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- `libcnb-cargo`
+  - The default for the `--target` argument of `cargo libcnb package` will now be based on an architecture that matches the host machine instead of `x86_64-unknown-linux-musl`. ([#922](https://github.com/heroku/libcnb.rs/pull/922))
 
 ## [0.28.1] - 2025-03-25
 

libcnb-cargo/src/cli.rs

@@ -24,8 +24,8 @@ pub(crate) struct PackageArgs {
     #[arg(long)]
     pub(crate) release: bool,
     /// Build for the target triple
-    #[arg(long, default_value = "x86_64-unknown-linux-musl")]
-    pub(crate) target: String,
+    #[arg(long)]
+    pub(crate) target: Option<String>,
     /// Directory for packaged buildpacks, defaults to 'packaged' in Cargo workspace root
     #[arg(long)]
     pub(crate) package_dir: Option<PathBuf>,

libcnb-cargo/src/package/command.rs

@@ -2,7 +2,10 @@ use crate::cli::PackageArgs;
 use crate::package::error::Error;
 use libcnb_data::buildpack::BuildpackId;
 use libcnb_package::buildpack_dependency_graph::build_libcnb_buildpacks_dependency_graph;
-use libcnb_package::cross_compile::{CrossCompileAssistance, cross_compile_assistance};
+use libcnb_package::cross_compile::{
+    AARCH64_UNKNOWN_LINUX_MUSL, CrossCompileAssistance, X86_64_UNKNOWN_LINUX_MUSL,
+    cross_compile_assistance,
+};
 use libcnb_package::dependency_graph::get_dependencies;
 use libcnb_package::output::create_packaged_buildpack_dir_resolver;
 use libcnb_package::util::absolutize_path;
@@ -21,6 +24,11 @@ pub(crate) fn execute(args: &PackageArgs) -> Result<(), Error> {
         CargoProfile::Dev
     };
 
+    let target_triple = match &args.target {
+        Some(target) => target.to_string(),
+        None => determine_target_triple_from_host_arch()?,
+    };
+
     let workspace_root_path =
         find_cargo_workspace_root_dir(&current_dir).map_err(Error::CannotFindCargoWorkspaceRoot)?;
 
@@ -37,18 +45,17 @@ pub(crate) fn execute(args: &PackageArgs) -> Result<(), Error> {
         .map_err(|error| Error::CannotCreatePackageDirectory(package_dir.clone(), error))?;
 
     let buildpack_dir_resolver =
-        create_packaged_buildpack_dir_resolver(&package_dir, cargo_profile, &args.target);
+        create_packaged_buildpack_dir_resolver(&package_dir, cargo_profile, &target_triple);
 
-    eprintln!("🖥️ Gathering Cargo configuration (for {})", args.target);
+    eprintln!("🖥️ Gathering Cargo configuration (for {target_triple})");
     let cargo_build_env = if args.no_cross_compile_assistance {
         Vec::new()
     } else {
-        match cross_compile_assistance(&args.target) {
+        match cross_compile_assistance(&target_triple) {
             CrossCompileAssistance::Configuration { cargo_env } => cargo_env,
             CrossCompileAssistance::NoAssistance => {
                 eprintln!(
-                    "Couldn't determine automatic cross-compile settings for target triple {}.",
-                    args.target
+                    "Couldn't determine automatic cross-compile settings for target triple {target_triple}."
                 );
                 eprintln!(
                     "This is not an error, but without proper cross-compile settings in your Cargo manifest and locally installed toolchains, compilation might fail."
@@ -110,7 +117,7 @@ pub(crate) fn execute(args: &PackageArgs) -> Result<(), Error> {
         libcnb_package::package::package_buildpack(
             &node.path,
             cargo_profile,
-            &args.target,
+            &target_triple,
             &cargo_build_env,
             &buildpack_destination_dir,
             &packaged_buildpack_dirs,
@@ -195,3 +202,14 @@ fn calculate_dir_size(path: impl AsRef<Path>) -> std::io::Result<u64> {
 
     Ok(size_in_bytes)
 }
+
+// NOTE: The target OS is always assumed to be linux based
+fn determine_target_triple_from_host_arch() -> Result<String, Error> {
+    match std::env::consts::ARCH {
+        "amd64" | "x86_64" => Ok(X86_64_UNKNOWN_LINUX_MUSL.to_string()),
+        "arm64" | "aarch64" => Ok(AARCH64_UNKNOWN_LINUX_MUSL.to_string()),
+        arch => Err(Error::CouldNotDetermineDefaultTargetForArch(
+            arch.to_string(),
+        )),
+    }
+}

libcnb-cargo/src/package/error.rs

@@ -24,4 +24,8 @@ pub(crate) enum Error {
     CannotConfigureCrossCompilation,
     #[error("No buildpacks found!")]
     NoBuildpacksFound,
+    #[error(
+        "Could not determine a default target triple from the current architecture ({0}), you must explicitly provide the --target argument"
+    )]
+    CouldNotDetermineDefaultTargetForArch(String),
 }

libcnb-package/src/cross_compile.rs

@@ -108,8 +108,8 @@ pub enum CrossCompileAssistance {
 }
 
 // Constants for supported target triples
-const AARCH64_UNKNOWN_LINUX_MUSL: &str = "aarch64-unknown-linux-musl";
-const X86_64_UNKNOWN_LINUX_MUSL: &str = "x86_64-unknown-linux-musl";
+pub const AARCH64_UNKNOWN_LINUX_MUSL: &str = "aarch64-unknown-linux-musl";
+pub const X86_64_UNKNOWN_LINUX_MUSL: &str = "x86_64-unknown-linux-musl";
 
 // Constants for `std::env::consts::OS` and `std::env::consts::ARCH`
 const OS_LINUX: &str = "linux";