Cross-compilation assistance: Env vars not set when targeting x86_64_unknown_linux_musl from Linux

[edmorley]

Currently the cross-compile assistance does not set any env vars in this branch:

libcnb.rs/libcnb-package/src/cross_compile.rs

Lines 49 to 53 in 4ec39b6

	} else if target_triple.as_ref() == X86_64_UNKNOWN_LINUX_MUSL && cfg!(target_os = "linux") {
	match which("musl-gcc") {
	Ok(_) => CrossCompileAssistance::Configuration {
	cargo_env: Vec::new(),
	},

Whilst some compiles will succeed with that implementation (since Rust will automatically use musl-gcc if it's found), it breaks when cross-compiling for a different architecture.

All musl-gcc does is compile using musl rather than glibc, using the same architecture as the host. (As opposed to the wrappers like x86_64-unknown-linux-musl which also target the different architecture.)

In addition, since none of the CC_* type env vars are set, if the buildpack uses any Rust dependencies that contain C components (such as the ring crate used for TLS), then the build will fail like so:

The following warnings were emitted during compilation:

warning: cc1: error: unrecognized command-line option '-m64'

error: failed to run custom build command for `libz-sys v1.1.12`

Caused by:
  process didn't exit successfully: `/buildpacks-python/target/debug/build/libz-sys-bfd25c9fd11b1325/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-env-changed=LIBZ_SYS_STATIC
  cargo:rerun-if-changed=build.rs
  cargo:rerun-if-env-changed=ZLIB_NO_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS_x86_64-unknown-linux-musl
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS_x86_64_unknown_linux_musl
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG_ALLOW_CROSS
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS
  cargo:rerun-if-env-changed=PKG_CONFIG_x86_64-unknown-linux-musl
  cargo:rerun-if-env-changed=PKG_CONFIG_x86_64_unknown_linux_musl
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-unknown-linux-musl
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_unknown_linux_musl
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG_SYSROOT_DIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR
  cargo-warning=pkg-config has not been configured to support cross-compilation.

  Install a sysroot for the target platform and configure it via
  PKG_CONFIG_SYSROOT_DIR and PKG_CONFIG_PATH, or install a
  cross-compiling wrapper for pkg-config and set it via
  PKG_CONFIG environment variable.
  TARGET = Some("x86_64-unknown-linux-musl")
  OPT_LEVEL = Some("0")
  HOST = Some("aarch64-unknown-linux-gnu")
  cargo:rerun-if-env-changed=CC_x86_64-unknown-linux-musl
  CC_x86_64-unknown-linux-musl = None
  cargo:rerun-if-env-changed=CC_x86_64_unknown_linux_musl
  CC_x86_64_unknown_linux_musl = None
  cargo:rerun-if-env-changed=TARGET_CC
  TARGET_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  RUSTC_LINKER = None
  cargo:rerun-if-env-changed=CROSS_COMPILE
  CROSS_COMPILE = None
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("false")
  CARGO_CFG_TARGET_FEATURE = Some("fxsr,sse,sse2")
  cargo:rerun-if-env-changed=CFLAGS_x86_64-unknown-linux-musl
  CFLAGS_x86_64-unknown-linux-musl = None
  cargo:rerun-if-env-changed=CFLAGS_x86_64_unknown_linux_musl
  CFLAGS_x86_64_unknown_linux_musl = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  running: "musl-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-m64" "-I" "src/zlib" "-fvisibility=hidden" "-DZ_SOLO" "-DSTDC" "-D_LARGEFILE64_SOURCE" "-D_POSIX_SOURCE" "-o" "/buildpacks-python/target/x86_64-unknown-linux-musl/debug/build/libz-sys-672d486ba2008913/out/lib/src/zlib/adler32.o" "-c" "src/zlib/adler32.c"
  cargo:warning=cc1: error: unrecognized command-line option '-m64'

  exit status: 1

  --- stderr


  error occurred: Command "musl-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-m64" "-I" "src/zlib" "-fvisibility=hidden" "-DZ_SOLO" "-DSTDC" "-D_LARGEFILE64_SOURCE" "-D_POSIX_SOURCE" "-o" "/buildpacks-python/target/x86_64-unknown-linux-musl/debug/build/libz-sys-672d486ba2008913/out/lib/src/zlib/adler32.o" "-c" "src/zlib/adler32.c" with args "musl-gcc" did not execute successfully (status code exit status: 1).