docker: split rust into own layer to flatten distribution

Real 'docker history' showed native-toolchain=334MB poles.
- rustc/cargo pull libstd-rust-dev; move them to a new 'rust' group ordered
  *after* native-toolchain (so shared gcc/build-essential stay there).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rpardini

extensions/kernel-rust.sh

@@ -51,6 +51,9 @@ declare -g RUST_TOOL_SYSROOT=""
 function add_host_dependencies__add_rust_compiler() {
 	display_alert "Adding Rust kernel build dependencies" "${EXTENSION}" "info"
 	# bindgen needs libclang for dlopen; available on all target distros.
+	# NB: do NOT pre-pull llvm-dev into an earlier layer to "split" this — measured, llvm-dev is a
+	# superset far larger than what libclang-dev needs (~+560MB total, bigger max layer). Keep
+	# libclang-dev as a single ~633MB 'clang' layer instead.
 	EXTRA_BUILD_DEPS+=("clang::libclang-dev")
 }
 

extensions/mtkflash.sh

@@ -14,7 +14,9 @@
 
 function add_host_dependencies__mtkflash() {
 	display_alert "Preparing mtkflash host-side dependencies" "${EXTENSION}" "info"
-	EXTRA_BUILD_DEPS+=("native-toolchain::rustc" "native-toolchain::cargo" "native-toolchain::build-essential")
+	# rustc/cargo pull the bulky libstd-rust-dev; keep them in their own 'rust' layer (after
+	# native-toolchain, so the shared gcc/build-essential stay in native-toolchain).
+	EXTRA_BUILD_DEPS+=("rust::rustc" "rust::cargo" "native-toolchain::build-essential")
 }
 
 function extension_finish_config__900_mtkflash() {

lib/functions/host/docker.sh

@@ -273,7 +273,11 @@ function docker_create_dockerfile_apt_install_runs() {
 
 	# Preferred layer order: heaviest/most-stable first, so they stay cached longest.
 	# Any unknown (e.g. extension-defined) group is appended after, in first-seen order.
-	declare -a preferred_order=(clang qemu cross-other cross-amd64 cross-arm64 cross-armhf native-toolchain build-tools imaging python fs-tools core)
+	# NOTE on ordering: groups that explicitly pre-install a heavy *shared* dependency of a later
+	# group must come first, so that dependency lands in their (smaller, parallel-pullable) layer
+	# instead of inflating the later one. Specifically: 'llvm' before 'clang' (llvm-dev is the bulk
+	# of libclang-dev), and 'native-toolchain' before 'rust' (so gcc stays in native-toolchain).
+	declare -a preferred_order=(llvm clang qemu cross-other cross-amd64 cross-arm64 cross-armhf native-toolchain rust build-tools imaging python fs-tools core)
 	declare -a final_order=()
 	for group in "${preferred_order[@]}"; do
 		[[ -n "${group_pkgs[${group}]:-}" ]] && final_order+=("${group}")

lib/functions/host/prepare-host.sh

@@ -280,8 +280,10 @@ function adaptative_prepare_host_dependencies() {
 	fi
 
 	if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+		# llvm goes in its own (earlier) layer; clang/lld depend on libllvm, so splitting it out
+		# keeps the clang layer smaller and gives llvm its own parallel-pullable layer.
+		host_dependencies+=("llvm::llvm")
 		host_dependencies+=("clang::clang")
-		host_dependencies+=("clang::llvm")
 		host_dependencies+=("clang::lld")
 	fi