A bindgen Rust aport builds in minutes with cross-native2, once pmbootstrap sets cargo up for it

scope: generic · severity: finding · confidence: proven · subsystem: build

Evidence

2026-09-14, porthole workspace, pmbootstrap 3.11.1 + branch build/rust-cross-native2 (carried in sandbox/patches), aport testing/obscura (gtk4-rs, libadwaita-rs, gstreamer-rs, libcamera-rs with bindgen; 213 crates). options=!pmb:crossdirect (QEMU only): 140 of 213 crates after 7 min, still compiling at 40 min. options=pmb:cross-native2: whole package, obscura-0.1.0-r1.apk 1137 KiB, in 2m35s; aarch64 PIE, interpreter /lib/ld-musl-aarch64.so.1, so: deps on libcamera.so.0.7, libgtk-4, libadwaita-1. chroot_native and chroot_buildroot_aarch64 both rust 1.98.1-r0 (48a229cea); native rustc lists aarch64-alpine-linux-musl.

The question — can a Rust aport whose build script runs bindgen (every libcamera-rs, and most *-sys crates that generate bindings) be built fast for aarch64 on an x86 host, without hacks in the APKBUILD?

The answer — yes, with options="pmb:cross-native2", in about 2.5 minutes instead of the better part of an hour, once pmbootstrap exports four things for cross-native2 (it already does the Go equivalent):

CARGO_BUILD_TARGET=aarch64-alpine-linux-musl
CARGO_TARGET_AARCH64_ALPINE_LINUX_MUSL_LINKER=aarch64-alpine-linux-musl-gcc
RUSTFLAGS=--sysroot=/mnt/sysroot/usr -Clink-arg=--sysroot=/mnt/sysroot
BINDGEN_EXTRA_CLANG_ARGS_aarch64_alpine_linux_musl=--target=... --sysroot=/mnt/sysroot

and stops installing every build dependency of a Rust aport into the native chroot for cross-native2 (that crossdirect rule dragged libcamera-dev in as an x86 package and set off a nested strict build, which a rootless workspace cannot zap).

Why each dead theory is dead

• QEMU or crossdirect only. crossdirect builds build scripts as native binaries that then dlopen() the target chroot’s libclang (RPATH /usr/lib/llvmNN/lib) and die with “unsupported relocation type 1026”; QEMU-only works and is ~20x slower.
• LIBCLANG_PATH at /native. The x86 libclang then loads the aarch64 libLLVM through its RPATH. Pregenerated bindings dodge the problem instead of building the way Alpine does.
• target-scoped rustflags. abuild’s default.conf exports RUSTFLAGS, and cargo lets RUSTFLAGS replace target.<triple>.rustflags: rustc never saw the sysroot and every target crate failed “can’t find crate for core”. RUSTFLAGS itself is safe because with a build target set cargo passes it to target artifacts only; build scripts and proc-macros stay native.
• no cross std. The target standard library comes from the sysroot’s own rust package (put rust in makedepends_host); it is the same toolchain release as the native one, the same match crossdirect already requires.

How to apply — split the aport into makedepends_build (cargo, cargo-auditable, clang-libclang, meson, pkgconf, …) and makedepends_host (rust plus the target -dev packages), add pmb:cross-native2 (and !check: cross-built tests cannot run on the build machine), and make sure a meson wrapper copies cargo’s output from target/$CARGO_BUILD_TARGET/release. Until pmbootstrap carries the change, the workspace image applies it; editing the patch now rebuilds the image (the recipe hash covers the build context).