cross/crossdirect: support rust (!845)

Use the native rustc cross compiler where possible, and fall back to the
qemu rustc if necessary. This should improve speed and reliability.
Without this patch, building squeekbox 1.4.0 for aarch64 with the
pmbootstrap stack just hangs forever.

Related: pmbootstrap#1861

cross/crossdirect/APKBUILD

@@ -5,22 +5,25 @@
 #
 # /native/usr/lib/crossdirect/armhf/gcc
 # -> /native/usr/bin/armv6-alpine-linux-muslgnueabihf-gcc
+# /native/usr/lib/crossdirect/armhf/rustc
+# -> /native/usr/bin/rustc --target=armv6-alpine-linux-musleabihf ...
 #
 # When building packages in the armhf chroot, PATH will get prepended with
 # "/native/usr/lib/crossdirect/armhf". The end game is of course invoking the
 # cross compiler from the native chroot, running at native speed, whenever
 # calling the compiler from the foreign arch chroot. See crossdirect.c for
-# implementation details (llvm, fakeroot, rpath).
+# implementation details of the C version (llvm, fakeroot, rpath). The rust
+# version is implemented as simple shell wrappers.
 
 pkgname=crossdirect
-pkgver=3
+pkgver=4
-pkgrel=1
+pkgrel=0
 pkgdesc="Wrappers to launch native cross compilers in foreign chroots"
 url="https://postmarketOS.org"
 arch="all"
 license="MIT"
-options=""
+options="!check"
-source="crossdirect.c"
+source="crossdirect.c rustc.sh rust-qemu-linker.sh"
 
 build() {
 	cd "$srcdir"
@@ -37,6 +40,9 @@ build() {
 			-Werror \
 			-DHOSTSPEC="\"$_hostspec\"" \
 			crossdirect.c
+
+		# rustc
+		sed "s/@ARCH@/$_arch/g" rustc.sh >rustc-"$_arch"
 	done
 }
 
@@ -45,22 +51,32 @@ package() {
 	_archs="x86_64 armhf armv7 aarch64"
 	_bins="c++ cc cpp g++ gcc clang clang++"
 
+	# Rust: qemu-linker
+	install -Dm755 "$srcdir/rust-qemu-linker.sh" \
+		"$pkgdir/usr/lib/crossdirect/rust-qemu-linker"
+
 	# Iterate over architectures
 	for _arch in $_archs; do
 		[ "$_arch" == "$CARCH" ] && continue
 
-		# Put arch-specific crossdirect wrapper in arch-specific bin folder
+		# GCC: put arch-specific crossdirect wrapper in arch-specific
+		# bin folder
 		_bindir="$pkgdir/usr/lib/crossdirect/$_arch"
 		_hostspec="$(arch_to_hostspec $_arch)"
 		mkdir -p "$_bindir"
 		cd "$_bindir"
 		cp "$srcdir/crossdirect-$_arch" "./"
 
-		# Create compiler symlinks
+		# GCC: create compiler symlinks
 		for _bin in $_bins; do
 			ln -s "crossdirect-$_arch" "$_bin"
 			ln -s "crossdirect-$_arch" "$_hostspec-$_bin"
 		done
+
+		# Rust: arch-specific rustc wrapper
+		install -Dm755 "$srcdir/rustc-$_arch" "$_bindir/rustc"
 	done
 }
-sha512sums="20b963322820de038257304c1eefa85767b78e242eda7459f06d70a1cfae5540a445aa7d5587024bf4d88a4bee28120ef9f5c2d24a648e71b542b9618318deb2  crossdirect.c"
+sha512sums="20b963322820de038257304c1eefa85767b78e242eda7459f06d70a1cfae5540a445aa7d5587024bf4d88a4bee28120ef9f5c2d24a648e71b542b9618318deb2  crossdirect.c
+c3492cf2bbf282d22585fb485a8a8950ddbd6e47faff472931072defa9c33ea757b72d455b2d608304c7e40f9c736fac2b9101b65accd48ad896bc7055ef0436  rustc.sh
+ea9bf8db3810d03d0a7395057f3d6e57f7127d87c55deaedc171c255288f5f3cc6fbcc680a5a1b92786cf573875d5dc22521173799fe2639acc97d0715ff905b  rust-qemu-linker.sh"

cross/crossdirect/rust-qemu-linker.sh

@@ -0,0 +1,8 @@
+#!/bin/sh -e
+# This wrapper gets called from the native rustc, and runs the qemu gcc for
+# linking, with LD_LIBRARY_PATH reset so it does not point to /native anymore.
+# It isn't possible to use the native cross compiler's linker with crossdirect
+# currently (pmaports#233).
+
+LD_LIBRARY_PATH=/lib:/usr/lib \
+	gcc "$@"

cross/crossdirect/rustc.sh

@@ -0,0 +1,48 @@
+#!/bin/sh -e
+
+rust_triplet() {
+	# Find the triplets in Alpine's rust APKBUILD or with:
+	# pmbootstrap chroot -barmhf --add=rust -- ls /usr/lib/rustlib
+	case "$1" in
+		x86_64)
+			echo "x86_64-alpine-linux-musl"
+			;;
+		armhf)
+			echo "arm-unknown-linux-musleabihf"
+			;;
+		armv7)
+			echo "armv7-unknown-linux-musleabihf"
+			;;
+		aarch64)
+			echo "aarch64-unknown-linux-musl"
+			;;
+		*)
+			echo "ERROR: don't know the rust triple for $1!" >&2
+			exit 1
+			;;
+	esac
+}
+
+arch="@ARCH@" # filled in by APKBUILD
+
+if ! LD_LIBRARY_PATH=/native/lib:/native/usr/lib \
+	/native/usr/bin/rustc \
+		-Clinker=/native/usr/lib/crossdirect/rust-qemu-linker \
+		--target=$(rust_triplet "$arch") \
+		--sysroot=/usr \
+		"$@"; then
+	echo "---" >&2
+	echo "WARNING: crossdirect: cross compiling with rustc failed, trying"\
+		"again with rustc + qemu" >&2
+	echo "---" >&2
+	# Usually the crossdirect approach works; however, when passing
+	# --extern to rustc with a dynamic library (.so), it fails with an
+	# error like 'can't find crate for `serde_derive`' (although the crate
+	# does exist). I think it fails to parse the metadata of the so file
+	# for some reason. We probably need to adjust rustc's
+	# librustc_metadata/locator.rs or something (and upstream that
+	# change!), but I've spent enough time on this already. Let's simply
+	# fall back to compiling in qemu in the very few cases where this is
+	# necessary.
+	/usr/bin/rustc "$@"
+fi