crossdev: Build a cross toolchain for Rust

To cross-compile Rust packages, we need a standard library compiled for
the target system.

lib/crossdev.sh

@@ -2,6 +2,8 @@
 
 set -e
 
+target=$1
+
 mkdir -p /var/db/repos/crossdev/profiles /var/db/repos/crossdev/metadata
 echo crossdev > /var/db/repos/crossdev/profiles/repo_name
 echo 'masters = gentoo' > /var/db/repos/crossdev/metadata/layout.conf
@@ -18,3 +20,33 @@ EOF
 emerge -kvnuUDj --rebuilt-binaries=y --color=y sys-devel/crossdev
 
 crossdev --stable -t "$1"
+
+ln -s /var/db/repos/gentoo/sys-devel/rust-std /var/db/repos/crossdev/cross-${target}/
+
+mkdir -p
+    /etc/portage/env/dev-lang \
+    /etc/portage/package.accept_keywords \
+    /etc/portage/package.use \
+    && :
+printf 'cross-%s/rust-std **\n' "${target}" \
+    > /etc/portage/package.accept-keywords/rust-cross
+echo 'dev-lang/rust rust-src' \
+    > /etc/portage/package.use/rust-src
+case ${target%%-*} in
+aarch64)
+    llvm_target=AArch64
+    ;;
+arm*)
+    llvm_target=ARM
+    ;;
+*)
+    printf 'Unknown LLVM target: %s' "${target%%-*}"
+    exit 1
+    ;;
+esac
+printf '( %s:%s:%s )\n' \
+    "${llvm_target}" \
+    "${target}" \
+    "${target}" \
+    >> /etc/portage/env/dev-lang/rust
+emerge -vbknuUj cross-${target}/rust-std