Bootstrapping Rust via cross compilation

This article covers using crossdev to bootstrap Rust for a specific architecture.

Environment

Figure out the LLVM target name corresponding to the destination platform (look at e.g. emerge -pvO sys-devel/llvm for hints).
- This will be referred to as LLVM_TARGET just for the purposes of this page.
- For sparc, it's Sparc.

Figure out the Rust target name for the destination platform.
- Hint: try rustc --print target-list | grep ${ARCH}.
- For sparc, it's sparc64-unknown-linux-gnu.

Figure out CHOST for use with both crossdev and later, Rust as CTARGET.
- This will be called CTARGET.
- Hint: look at /var/db/repos/gentoo/profiles/arch/${ARCH}/make.defaults.
- For sparc, it's sparc64-unknown-linux-gnu as 32-bit will not be covered (at least for now!)

crossdev

Install crossdev:

root #emerge --ask sys-devel/crossdev

Create an overlay for it to dump its ebuilds into:

root #eselect repository create crossdev

Create a cross toolchain for the chosen CHOST. For sparc, it'll be:

amd64 #crossdev --ov-output /var/db/repos/local/crossdev sparc64-unknown-linux-gnu

Rust

Grab repo

The git repo is used because the tag tarballs lack the required submodules:

amd64 #

git clone https://github.com/rust-lang/rust.git -b stable && cd rust

amd64 #git submodule update --init --recursive

Build it

Install dev-libs/openssl for the openssl-sys create:

amd64 #sparc64-unknown-linux-gnu-emerge -v1 openssl

Note
Versions of Rust released before or slightly after May 15, 2023 may depend on OpenSSL 1.1.1 instead of the latest version, which is EOL and masked as of the time of writing. For more details see https://github.com/sfackler/rust-openssl/issues/1645.

Configure:

FILE config.toml

# Includes one of the default files in src/bootstrap/defaults
profile = "user"
changelog-seen = 2

[rust]
channel = "stable"

[target.sparc64-unknown-linux-gnu]
# Needed because of some mixup between Rust target names and defaults(?)
# for it -- seems to assume Debian naming of sparc64-linux-gnu otherwise?
cc = "sparc64-unknown-linux-gnu-gcc"
cxx = "sparc64-unknown-linux-gnu-g++"

Run the mono x.py build script:

amd64 #

OPENSSL_INCLUDE_DIR=/usr/sparc64-unknown-linux-gnu/usr/include/ OPENSSL_LIB_DIR=/usr/sparc64-unknown-linux-gnu/usr/lib PKG_CONFIG_PATH=/usr/sparc64-unknown-linux-gnu/usr/lib/pkgconfig ./x.py dist --build x86_64-unknown-linux-gnu --host sparc64-unknown-linux-gnu --target sparc64-unknown-linux-gnu

All going well, when it terminates, there should be a fresh rustc binary:

amd64 #file build/sparc64-unknown-linux-gnu/stage2/bin/rustc

build/sparc64-unknown-linux-gnu/stage2/bin/rustc: ELF 64-bit MSB pie executable, SPARC V9, total store ordering, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux.so.2, for GNU/Linux 3.2.0, with debug_info, not stripped

On the other side

Install built Rust temporarily

Warning
Use a path like /usr/local where it's easy to delete the files afterwards and be sure nothing is lingering "contaminating" later builds. In fact, this whole procedure should be done in a clean chroot.

Copy the files over to the destination box (sparc in this case):

Copy over:
- build/dist/cargo-1.61.0-sparc64-unknown-linux-gnu.tar.xz
- build/dist/rust-std-1.61.0-sparc64-unknown-linux-gnu.tar.xz
- build/dist/rustc-1.61.0-sparc64-unknown-linux-gnu.tar.xz
For each: on the other side, untar it to a temporary location, and run inside ./install.sh --prefix=/usr/local/lib/rust/1.61.0/

Ebuild mangling

Some changes are needed to /etc/portage:

FILE /etc/portage/package.unmask

app-eselect/eselect-rust
dev-lang/rust
virtual/rust

FILE /etc/portage/package.accept_keywords

app-eselect/eselect-rust * ~*
dev-lang/rust * ~*
virtual/rust * ~*

FILE /etc/portage/package.use

# This is our LLVM_TARGET from earlier.
dev-lang/rust RUST_TARGETS: Sparc

Copy the Rust ebuilds to a temporary location:

sparc #

mkdir -p /tmp/dev-lang/rust && cd /tmp/dev-lang/rust

sparc #cp -rv /var/db/repos/gentoo/dev-lang/rust/* .

Modify the Rust ebuild corresponding to the current latest Rust release (must match the version built from git):

Put 'python-any-r1_pkg_setup' at the top of pkg_setup, then add 'return' immediately below it in pkg_setup (re eselect-rust checks) (so only those two lines are run)
Replace the rustc call in src_configure by just setting rust_stage0_root=/usr/local/lib/rust/1.61.0/
May need to comment out the [[ -d ${rust_stage0_root} ]] check (?)

Todo: Todo:

Check if previous point should be commented out or not.

Then try to emerge using it:

sparc #USE=system-bootstrap ebuild rust-1.61.0.ebuild clean merge

Once done, some manual fiddling may be needed to get LDPATH right initially and make eselect rust happy.

Run eselect rust update and placate any complaints it has if there's stale stuff left over from the /usr/local bits. Try env-update && . /etc/profile.

Remember to clean up /usr/local/{bin,lib/*}.

Now do it all again using a vanilla ebuild:

sparc #USE=system-bootstrap ebuild /var/db/repos/gentoo/dev-lang/rust/rust-1.61.0.ebuild clean merge

Distribution tarball

Note
See bug #671736 and bug #842246 which will make it easier to add non-upstream bootstrap tarballs into dev-lang/rust-bin.

Now re-emerge dev-lang/rust with the dist USE flag:

sparc #USE="dist system-bootstrap" ebuild rust-1.61.0.ebuild clean merge

Once done, the binaries should be at $(rustc --print sysroot)/dist (in this case, that's /usr/lib/rust/1.61.0/dist):

sparc #ls -al $(rustc --print sysroot)/dist

total 61732
drwxr-xr-x 2 root root     4096 Jun 22 19:19 .
drwxr-xr-x 7 root root     4096 Jun 22 19:19 ..
-rw-r--r-- 1 root root  4134536 Jun 22 19:18 cargo-1.61.0-sparc64-unknown-linux-gnu.tar.xz
-rw-r--r-- 1 root root 20586916 Jun 22 19:18 rust-std-1.61.0-sparc64-unknown-linux-gnu.tar.xz
-rw-r--r-- 1 root root 38477600 Jun 22 19:18 rustc-1.61.0-sparc64-unknown-linux-gnu.tar.xz

External references