Edit: It was a PAINSTAKING process that took an entire day of failing numerous times but persevering, so I managed to do it. Here's how I did it in the end:

First I of course had to install cross:

$ cargo install cross

Then I installed Docker Desktop and got the daemon running. After that I added Windows as a target with Rustup:

$ rustup target add x86_64-pc-windows-msvc

Then I had to build the Docker image for my target, following the instructions at https://github.com/cross-rs/cross-toolchains.

$ cd /path/to/dir # any directory you prefer
$ git clone https://github.com/cross-rs/cross
$ cd cross
$ git submodule update --init --remote

Then I followed the instructions to configure:

$ cargo xtask configure-crosstool

I was looking to build for x86_64-pc-windows-msvc, so I checked the table in README.md and built the appropriate image with the local tag:

$ cargo build-docker-image x86_64-pc-windows-msvc-cross --tag local

I then created a file called Cross.toml in my project directory and added the following lines to it:

[target.x86_64-pc-windows-msvc]
image = "ghcr.io/cross-rs/x86_64-pc-windows-msvc-cross:local"

Finally,

$ cross build --release --target x86_64-pc-windows-msvc --verbose

did the job. To be honest, I have no idea how to use Docker and what it's for, and I was just improvising here, which is probably why it took an entire day. Albeit, I'm very happy I managed to get it working, and the resulting .exe works on Windows (I had my friend test it).

I'm on macOS, and I just wrote my first Rust project. My friend and I are both learning different languages (he's learning Python and I'm learning Rust; dw Rust isn't my first language lol). I want to compile my program into an executable that my friend can run on his Windows machine. I would also like if I could compile for Linux so I can have my dad try it out.

I'm joining a project and I was advised that to compile it, I need to run

cargo build --release --target arm-unknown-linux-gnueabihf

I'm on Mac OS (Sonoma 14.2.1). I know I need an ARM toolchain for this but I'm not sure what I need or, more accurately, where/how to install it.

In the project directory, I have been advised to create `.cargo/config.toml`
with this content

[target.arm-unknown-linux-gnueabihf]
linker = "gcc-ar-13" 

I assume I need to change the value for the linker but, as mentioned, I'm unsure what option to put there.

If it matters, I have no issue with cargo build --release by itself.

Here are more details from the output. I recognize in this case that it seems to be issues with the command line arguments but I dont think this is the right linker so Im not too concerned about this specifically. This is lightly redacted

  Compiling cfg-if v1.0.0
  # Omitting lots of lines that start with "Compiling"
  Compiling project v0.1.19 (/Users/USER/repos/project)

error: linking with `gcc-ar-13` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="PATH_REDACTED" VSLANG="1033" "gcc-ar-13" "/var/folders/3z/z7gj1knj2pqf_q17jz7yzc3h0000gq/T/rustcNChRuA/symbols.o" "/Users/USER/repos/project/target/arm-unknown-linux-gnueabihf/release/deps/project-f18b9ac4a98f6089.project.f25b3891c1c0fef8-cgu.15.rcgu.o" "-Wl,--as-needed" "-L" "/Users/USER/repos/project/target/arm-unknown-linux-gnueabihf/release/deps" "-L" "/Users/USER/repos/project/target/release/deps" "-L" "/Users/USER/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/arm-unknown-linux-gnueabihf/lib" "-Wl,-Bstatic" "/Users/USER/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/arm-unknown-linux-gnueabihf/lib/libcompiler_builtins-31fb99244964ed82.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/Users/USER/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/arm-unknown-linux-gnueabihf/lib" "-o" "/Users/USER/repos/project/target/arm-unknown-linux-gnueabihf/release/deps/project-f18b9ac4a98f6089" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: /Library/Developer/CommandLineTools/usr/bin/ar: illegal option -- /           
       usage:
             ar -d [-TLsv] archive file ...
             ar -m [-TLsv] archive file ...
             ar -m [-abiTLsv] position archive file ...
             ar -p [-TLsv] archive [file ...]
             ar -q [-cTLsv] archive file ...
             ar -r [-cuTLsv] archive file ...
             ar -r [-abciuTLsv] position archive file ...
             ar -t [-TLsv] archive [file ...]
             ar -x [-ouTLsv] archive [file ...]
error: could not compile `project` (bin "project") due to previous error

I had just posted how to cross compile a gtk program from linux for windows, so thought I'd try to figure out the same process to compile to Mac. After I got part way through cross compiling gtk, I realized you need to sign gui apps for them to run in OSX, and have to do that from a mac anyway. So this process is kind of pointless for that. If you are developing a cli tool though, this can still be useful.

first install the toolchain

rustup target add x86_64-apple-darwin

setup the linker

~/.cargo/config

[target.x86_64-apple-darwin]
linker = "x86_64-apple-darwin15-gcc"
ar = "x86_64-apple-darwin15-ar"

install osxcross. taken from the aur pkgbuild for osxcross-git, you can just install that if you're on arch, but you'll need to modify the pkgbuild and set OSX_MIN_VERSION to 10.7.

git clone https://github.com/tpoechtrager/osxcross
cd osxcross
wget https://s3.dockerproject.org/darwin/v2/MacOSX10.11.sdk.tar.xz
mv MacOSX10.11.sdk.tar.xz tarballs/
sed -i -e 's|-march=native||g' build_clang.sh wrapper/build.sh
UNATTENDED=yes OSX_VERSION_MIN=10.7 ./build.sh
sudo mkdir -p /usr/local/osx-ndk-x86
sudo mv target/* /usr/local/osx-ndk-x86

then to compile

export PATH=/usr/local/osx-ndk-x86/bin:$PATH
export PKG_CONFIG_ALLOW_CROSS=1
cargo build --target=x86_64-apple-darwin --release

if anyone wants to try to get gtk working you can see the steps I took to build it here, but I'm going to give up on it since you can't sign from linux.