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Reference: Cross compilation

You're working with a macOS or Linux system, and everything is going great.

Satisfied with your work, you push a PR, and sit back to get a cup of coff- wait, what's that Windows build issue?

error[E0793]: reference to packed field is unaligned
    --> C:\Users\runneradmin\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ntapi-0.3.7\src\ntexapi.rs:2783:52
     |
2783 |         *tick_count.QuadPart_mut() = read_volatile(&(*USER_SHARED_DATA).u.TickCountQuad);
     |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
     = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)

You then spend several PR cycles trying to get this to be fixed, each time with a delay (and a bunch of wasteful parallel CI jobs running, whose results you don't care about).

Wouldn't it be better if instead, you could just build for Windows locally?

The better way

You can! ✨

Steps:

  1. Install cross: cargo binstall cross
  2. cross build --target x86_64-pc-windows-gnu

Now your build still fails but at least your testing loop is faster 🥲

Cross version compile

You can also do this across versions:

; rustup install 1.68
; rustup run 1.68 cross build --target x86_64-pc-windows-gnu

Native cross compilation

cross uses Docker to do its thing.

If you don't like that, you can cross compile by installing dependencies yourself, assuming that's supported without emulation:

  1. Install the mingw-64 package.
  • On macOS, that's brew install mingw-w64
  • On Debian/Ubuntu, that's sudo apt-get install mingw-w64
  • On Arch, that's sudo pacman -S mingw-w64, install all
  1. rustup target add x86_64-pc-windows-gnu
  2. cargo build --target x86_64-pc-windows-gnu

And that still works across versions:

; rustup install 1.68
; rustup target add x86_64-pc-windows-gnu --toolchain 1.68
; rustup run 1.68 cargo build --target x86_64-pc-windows-gnu