Installation fails on Windows ARM64 (aarch64) due to yaml12 Rust compilation target error

[benediktschfr]

Description

Hi! When trying to install Rapp (v0.4.0) on a Windows ARM64 system (aarch64), the installation fails during the compilation of the new dependency yaml12.

The yaml12 build script (Makevars.win) likely forces Cargo to compile for a GNU target, but it drops the architecture prefix, resulting in --target -pc-windows-gnu instead of aarch64-pc-windows-gnu. Furthermore, the official stable Rust toolchain for Windows ARM64 (stable-aarch64-pc-windows-msvc) does not inherently support the GNU target, if I understand it correctly.

Standard Output / Error Message

During devtools::install() or pak::pak(), the build fails with the following Cargo error:

RUSTFLAGS=" --print=native-static-libs" cargo build -j 2 --offline --target=-pc-windows-gnu --lib --release --manifest-path=rust/Cargo.toml --target-dir=./rust/target
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `...\bin\rustc.exe - --crate-name ___ --print=file-names --print=native-static-libs --target -pc-windows-gnu ...` (exit code: 1)
  --- stderr
  error: error loading target specification: could not find specification for target "-pc-windows-gnu"

Environment

• OS: Windows 11 (ARM64 / aarch64)
• R Version: 4.5.x (aarch64-w64-mingw32)
• Rust Toolchain: stable-aarch64-pc-windows-msvc
• Rapp Version: 0.4.0 (relies on yaml12 v0.1.0)

Suggested Fix / Workaround

For now, rolling back to Rapp v0.3.0 bypasses the issue since it doesn't depend on yaml12.
For v0.4.0+, the configuration script in yaml12 needs, if possible, to properly detect the aarch64 architecture on Windows when setting up the --target flag for Cargo, or fallback gracefully if the required target isn't supported by the host's MSVC toolchain.

[t-kalinowski]

Thanks for the report and the detailed diagnosis.

This is a yaml12 packaging issue, not an Rapp issue directly. yaml12 is currently using the Windows Makevars scaffold generated by rextendr. In both places, the Rust target is derived from $(WIN):

• yaml12: https://github.com/posit-dev/r-yaml12/blob/bbf9b2b7112a7994da8a0ca9fecf7383f373a434/src/Makevars.win.in#L1
• rextendr template: https://github.com/extendr/rextendr/blob/3f48f4932b7bbf7b426282baf4186a4fdf01e53a/inst/templates/Makevars.win.in#L1

On ARM Windows, WIN is empty, so this becomes -pc-windows-gnu, which explains the Cargo error.

The target-detection helper in rextendr also only handles x86_64 and i386 Windows today:
https://github.com/extendr/rextendr/blob/3f48f4932b7bbf7b426282baf4186a4fdf01e53a/R/source.R#L766-L783

The right Rust target for native ARM Windows with Rtools45 is:

rustup target add aarch64-pc-windows-gnullvm

and yaml12 should build with:

--target=aarch64-pc-windows-gnullvm

I’ll fix this in yaml12, and we should also patch the upstream rextendr template so future generated packages handle ARM Windows correctly. Until then, using Rapp 0.3.0 is the right workaround because it avoids the new yaml12 dependency.

[t-kalinowski]

I have a draft yaml12 fix here: posit-dev/r-yaml12#10

Could you try installing that branch on your Windows ARM64 machine with pak?

pak::pak("posit-dev/r-yaml12@fix-windows-arm64-rust-target")

If you have not already installed the ARM Windows Rust target, you should be prompted to run this first from a terminal:

rustup target add aarch64-pc-windows-gnullvm

Then try installing Rapp again:

pak::pak("r-lib/Rapp")

The branch replaces the old $(WIN)-based target selection with an R helper that maps native Windows ARM to aarch64-pc-windows-gnullvm:
https://github.com/posit-dev/r-yaml12/blob/fix-windows-arm64-rust-target/tools/windows-rust-target.R

This was inherited from the current rextendr Windows Makevars template, which still derives the target from $(WIN):
https://github.com/extendr/rextendr/blob/main/inst/templates/Makevars.win.in#L1

If this works on your machine, I’ll merge the yaml12 fix and then follow up upstream in rextendr.