Fix build on Windows ARM64 (#52)

* Fix build on Windows ARM64

* Make Rust install guidance arch-aware and fail fast on missing target

Infer CARGO_TARGET before the toolchain checks so the install
instructions can name the correct target for the architecture, and
verify the target stdlib is present via rustc --print target-libdir
before starting the lengthy vendor extraction and build.

---------

Co-authored-by: shikokuchuo <53399081+shikokuchuo@users.noreply.github.com>

Author: jeroen

.github/workflows/R-CMD-check.yaml

@@ -21,6 +21,7 @@ jobs:
         config:
           - {os: macos-latest,   r: 'release'}
           - {os: windows-latest, r: 'release'}
+          - {os: windows-11-arm, r: 'release'}
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
           - {os: ubuntu-latest,   r: 'oldrel-1'}
@@ -32,6 +33,9 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - if: ${{ runner.os == 'windows' && runner.arch == 'ARM64' }}
+        run: rustup target add aarch64-pc-windows-gnullvm
+
       - uses: r-lib/actions/setup-pandoc@v2
 
       - uses: r-lib/actions/setup-r@v2

configure.win

@@ -51,6 +51,17 @@ if [ ${AUTOMERGE_FOUND} -eq 0 ]; then
     # Minimum Rust version required (MSRV)
     RUST_MSRV="1.85"
 
+    # Infer the Rust target from the C toolchain
+    case `${CC} -dumpmachine 2>/dev/null` in
+        aarch64*)
+            CARGO_TARGET="aarch64-pc-windows-gnullvm"
+            ;;
+        *)
+            CARGO_TARGET="x86_64-pc-windows-gnu"
+            ;;
+    esac
+    echo "Using Rust target: ${CARGO_TARGET}"
+
     # Check for Rust toolchain (cargo and rustc)
     # CRAN policy: check both system PATH and ~/.cargo/bin
     CARGO_CMD=""
@@ -82,7 +93,7 @@ if [ ${AUTOMERGE_FOUND} -eq 0 ]; then
         echo ""
         echo "Install Rust from https://rustup.rs/:"
         echo "  1. Download and run rustup-init.exe"
-        echo "  2. Select: x86_64-pc-windows-gnu toolchain"
+        echo "  2. After installation run: rustup target add ${CARGO_TARGET}"
         echo ""
         echo "Or via winget:  winget install Rustlang.Rustup"
         echo "Or via scoop:   scoop install rustup"
@@ -110,9 +121,15 @@ if [ ${AUTOMERGE_FOUND} -eq 0 ]; then
         exit 1
     fi
 
-    # Ensure GNU toolchain is used (required for Rtools compatibility)
-    if ! $RUSTC_CMD --version --verbose 2>/dev/null | grep -q "host:.*windows-gnu"; then
-        echo "Note: Configuring Rust to use GNU toolchain for Rtools compatibility"
+    # Fail before the lengthy build if the target stdlib is absent
+    TARGET_LIBDIR=$($RUSTC_CMD --print target-libdir --target "${CARGO_TARGET}" 2>/dev/null)
+    if [ ! -d "$TARGET_LIBDIR" ]; then
+        echo "-------------------- RUST TARGET NOT INSTALLED --------------------"
+        echo "The Rust standard library for ${CARGO_TARGET} is not installed."
+        echo ""
+        echo "Install it with: rustup target add ${CARGO_TARGET}"
+        echo "--------------------------------------------------------------------"
+        exit 1
     fi
 
     echo "Building automerge-c from bundled source..."
@@ -153,13 +170,10 @@ EOF
         xz -dc src/automerge/rust/vendor.tar.xz | tar -xf - -C src/automerge/rust
     fi
 
-    # Force GNU target to match MinGW toolchain from Rtools
-    export RUSTUP_TOOLCHAIN="stable-x86_64-pc-windows-gnu"
-
     echo "Building (this may take several minutes)..."
     # Default features select UTF-32 indexing; -j2 respects CRAN's core limit.
     # build.rs runs cbindgen to emit ${CBINDGEN_TARGET_DIR}/automerge.h.
-    "$CARGO_CMD" build --release -j2 \
+    "$CARGO_CMD" build --release -j2 --target "${CARGO_TARGET}" \
         --manifest-path src/automerge/rust/automerge-c/Cargo.toml \
         || { echo "ERROR: cargo build failed"; exit 1; }
 
@@ -178,7 +192,7 @@ EOF
         "$HEADER" > "${HEADER}.tmp" && mv "${HEADER}.tmp" "$HEADER"
 
     # Stage the static library where Makevars expects it
-    cp am-build/target/release/libautomerge_core.a am-install/lib/libautomerge.a \
+    cp "am-build/target/${CARGO_TARGET}/release/libautomerge_core.a" am-install/lib/libautomerge.a \
         || { echo "ERROR: failed to stage static library"; exit 1; }
 
     rm -rf am-build