Fix support for arm64 on Windows

Author: jeroen

src/Makevars.ucrt

@@ -1,4 +1,4 @@
-TARGET = x86_64-pc-windows-gnu
+TARGET = $(shell "$(R_HOME)/bin$(R_ARCH_BIN)/Rscript.exe" "../tools/rustarch.R")
 LIBDIR = myrustlib/target/$(TARGET)/release
 STATLIB = $(LIBDIR)/libmyrustlib.a
 PKG_LIBS = -L$(LIBDIR) -lmyrustlib -lws2_32 -ladvapi32 -luserenv -ldbghelp -lbcrypt -lntdll

tools/rustarch.R

@@ -0,0 +1,13 @@
+# See notes in FAQ about ARM64 support:
+# https://github.com/r-rust/faq#does-rust-support-windows-on-arm64-aarch64
+arch <- if(grepl("aarch", R.version$platform)){
+  "aarch64-pc-windows-gnullvm"
+} else if(grepl("clang", Sys.getenv('R_COMPILED_BY'))){
+  "x86_64-pc-windows-gnullvm"
+} else if(grepl("i386", R.version$platform)){
+  "i686-pc-windows-gnu"
+} else {
+  "x86_64-pc-windows-gnu"
+}
+
+cat(arch)