Add shim for cargo

Author: jeroen

R/build.R

@@ -161,6 +161,7 @@ wasm_build <- function(pkg, tarball_path, contrib_bin, compress) {
   webr_profile <- system.file("webr-profile", package = "rwasm")
   sys_bin <- system.file("bin", package = "rwasm")
   webr_env <- c(
+    paste0("HOST_CARGO"=, Sys.which('cargo')),
     paste0("BUILD_PLATFORM=", R.Version()$platform),
     paste0("R_PROFILE_USER=", webr_profile),
     paste0("R_MAKEVARS_USER=", webr_vars),

inst/bin/cargo

@@ -0,0 +1,32 @@
+#!/bin/sh
+set -e
+
+for arg in "$@"; do
+  case "$arg" in
+    "--manifest-path="*)
+    manifest="${arg:16}"
+    ;;
+    "--target="*)
+    target="$arg"
+    ;;
+  esac
+done
+
+# Invoke real cargo
+if [ -z "${HOST_CARGO}" ]; then
+  echo "cargo not found"
+  exit 1
+fi
+
+# Make cargo cross-compile by default. Artifacts then land under target/<triple>/
+# instead of target/release where package Makevars expect them, so copy them
+# back up after the build.
+export CARGO_BUILD_TARGET="wasm32-unknown-emscripten"
+echo "Cross compiling for $CARGO_BUILD_TARGET..."
+${HOST_CARGO} "$@"
+
+if [ "$1" = "build" ] && [ -z "$target" ]; then
+  echo "Copying $CARGO_BUILD_TARGET artifacts to target dir"
+  cd "$(dirname ${manifest:-.})/target"
+  cp -Rf "$CARGO_BUILD_TARGET"/* . || true
+fi