Wire Apple KMS release signing

Apple/NetworkExtension/libburrow/build-rust.sh

@@ -45,6 +45,27 @@ for ARCH in "${BURROW_ARCHS[@]}"; do
     esac
 done
 
+if [[ -x "$(command -v rustup)" ]]; then
+    INSTALLED_TARGETS="$(rustup target list --installed)"
+    MISSING_TARGETS=()
+    for TARGET in "${RUST_TARGETS[@]}"; do
+        if ! grep -qx "$TARGET" <<< "$INSTALLED_TARGETS"; then
+            MISSING_TARGETS+=("$TARGET")
+        fi
+    done
+    if (( ${#MISSING_TARGETS[@]} > 0 )); then
+        rustup target add "${MISSING_TARGETS[@]}"
+    fi
+else
+    SYSROOT="$(rustc --print sysroot)"
+    for TARGET in "${RUST_TARGETS[@]}"; do
+        if [[ ! -d "${SYSROOT}/lib/rustlib/${TARGET}" ]]; then
+            echo "error: Rust target ${TARGET} is not installed and rustup is unavailable" >&2
+            exit 1
+        fi
+    done
+fi
+
 # Pass all RUST_TARGETS in a single invocation
 CARGO_ARGS=()
 for TARGET in "${RUST_TARGETS[@]}"; do