build: cache Rust/Go downloads when required

.repo/build

@@ -15,6 +15,7 @@ traperr() {
 trap traperr ERR
 
 PACKAGES=("$@")
+EXTRA_ARGS=()
 
 if [ ${#PACKAGES[@]} -eq 0 ]; then
   echo "${BOLD}${RED}ERROR:${RESET}${BOLD} no packages specified${RESET}"
@@ -24,7 +25,20 @@ fi
 for package in "${PACKAGES[@]}"; do
   (
     # change to package directory
-    cd "$package" || exit 1;
+    cd "$package" || exit 1
+
+    # capture output for parsing dependencies
+    makepkg_output=$(makepkg --printsrcinfo)
+
+    # cache downloaded crates
+    if grep -q "depends = rust\|cargo" <<<"$makepkg_output"; then
+      EXTRA_ARGS+=(--bind-rw /var/cache/cargo:/build/.cargo)
+    fi
+
+    # cache downloaded modules
+    if grep -q "depends = go" <<<"$makepkg_output"; then
+      EXTRA_ARGS+=(--bind-rw /var/cache/golang:/build/go)
+    fi
 
     # build signed package in chroot & add to local repo
     aur build \
@@ -32,7 +46,8 @@ for package in "${PACKAGES[@]}"; do
       --gpg-sign \
       --remove \
       --force \
-      --makepkg-conf=/etc/aurutils/makepkg-grawlinson.conf
+      --makepkg-conf=/etc/aurutils/makepkg-grawlinson.conf \
+      "${EXTRA_ARGS[@]}"
   )
 done