Prefer rustup on --target completion

Author: Angelo Ross

src/etc/_cargo

@@ -407,9 +407,15 @@ _cargo_cmds() {
 }
 
 _cargo_target_triple() {
-    local -a targets
-	targets=( ${(f)"$(rustc --print target-list)"} )
-	_describe 'target triple' targets
+    local -a result
+
+    if (( $+commands[rustup] )); then
+        result=( ${(f)"$(rustup target list --installed)"} )
+    else
+        result=( ${(f)"$(rustc --print target-list)"} )
+    fi
+
+    _describe 'target triple' result
 }
 
 #FIXME: Disabled until fixed