Update ui tests to 2018 edition

test_suite/tests/compiletest.rs

@@ -7,7 +7,12 @@ fn ui() {
     let config = compiletest::Config {
         mode: compiletest::common::Mode::Ui,
         src_base: std::path::PathBuf::from("tests/ui"),
-        target_rustcflags: Some("-L deps/target/debug/deps".to_owned()),
+        target_rustcflags: Some(String::from("\
+            --edition=2018 \
+            -L deps/target/debug/deps \
+            -Z unstable-options \
+            --extern serde_derive \
+        ")),
         ..Default::default()
     };