Update quiz questions to 2018 edition

Author: dtolnay

docs/quiz.js

@@ -214,7 +214,7 @@ function showExplanation() {
         textOutput.value = questions[q].answer;
     }
     buttonPlayground.href =
-        "https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&code="
+        "https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&code="
         + encodeURIComponent(questions[q].code.trim());
     show(buttonPlayground);
     show(explanationAlert);

src/main.rs

@@ -138,8 +138,8 @@ fn check_answer(path: &Path, expected: &str) {
 
     let status = Command::new("rustc")
         .arg(path)
-        .arg("--out-dir")
-        .arg("/tmp/rust-quiz")
+        .arg("--edition=2018")
+        .arg("--out-dir=/tmp/rust-quiz")
         .stderr(Stdio::null())
         .status()
         .expect("failed to execute rustc");