Use slicing syntax instead of .as_slice()

tests/json_struct.rs

@@ -24,7 +24,7 @@ fn test_json_struct() {
     };
 
     let s = serde::json::to_string(&input).unwrap();
-    assert_eq!(s.as_slice(), r#"{"$schema":"a","title":"b","type":3}"#);
+    assert_eq!(&s[], r#"{"$schema":"a","title":"b","type":3}"#);
 
     let output: Test = serde::json::from_str(&s).unwrap();
     assert_eq!(input, output);