Switch to the Rust 2018 edition to use `async fn`.
"##,
+E0744: r##"
+Control-flow expressions are not allowed inside a const context.
+
+At the moment, `if` and `match`, as well as the looping constructs `for`,
+`while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
+
+```compile_fail,E0744
+const _: {
+ let mut x = 0;
+ loop {
+ x += 1;
+ if x == 4 {
+ break;
+ }
+ }
+
+ x
+};
+```
+
+"##,
+
;
E0226, // only a single explicit lifetime bound is permitted
E0472, // asm! is unsupported on this target