Add a missing test

Author: saethlin

tests/ui/precondition-checks/as_ascii_unchecked.rs

@@ -0,0 +1,17 @@
+//@ run-crash
+//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
+//@ error-pattern: as_ascii_unchecked requires that the
+//@ revisions: char str
+
+#![feature(ascii_char)]
+
+use std::ascii::Char;
+
+fn main() {
+    unsafe {
+        #[cfg(char)]
+        let _c: Char = '🦀'.as_ascii_unchecked();
+        #[cfg(str)]
+        let _c: &[Char] = "🦀".as_ascii_unchecked();
+    }
+}