add test

fix tidy

remove dir

tests/ui/async-await/const-async-fn-in-main.rs

@@ -0,0 +1,7 @@
+// edition:2021
+// Check what happens when a const async fn is in the main function (#102796)
+
+fn main() {
+    const async fn a() {}
+//~^ ERROR functions cannot be both `const` and `async`
+}

tests/ui/async-await/const-async-fn-in-main.stderr

@@ -0,0 +1,11 @@
+error: functions cannot be both `const` and `async`
+  --> $DIR/const-async-fn-in-main.rs:5:5
+   |
+LL |     const async fn a() {}
+   |     ^^^^^-^^^^^----------
+   |     |     |
+   |     |     `async` because of this
+   |     `const` because of this
+
+error: aborting due to previous error
+