Add type-mismatch ui test

Author: Veykril

tests/ui/type-mismatch.rs

@@ -0,0 +1,21 @@
+use async_trait::async_trait;
+
+#[async_trait]
+trait Interface {
+    async fn f(&self);
+    async fn g(&self) -> ();
+}
+
+struct Thing;
+
+#[async_trait]
+impl Interface for Thing {
+    async fn f(&self) {
+        0
+    }
+    async fn g(&self) {
+        0
+    }
+}
+
+fn main() {}

tests/ui/type-mismatch.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> tests/ui/type-mismatch.rs:13:23
+   |
+13 |       async fn f(&self) {
+   |  _______________________^
+14 | |         0
+15 | |     }
+   | |_____^ expected integer, found `()`
+
+error[E0308]: mismatched types
+  --> tests/ui/type-mismatch.rs:16:23
+   |
+16 |       async fn g(&self) {
+   |  _______________________^
+17 | |         0
+18 | |     }
+   | |_____^ expected integer, found `()`