Add completion test

Author: Jonas Schievink

crates/ide-completion/src/completions/dot.rs

@@ -785,4 +785,24 @@ fn main() {
 ",
         )
     }
+
+    #[test]
+    fn tuple_index_completion() {
+        check(
+            r#"
+struct I;
+impl I {
+    fn i_method(&self) {}
+}
+struct S((), I);
+
+fn f(s: S) {
+    s.1.$0
+}
+"#,
+            expect![[r#"
+                me i_method() fn(&self)
+            "#]],
+        );
+    }
 }