Goto definition works for S { a: } case

What happens here is that we lower `: ` to a missing expression, and
then correctly record that the corresponding field expression resolves
to a specific field. Where we fail is in the mapping of syntax to this
missing expression. Doing it via `ast_field.expr()` fails, as that
expression is `None`. Instead, we go in the opposite direcition and ask
each lowered field about its source.

This works, but has wrong complexity `O(N)` and, really, the
implementation is just too complex. We need some better management of
data here.

crates/ide/src/goto_definition.rs

@@ -1158,6 +1158,17 @@ struct S;
 
 //- /m.rs
 //! [`super::S$0`]
+"#,
+        )
+    }
+
+    #[test]
+    fn goto_incomplete_field() {
+        check(
+            r#"
+struct A { a: u32 }
+         //^
+fn foo() { A { a$0: }; }
 "#,
         )
     }