Fix a panic with a diagnostics fix when a keyword is used as a field

I found it easiest to fix in the quickfix code, and not deeper (e.g. body lowering).

crates/ide-diagnostics/src/handlers/unresolved_field.rs

@@ -90,7 +90,9 @@ fn field_fix(ctx: &DiagnosticsContext<'_>, d: &hir::UnresolvedField) -> Option<A
         make::ty("()")
     };
 
-    if !is_editable_crate(target_module.krate(), ctx.sema.db) {
+    if !is_editable_crate(target_module.krate(), ctx.sema.db)
+        || SyntaxKind::from_keyword(field_name, ctx.edition).is_some()
+    {
         return None;
     }
 
@@ -501,4 +503,19 @@ fn main() {}
             "#,
         )
     }
+
+    #[test]
+    fn regression_18683() {
+        check_diagnostics(
+            r#"
+struct S;
+impl S {
+    fn f(self) {
+        self.self
+          // ^^^^ error: no field `self` on type `S`
+    }
+}
+        "#,
+        );
+    }
 }