message: Use box for Diagnostic
diff: @@ -6,7 +6,7 @@ use std::fmt;
  #[must_use]
  pub struct Diagnostic<'a> {
  - db: Builder<'a>,
  + db: Box<Builder<'a>>,
  }
  impl<'a> Diagnostic<'a> {
  @@ -21,7 +21,7 @@ impl<'a> Diagnostic<'a> {
  msg: &str,
  ) -> Self {
  Diagnostic {
  - db: Builder::new_diagnostic(
  + db: box Builder::new_diagnostic(
  &handler.inner,
  RustcDiagnostic::new_with_code(level, code, msg),
  ),
  