Some diagnostic-related nits 1. Use `&mut Diagnostic` instead of `&mut DiagnosticBuilder<'_, T>` 2. Make `diag.span_suggestions` take an `IntoIterator` instead of `Iterator`, just to remove some `.into_iter` calls on the caller. idk if I should add a lint to make sure people use `&mut Diagnostic` instead of `&mut DiagnosticBuilder<'_, T>` in cases where we're just, e.g., adding subdiagnostics to the diagnostic... maybe a followup.