Fetch diagnostic item later.

Author: cjgillot

Diff for: compiler/rustc_lint/src/builtin.rs

@@ -756,14 +756,14 @@ impl<'tcx> LateLintPass<'tcx> for MissingDebugImplementations {
             _ => return,
         }
 
-        let Some(debug) = cx.tcx.get_diagnostic_item(sym::Debug) else { return };
-
         // Avoid listing trait impls if the trait is allowed.
         let (level, _) = cx.tcx.lint_level_at_node(MISSING_DEBUG_IMPLEMENTATIONS, item.hir_id());
         if level == Level::Allow {
             return;
         }
 
+        let Some(debug) = cx.tcx.get_diagnostic_item(sym::Debug) else { return };
+
         let has_impl = cx
             .tcx
             .non_blanket_impls_for_ty(debug, cx.tcx.type_of(item.owner_id).instantiate_identity())