make import.vis is not mutable

compiler/rustc_resolve/src/check_unused.rs

@@ -382,7 +382,7 @@ impl Resolver<'_, '_> {
         for import in self.potentially_unused_imports.iter() {
             match import.kind {
                 _ if import.used.get().is_some()
-                    || import.expect_vis().is_public()
+                    || import.vis.is_public()
                     || import.span.is_dummy() =>
                 {
                     if let ImportKind::MacroUse { .. } = import.kind {