replace_box: clean-up a bit (#15834)

changelog: none

clippy_lints/src/replace_box.rs

@@ -3,9 +3,8 @@
 use clippy_utils::ty::implements_trait;
 use clippy_utils::{is_default_equivalent_call, local_is_initialized, path_def_id, path_to_local};
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, LangItem, QPath};
+use rustc_hir::{Expr, ExprKind, QPath};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty::{self, Ty};
 use rustc_session::declare_lint_pass;
 use rustc_span::sym;
 
@@ -40,20 +39,11 @@ fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
         if let ExprKind::Assign(lhs, rhs, _) = &expr.kind
             && !lhs.span.from_expansion()
             && !rhs.span.from_expansion()
-        {
-            let lhs_ty = cx.typeck_results().expr_ty(lhs);
-
+            && let lhs_ty = cx.typeck_results().expr_ty(lhs)
             // No diagnostic for late-initialized locals
-            if let Some(local) = path_to_local(lhs)
-                && !local_is_initialized(cx, local)
-            {
-                return;
-            }
-
-            let Some(inner_ty) = get_box_inner_type(cx, lhs_ty) else {
-                return;
-            };
-
+            && path_to_local(lhs).is_none_or(|local| local_is_initialized(cx, local))
+            && let Some(inner_ty) = lhs_ty.boxed_ty()
+        {
             if let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default)
                 && implements_trait(cx, inner_ty, default_trait_id, &[])
                 && is_default_call(cx, rhs)
@@ -103,16 +93,6 @@ fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
     }
 }
 
-fn get_box_inner_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
-    if let ty::Adt(def, args) = ty.kind()
-        && cx.tcx.is_lang_item(def.did(), LangItem::OwnedBox)
-    {
-        Some(args.type_at(0))
-    } else {
-        None
-    }
-}
-
 fn is_default_call(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
     matches!(expr.kind, ExprKind::Call(func, _args) if is_default_equivalent_call(cx, func, Some(expr)))
 }