Revert "Use a more straight-forward size check"

This reverts commit 2c642b5.

Author: botahamec

clippy_lints/src/unnecessary_box_returns.rs

@@ -1,4 +1,4 @@
-use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::{diagnostics::span_lint_and_then, ty::implements_trait};
 use rustc_errors::Applicability;
 use rustc_hir::{def_id::LocalDefId, FnDecl, FnRetTy, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind};
 use rustc_hir_analysis::hir_ty_to_ty;
@@ -63,9 +63,10 @@ impl UnnecessaryBoxReturns {
         }
 
         let boxed_ty = return_ty.boxed_ty();
+        let Some(sized_trait) = cx.tcx.lang_items().sized_trait() else { return };
 
         // it's sometimes useful to return Box<T> if T is unsized, so don't lint those
-        if boxed_ty.is_sized(cx.tcx, cx.param_env) {
+        if implements_trait(cx, boxed_ty, sized_trait, &[]) {
             span_lint_and_then(
                 cx,
                 UNNECESSARY_BOX_RETURNS,