Rollup merge of #129969 - GrigorenkoPV:boxed-ty, r=compiler-errors

Make `Ty::boxed_ty` return an `Option`

Looks like a good place to use Rust's type system.

---

Most of 4ac7bcbaad/compiler/rustc_middle/src/ty/sty.rs (L971-L1963) looks like it could be moved to `TyKind` (then I guess  `Ty` should be made to deref to `TyKind`).

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -348,8 +348,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     }
                 }
                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found
-                    && ty.is_box()
-                    && ty.boxed_ty() == found
+                    && ty.boxed_ty() == Some(found)
                     && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
                 {
                     err.span_suggestion(