Make Ty::boxed_ty return an Option

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -662,9 +662,10 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, '_, 'tcx> {
                                                 // `&dyn Trait`
                                                 ty::Ref(_, ty, _) if ty.is_trait() => true,
                                                 // `Box<dyn Trait>`
-                                                _ if ty.is_box() && ty.boxed_ty().is_trait() => {
+                                                _ if ty.boxed_ty().is_some_and(Ty::is_trait) => {
                                                     true
                                                 }
+
                                                 // `dyn Trait`
                                                 _ if ty.is_trait() => true,
                                                 // Anything else.

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -345,9 +345,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, '_, 'infcx, 'tcx> {
         variant_index: Option<VariantIdx>,
         including_tuple_field: IncludingTupleField,
     ) -> Option<String> {
-        if ty.is_box() {
+        if let Some(boxed_ty) = ty.boxed_ty() {
             // If the type is a box, the field is described from the boxed type
-            self.describe_field_from_ty(ty.boxed_ty(), field, variant_index, including_tuple_field)
+            self.describe_field_from_ty(boxed_ty, field, variant_index, including_tuple_field)
         } else {
             match *ty.kind() {
                 ty::Adt(def, _) => {

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -456,7 +456,7 @@ pub(crate) fn type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) ->
             if def.is_box()
                 && args.get(1).map_or(true, |arg| cx.layout_of(arg.expect_ty()).is_1zst()) =>
         {
-            build_pointer_or_reference_di_node(cx, t, t.boxed_ty(), unique_type_id)
+            build_pointer_or_reference_di_node(cx, t, t.expect_boxed_ty(), unique_type_id)
         }
         ty::FnDef(..) | ty::FnPtr(..) => build_subroutine_type_di_node(cx, unique_type_id),
         ty::Closure(..) => build_closure_env_di_node(cx, unique_type_id),

compiler/rustc_const_eval/src/interpret/call.rs

@@ -189,7 +189,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 ty::Ref(_, ty, _) => *ty,
                 ty::RawPtr(ty, _) => *ty,
                 // We only accept `Box` with the default allocator.
-                _ if ty.is_box_global(*self.tcx) => ty.boxed_ty(),
+                _ if ty.is_box_global(*self.tcx) => ty.expect_boxed_ty(),
                 _ => return Ok(None),
             }))
         };

compiler/rustc_hir_typeck/src/method/prelude_edition_lints.rs

@@ -63,8 +63,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // Instead, the problem is that the array-into_iter hack will no longer
                     // apply in Rust 2021.
                     (ARRAY_INTO_ITER, "2021")
-                } else if self_ty.is_box()
+                } else if self_ty.boxed_ty().is_some_and(Ty::is_slice)
-                    && self_ty.boxed_ty().is_slice()
                     && !span.at_least_rust_2024()
                 {
                     // In this case, it wasn't really a prelude addition that was the problem.

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1485,8 +1485,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
                         // Some trait methods are excluded for boxed slices before 2024.
                         // (`boxed_slice.into_iter()` wants a slice iterator for compatibility.)
-                        if self_ty.is_box()
+                        if self_ty.boxed_ty().is_some_and(Ty::is_slice)
-                            && self_ty.boxed_ty().is_slice()
                             && !method_name.span.at_least_rust_2024()
                         {
                             let trait_def = self.tcx.trait_def(poly_trait_ref.def_id());

compiler/rustc_lint/src/shadowed_into_iter.rs

@@ -94,12 +94,9 @@ impl<'tcx> LateLintPass<'tcx> for ShadowedIntoIter {
         fn is_ref_to_array(ty: Ty<'_>) -> bool {
             if let ty::Ref(_, pointee_ty, _) = *ty.kind() { pointee_ty.is_array() } else { false }
         }
-        fn is_boxed_slice(ty: Ty<'_>) -> bool {
-            ty.is_box() && ty.boxed_ty().is_slice()
-        }
         fn is_ref_to_boxed_slice(ty: Ty<'_>) -> bool {
             if let ty::Ref(_, pointee_ty, _) = *ty.kind() {
-                is_boxed_slice(pointee_ty)
+                pointee_ty.boxed_ty().is_some_and(Ty::is_slice)
             } else {
                 false
             }
@@ -119,7 +116,7 @@ impl<'tcx> LateLintPass<'tcx> for ShadowedIntoIter {
                     .iter()
                     .copied()
                     .take_while(|ty| !is_ref_to_boxed_slice(*ty))
-                    .position(|ty| is_boxed_slice(ty))
+                    .position(|ty| ty.boxed_ty().is_some_and(Ty::is_slice))
             {
                 (BOXED_SLICE_INTO_ITER, "Box<[T]>", "2024", idx == 0)
             } else {

compiler/rustc_lint/src/types.rs

@@ -1304,8 +1304,10 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
         match *ty.kind() {
             ty::Adt(def, args) => {
-                if def.is_box() && matches!(self.mode, CItemKind::Definition) {
+                if let Some(boxed) = ty.boxed_ty()
-                    if ty.boxed_ty().is_sized(tcx, self.cx.param_env) {
+                    && matches!(self.mode, CItemKind::Definition)
+                {
+                    if boxed.is_sized(tcx, self.cx.param_env) {
                         return FfiSafe;
                     } else {
                         return FfiUnsafe {

compiler/rustc_lint/src/unused.rs

@@ -283,9 +283,8 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             }
 
             match *ty.kind() {
-                ty::Adt(..) if ty.is_box() => {
+                ty::Adt(..) if let Some(boxed) = ty.boxed_ty() => {
-                    let boxed_ty = ty.boxed_ty();
+                    is_ty_must_use(cx, boxed, expr, span)
-                    is_ty_must_use(cx, boxed_ty, expr, span)
                         .map(|inner| MustUsePath::Boxed(Box::new(inner)))
                 }
                 ty::Adt(def, args) if cx.tcx.is_lang_item(def.did(), LangItem::Pin) => {

compiler/rustc_middle/src/ty/layout.rs

@@ -1075,11 +1075,13 @@ where
                 // the raw pointer, so size and align are set to the boxed type, but `pointee.safe`
                 // will still be `None`.
                 if let Some(ref mut pointee) = result {
-                    if offset.bytes() == 0 && this.ty.is_box() {
+                    if offset.bytes() == 0
+                        && let Some(boxed_ty) = this.ty.boxed_ty()
+                    {
                         debug_assert!(pointee.safe.is_none());
                         let optimize = tcx.sess.opts.optimize != OptLevel::No;
                         pointee.safe = Some(PointerKind::Box {
-                            unpin: optimize && this.ty.boxed_ty().is_unpin(tcx, cx.param_env()),
+                            unpin: optimize && boxed_ty.is_unpin(tcx, cx.param_env()),
                             global: this.ty.is_box_global(tcx),
                         });
                     }

compiler/rustc_middle/src/ty/sty.rs

@@ -1170,14 +1170,19 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
-    /// Panics if called on any type other than `Box<T>`.
+    pub fn boxed_ty(self) -> Option<Ty<'tcx>> {
-    pub fn boxed_ty(self) -> Ty<'tcx> {
         match self.kind() {
-            Adt(def, args) if def.is_box() => args.type_at(0),
+            Adt(def, args) if def.is_box() => Some(args.type_at(0)),
-            _ => bug!("`boxed_ty` is called on non-box type {:?}", self),
+            _ => None,
         }
     }
 
+    /// Panics if called on any type other than `Box<T>`.
+    pub fn expect_boxed_ty(self) -> Ty<'tcx> {
+        self.boxed_ty()
+            .unwrap_or_else(|| bug!("`expect_boxed_ty` is called on non-box type {:?}", self))
+    }
+
     /// A scalar type is one that denotes an atomic datum, with no sub-components.
     /// (A RawPtr is scalar because it represents a non-managed pointer, so its
     /// contents are abstract to rustc.)
@@ -1323,7 +1328,7 @@ impl<'tcx> Ty<'tcx> {
     /// Some types -- notably unsafe ptrs -- can only be dereferenced explicitly.
     pub fn builtin_deref(self, explicit: bool) -> Option<Ty<'tcx>> {
         match *self.kind() {
-            Adt(def, _) if def.is_box() => Some(self.boxed_ty()),
+            _ if let Some(boxed) = self.boxed_ty() => Some(boxed),
             Ref(_, ty, _) => Some(ty),
             RawPtr(ty, _) if explicit => Some(ty),
             _ => None,

compiler/rustc_middle/src/ty/util.rs

@@ -1628,7 +1628,7 @@ impl<'tcx> ExplicitSelf<'tcx> {
             _ if is_self_ty(self_arg_ty) => ByValue,
             ty::Ref(region, ty, mutbl) if is_self_ty(ty) => ByReference(region, mutbl),
             ty::RawPtr(ty, mutbl) if is_self_ty(ty) => ByRawPointer(mutbl),
-            ty::Adt(def, _) if def.is_box() && is_self_ty(self_arg_ty.boxed_ty()) => ByBox,
+            _ if self_arg_ty.boxed_ty().is_some_and(is_self_ty) => ByBox,
             _ => Other,
         }
     }

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

@@ -62,11 +62,13 @@ impl<'tcx, 'a> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'tcx, 'a> {
         let base_ty = self.local_decls[place.local].ty;
 
         // Derefer ensures that derefs are always the first projection
-        if place.projection.first() == Some(&PlaceElem::Deref) && base_ty.is_box() {
+        if let Some(PlaceElem::Deref) = place.projection.first()
+            && let Some(boxed_ty) = base_ty.boxed_ty()
+        {
             let source_info = self.local_decls[place.local].source_info;
 
             let (unique_ty, nonnull_ty, ptr_ty) =
-                build_ptr_tys(tcx, base_ty.boxed_ty(), self.unique_did, self.nonnull_did);
+                build_ptr_tys(tcx, boxed_ty, self.unique_did, self.nonnull_did);
 
             let ptr_local = self.patch.new_temp(ptr_ty, source_info.span);
 
@@ -120,13 +122,15 @@ impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs {
                     for (base, elem) in place.iter_projections() {
                         let base_ty = base.ty(&body.local_decls, tcx).ty;
 
-                        if elem == PlaceElem::Deref && base_ty.is_box() {
+                        if let PlaceElem::Deref = elem
+                            && let Some(boxed_ty) = base_ty.boxed_ty()
+                        {
                             // Clone the projections before us, since now we need to mutate them.
                             let new_projections =
                                 new_projections.get_or_insert_with(|| base.projection.to_vec());
 
                             let (unique_ty, nonnull_ty, ptr_ty) =
-                                build_ptr_tys(tcx, base_ty.boxed_ty(), unique_did, nonnull_did);
+                                build_ptr_tys(tcx, boxed_ty, unique_did, nonnull_did);
 
                             new_projections.extend_from_slice(&build_projection(
                                 unique_ty, nonnull_ty, ptr_ty,

compiler/rustc_monomorphize/src/collector.rs

@@ -1041,8 +1041,11 @@ fn find_vtable_types_for_unsizing<'tcx>(
     match (source_ty.kind(), target_ty.kind()) {
         (&ty::Ref(_, a, _), &ty::Ref(_, b, _) | &ty::RawPtr(b, _))
         | (&ty::RawPtr(a, _), &ty::RawPtr(b, _)) => ptr_vtable(a, b),
-        (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
+        (_, _)
-            ptr_vtable(source_ty.boxed_ty(), target_ty.boxed_ty())
+            if let Some(source_boxed) = source_ty.boxed_ty()
+                && let Some(target_boxed) = target_ty.boxed_ty() =>
+        {
+            ptr_vtable(source_boxed, target_boxed)
         }
 
         // T as dyn* Trait

compiler/rustc_monomorphize/src/lib.rs

@@ -1,5 +1,7 @@
 // tidy-alphabetical-start
 #![feature(array_windows)]
+#![feature(if_let_guard)]
+#![feature(let_chains)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 

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() == Some(found)
-                    && ty.boxed_ty() == found
                     && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
                 {
                     err.span_suggestion(

src/tools/clippy/clippy_lints/src/escape.rs

@@ -50,7 +50,7 @@ declare_clippy_lint! {
 }
 
 fn is_non_trait_box(ty: Ty<'_>) -> bool {
-    ty.is_box() && !ty.boxed_ty().is_trait()
+    ty.boxed_ty().is_some_and(|boxed| !boxed.is_trait())
 }
 
 struct EscapeDelegate<'a, 'tcx> {
@@ -191,8 +191,8 @@ impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
 impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
     fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
         // Large types need to be boxed to avoid stack overflows.
-        if ty.is_box() {
+        if let Some(boxed_ty) = ty.boxed_ty() {
-            self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
+            self.cx.layout_of(boxed_ty).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
         } else {
             false
         }

src/tools/clippy/clippy_lints/src/methods/mod.rs

@@ -5187,8 +5187,8 @@ impl SelfKind {
         fn matches_value<'a>(cx: &LateContext<'a>, parent_ty: Ty<'a>, ty: Ty<'a>) -> bool {
             if ty == parent_ty {
                 true
-            } else if ty.is_box() {
+            } else if let Some(boxed_ty) = ty.boxed_ty() {
-                ty.boxed_ty() == parent_ty
+                boxed_ty == parent_ty
             } else if is_type_diagnostic_item(cx, ty, sym::Rc) || is_type_diagnostic_item(cx, ty, sym::Arc) {
                 if let ty::Adt(_, args) = ty.kind() {
                     args.types().next().map_or(false, |t| t == parent_ty)

src/tools/clippy/clippy_lints/src/methods/utils.rs

@@ -16,7 +16,7 @@ pub(super) fn derefs_to_slice<'tcx>(
     fn may_slice<'a>(cx: &LateContext<'a>, ty: Ty<'a>) -> bool {
         match ty.kind() {
             ty::Slice(_) => true,
-            ty::Adt(def, _) if def.is_box() => may_slice(cx, ty.boxed_ty()),
+            ty::Adt(..) if let Some(boxed) = ty.boxed_ty() => may_slice(cx, boxed),
             ty::Adt(..) => is_type_diagnostic_item(cx, ty, sym::Vec),
             ty::Array(_, size) => size.try_eval_target_usize(cx.tcx, cx.param_env).is_some(),
             ty::Ref(_, inner, _) => may_slice(cx, *inner),
@@ -33,7 +33,7 @@ pub(super) fn derefs_to_slice<'tcx>(
     } else {
         match ty.kind() {
             ty::Slice(_) => Some(expr),
-            ty::Adt(def, _) if def.is_box() && may_slice(cx, ty.boxed_ty()) => Some(expr),
+            _ if ty.boxed_ty().is_some_and(|boxed| may_slice(cx, boxed)) => Some(expr),
             ty::Ref(_, inner, _) => {
                 if may_slice(cx, *inner) {
                     Some(expr)

src/tools/clippy/clippy_lints/src/unnecessary_box_returns.rs

@@ -75,11 +75,9 @@ impl UnnecessaryBoxReturns {
             .instantiate_bound_regions_with_erased(cx.tcx.fn_sig(def_id).skip_binder())
             .output();
 
-        if !return_ty.is_box() {
+        let Some(boxed_ty) = return_ty.boxed_ty() else {
             return;
-        }
+        };
-
-        let boxed_ty = return_ty.boxed_ty();
 
         // It's sometimes useful to return Box<T> if T is unsized, so don't lint those.
         // Also, don't lint if we know that T is very large, in which case returning

src/tools/clippy/clippy_utils/src/ty.rs

@@ -704,8 +704,8 @@ pub fn expr_sig<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>) -> Option<ExprFnS
 
 /// If the type is function like, get the signature for it.
 pub fn ty_sig<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<ExprFnSig<'tcx>> {
-    if ty.is_box() {
+    if let Some(boxed_ty) = ty.boxed_ty() {
-        return ty_sig(cx, ty.boxed_ty());
+        return ty_sig(cx, boxed_ty);
     }
     match *ty.kind() {
         ty::Closure(id, subs) => {