src/value_and_place.rs

   1 use crate::prelude::*;
   2
   3 use cranelift_codegen::ir::immediates::Offset32;
   4
   5 fn codegen_field<'tcx>(
   6     fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
   7     base: Pointer,
   8     extra: Option<Value>,
   9     layout: TyLayout<'tcx>,
  10     field: mir::Field,
  11 ) -> (Pointer, TyLayout<'tcx>) {
  12     let field_offset = layout.fields.offset(field.index());
  13     let field_layout = layout.field(&*fx, field.index());
  14
  15     let simple = |fx: &mut FunctionCx<_>| {
  16         (
  17             base.offset_i64(fx, i64::try_from(field_offset.bytes()).unwrap()),
  18             field_layout,
  19         )
  20     };
  21
  22     if let Some(extra) = extra {
  23         if !field_layout.is_unsized() {
  24             return simple(fx);
  25         }
  26         match field_layout.ty.kind {
  27             ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(fx),
  28             ty::Adt(def, _) if def.repr.packed() => {
  29                 assert_eq!(layout.align.abi.bytes(), 1);
  30                 return simple(fx);
  31             }
  32             _ => {
  33                 // We have to align the offset for DST's
  34                 let unaligned_offset = field_offset.bytes();
  35                 let (_, unsized_align) = crate::unsize::size_and_align_of_dst(fx, field_layout, extra);
  36
  37                 let one = fx.bcx.ins().iconst(pointer_ty(fx.tcx), 1);
  38                 let align_sub_1 = fx.bcx.ins().isub(unsized_align, one);
  39                 let and_lhs = fx.bcx.ins().iadd_imm(align_sub_1, unaligned_offset as i64);
  40                 let zero = fx.bcx.ins().iconst(pointer_ty(fx.tcx), 0);
  41                 let and_rhs = fx.bcx.ins().isub(zero, unsized_align);
  42                 let offset = fx.bcx.ins().band(and_lhs, and_rhs);
  43
  44                 (
  45                     base.offset_value(fx, offset),
  46                     field_layout,
  47                 )
  48             }
  49         }
  50     } else {
  51         simple(fx)
  52     }
  53 }
  54
  55 fn scalar_pair_calculate_b_offset(tcx: TyCtxt<'_>, a_scalar: &Scalar, b_scalar: &Scalar) -> Offset32 {
  56     let b_offset = a_scalar
  57         .value
  58         .size(&tcx)
  59         .align_to(b_scalar.value.align(&tcx).abi);
  60     Offset32::new(b_offset.bytes().try_into().unwrap())
  61 }
  62
  63 /// A read-only value
  64 #[derive(Debug, Copy, Clone)]
  65 pub struct CValue<'tcx>(CValueInner, TyLayout<'tcx>);
  66
  67 #[derive(Debug, Copy, Clone)]
  68 enum CValueInner {
  69     ByRef(Pointer),
  70     ByVal(Value),
  71     ByValPair(Value, Value),
  72 }
  73
  74 impl<'tcx> CValue<'tcx> {
  75     pub fn by_ref(ptr: Pointer, layout: TyLayout<'tcx>) -> CValue<'tcx> {
  76         CValue(CValueInner::ByRef(ptr), layout)
  77     }
  78
  79     pub fn by_val(value: Value, layout: TyLayout<'tcx>) -> CValue<'tcx> {
  80         CValue(CValueInner::ByVal(value), layout)
  81     }
  82
  83     pub fn by_val_pair(value: Value, extra: Value, layout: TyLayout<'tcx>) -> CValue<'tcx> {
  84         CValue(CValueInner::ByValPair(value, extra), layout)
  85     }
  86
  87     pub fn layout(&self) -> TyLayout<'tcx> {
  88         self.1
  89     }
  90
  91     // FIXME remove
  92     pub fn force_stack<'a>(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) -> Pointer {
  93         let layout = self.1;
  94         match self.0 {
  95             CValueInner::ByRef(ptr) => ptr,
  96             CValueInner::ByVal(_) | CValueInner::ByValPair(_, _) => {
  97                 let cplace = CPlace::new_stack_slot(fx, layout);
  98                 cplace.write_cvalue(fx, self);
  99                 cplace.to_ptr(fx)
 100             }
 101         }
 102     }
 103
 104     pub fn try_to_addr(self) -> Option<Value> {
 105         match self.0 {
 106             CValueInner::ByRef(ptr) => {
 107                 if let Some((base_addr, offset)) = ptr.try_get_addr_and_offset() {
 108                     if offset == Offset32::new(0) {
 109                         Some(base_addr)
 110                     } else {
 111                         None
 112                     }
 113                 } else {
 114                     None
 115                 }
 116             }
 117             CValueInner::ByVal(_) | CValueInner::ByValPair(_, _) => None,
 118         }
 119     }
 120
 121     /// Load a value with layout.abi of scalar
 122     pub fn load_scalar<'a>(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) -> Value {
 123         let layout = self.1;
 124         match self.0 {
 125             CValueInner::ByRef(ptr) => {
 126                 let clif_ty = match layout.abi {
 127                     layout::Abi::Scalar(ref scalar) => scalar_to_clif_type(fx.tcx, scalar.clone()),
 128                     layout::Abi::Vector { ref element, count } => {
 129                         scalar_to_clif_type(fx.tcx, element.clone())
 130                             .by(u16::try_from(count).unwrap()).unwrap()
 131                     }
 132                     _ => unreachable!(),
 133                 };
 134                 ptr.load(fx, clif_ty, MemFlags::new())
 135             }
 136             CValueInner::ByVal(value) => value,
 137             CValueInner::ByValPair(_, _) => bug!("Please use load_scalar_pair for ByValPair"),
 138         }
 139     }
 140
 141     /// Load a value pair with layout.abi of scalar pair
 142     pub fn load_scalar_pair<'a>(
 143         self,
 144         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 145     ) -> (Value, Value) {
 146         let layout = self.1;
 147         match self.0 {
 148             CValueInner::ByRef(ptr) => {
 149                 let (a_scalar, b_scalar) = match &layout.abi {
 150                     layout::Abi::ScalarPair(a, b) => (a, b),
 151                     _ => unreachable!("load_scalar_pair({:?})", self),
 152                 };
 153                 let b_offset = scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
 154                 let clif_ty1 = scalar_to_clif_type(fx.tcx, a_scalar.clone());
 155                 let clif_ty2 = scalar_to_clif_type(fx.tcx, b_scalar.clone());
 156                 let val1 = ptr.load(fx, clif_ty1, MemFlags::new());
 157                 let val2 = ptr.offset(fx, b_offset).load(fx, clif_ty2, MemFlags::new());
 158                 (val1, val2)
 159             }
 160             CValueInner::ByVal(_) => bug!("Please use load_scalar for ByVal"),
 161             CValueInner::ByValPair(val1, val2) => (val1, val2),
 162         }
 163     }
 164
 165     pub fn value_field<'a>(
 166         self,
 167         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 168         field: mir::Field,
 169     ) -> CValue<'tcx> {
 170         let layout = self.1;
 171         match self.0 {
 172             CValueInner::ByVal(val) => {
 173                 match layout.abi {
 174                     layout::Abi::Vector { element: _, count } => {
 175                         let count = u8::try_from(count).expect("SIMD type with more than 255 lanes???");
 176                         let field = u8::try_from(field.index()).unwrap();
 177                         assert!(field < count);
 178                         let lane = fx.bcx.ins().extractlane(val, field);
 179                         let field_layout = layout.field(&*fx, usize::from(field));
 180                         CValue::by_val(lane, field_layout)
 181                     }
 182                     _ => unreachable!("value_field for ByVal with abi {:?}", layout.abi),
 183                 }
 184             }
 185             CValueInner::ByRef(ptr) => {
 186                 let (field_ptr, field_layout) = codegen_field(fx, ptr, None, layout, field);
 187                 CValue::by_ref(field_ptr, field_layout)
 188             }
 189             _ => bug!("place_field for {:?}", self),
 190         }
 191     }
 192
 193     pub fn unsize_value<'a>(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>, dest: CPlace<'tcx>) {
 194         crate::unsize::coerce_unsized_into(fx, self, dest);
 195     }
 196
 197     /// If `ty` is signed, `const_val` must already be sign extended.
 198     pub fn const_val<'a>(
 199         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 200         layout: TyLayout<'tcx>,
 201         const_val: u128,
 202     ) -> CValue<'tcx> {
 203         let clif_ty = fx.clif_type(layout.ty).unwrap();
 204
 205         let val = match layout.ty.kind {
 206             ty::TyKind::Uint(UintTy::U128) | ty::TyKind::Int(IntTy::I128) => {
 207                 let lsb = fx.bcx.ins().iconst(types::I64, const_val as u64 as i64);
 208                 let msb = fx
 209                     .bcx
 210                     .ins()
 211                     .iconst(types::I64, (const_val >> 64) as u64 as i64);
 212                 fx.bcx.ins().iconcat(lsb, msb)
 213             }
 214             ty::TyKind::Bool => {
 215                 assert!(
 216                     const_val == 0 || const_val == 1,
 217                     "Invalid bool 0x{:032X}",
 218                     const_val
 219                 );
 220                 fx.bcx.ins().iconst(types::I8, const_val as i64)
 221             }
 222             ty::TyKind::Uint(_) | ty::TyKind::Ref(..) | ty::TyKind::RawPtr(..) => fx
 223                 .bcx
 224                 .ins()
 225                 .iconst(clif_ty, u64::try_from(const_val).expect("uint") as i64),
 226             ty::TyKind::Int(_) => fx.bcx.ins().iconst(clif_ty, const_val as i128 as i64),
 227             _ => panic!(
 228                 "CValue::const_val for non bool/integer/pointer type {:?} is not allowed",
 229                 layout.ty
 230             ),
 231         };
 232
 233         CValue::by_val(val, layout)
 234     }
 235
 236     pub fn unchecked_cast_to(self, layout: TyLayout<'tcx>) -> Self {
 237         CValue(self.0, layout)
 238     }
 239 }
 240
 241 /// A place where you can write a value to or read a value from
 242 #[derive(Debug, Copy, Clone)]
 243 pub struct CPlace<'tcx> {
 244     inner: CPlaceInner,
 245     layout: TyLayout<'tcx>,
 246 }
 247
 248 #[derive(Debug, Copy, Clone)]
 249 pub enum CPlaceInner {
 250     Var(Local),
 251     Addr(Pointer, Option<Value>),
 252     NoPlace,
 253 }
 254
 255 impl<'tcx> CPlace<'tcx> {
 256     pub fn layout(&self) -> TyLayout<'tcx> {
 257         self.layout
 258     }
 259
 260     pub fn inner(&self) -> &CPlaceInner {
 261         &self.inner
 262     }
 263
 264     pub fn no_place(layout: TyLayout<'tcx>) -> CPlace<'tcx> {
 265         CPlace {
 266             inner: CPlaceInner::NoPlace,
 267             layout,
 268         }
 269     }
 270
 271     pub fn new_stack_slot(
 272         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 273         layout: TyLayout<'tcx>,
 274     ) -> CPlace<'tcx> {
 275         assert!(!layout.is_unsized());
 276         if layout.size.bytes() == 0 {
 277             return CPlace {
 278                 inner: CPlaceInner::NoPlace,
 279                 layout,
 280             };
 281         }
 282
 283         let stack_slot = fx.bcx.create_stack_slot(StackSlotData {
 284             kind: StackSlotKind::ExplicitSlot,
 285             size: layout.size.bytes() as u32,
 286             offset: None,
 287         });
 288         CPlace {
 289             inner: CPlaceInner::Addr(Pointer::stack_slot(stack_slot), None),
 290             layout,
 291         }
 292     }
 293
 294     pub fn new_var(
 295         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 296         local: Local,
 297         layout: TyLayout<'tcx>,
 298     ) -> CPlace<'tcx> {
 299         fx.bcx
 300             .declare_var(mir_var(local), fx.clif_type(layout.ty).unwrap());
 301         CPlace {
 302             inner: CPlaceInner::Var(local),
 303             layout,
 304         }
 305     }
 306
 307     pub fn for_ptr(ptr: Pointer, layout: TyLayout<'tcx>) -> CPlace<'tcx> {
 308         CPlace {
 309             inner: CPlaceInner::Addr(ptr, None),
 310             layout,
 311         }
 312     }
 313
 314     pub fn for_ptr_with_extra(ptr: Pointer, extra: Value, layout: TyLayout<'tcx>) -> CPlace<'tcx> {
 315         CPlace {
 316             inner: CPlaceInner::Addr(ptr, Some(extra)),
 317             layout,
 318         }
 319     }
 320
 321     pub fn to_cvalue(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) -> CValue<'tcx> {
 322         let layout = self.layout();
 323         match self.inner {
 324             CPlaceInner::Var(var) => {
 325                 let val = fx.bcx.use_var(mir_var(var));
 326                 fx.bcx.set_val_label(val, cranelift_codegen::ir::ValueLabel::from_u32(var.as_u32()));
 327                 CValue::by_val(val, layout)
 328             }
 329             CPlaceInner::Addr(ptr, extra) => {
 330                 assert!(extra.is_none(), "unsized values are not yet supported");
 331                 CValue::by_ref(ptr, layout)
 332             }
 333             CPlaceInner::NoPlace => CValue::by_ref(
 334                 Pointer::const_addr(fx, i64::try_from(self.layout.align.pref.bytes()).unwrap()),
 335                 layout,
 336             ),
 337         }
 338     }
 339
 340     pub fn to_ptr(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) -> Pointer {
 341         match self.to_ptr_maybe_unsized(fx) {
 342             (ptr, None) => ptr,
 343             (_, Some(_)) => bug!("Expected sized cplace, found {:?}", self),
 344         }
 345     }
 346
 347     pub fn to_ptr_maybe_unsized(
 348         self,
 349         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 350     ) -> (Pointer, Option<Value>) {
 351         match self.inner {
 352             CPlaceInner::Addr(ptr, extra) => (ptr, extra),
 353             CPlaceInner::NoPlace => {
 354                 (
 355                     Pointer::const_addr(fx, i64::try_from(self.layout.align.pref.bytes()).unwrap()),
 356                     None,
 357                 )
 358             }
 359             CPlaceInner::Var(_) => bug!("Expected CPlace::Addr, found CPlace::Var"),
 360         }
 361     }
 362
 363     pub fn write_cvalue(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>, from: CValue<'tcx>) {
 364         #[cfg(debug_assertions)]
 365         {
 366             use cranelift_codegen::cursor::{Cursor, CursorPosition};
 367             let cur_ebb = match fx.bcx.cursor().position() {
 368                 CursorPosition::After(ebb) => ebb,
 369                 _ => unreachable!(),
 370             };
 371             fx.add_comment(
 372                 fx.bcx.func.layout.last_inst(cur_ebb).unwrap(),
 373                 format!("write_cvalue: {:?} <- {:?}",self, from),
 374             );
 375         }
 376
 377         let from_ty = from.layout().ty;
 378         let to_ty = self.layout().ty;
 379
 380         fn assert_assignable<'tcx>(
 381             fx: &FunctionCx<'_, 'tcx, impl Backend>,
 382             from_ty: Ty<'tcx>,
 383             to_ty: Ty<'tcx>,
 384         ) {
 385             match (&from_ty.kind, &to_ty.kind) {
 386                 (ty::Ref(_, t, Mutability::Not), ty::Ref(_, u, Mutability::Not))
 387                 | (ty::Ref(_, t, Mutability::Mut), ty::Ref(_, u, Mutability::Not))
 388                 | (ty::Ref(_, t, Mutability::Mut), ty::Ref(_, u, Mutability::Mut)) => {
 389                     assert_assignable(fx, t, u);
 390                     // &mut T -> &T is allowed
 391                     // &'a T -> &'b T is allowed
 392                 }
 393                 (ty::Ref(_, _, Mutability::Not), ty::Ref(_, _, Mutability::Mut)) => panic!(
 394                     "Cant assign value of type {} to place of type {}",
 395                     from_ty, to_ty
 396                 ),
 397                 (ty::FnPtr(_), ty::FnPtr(_)) => {
 398                     let from_sig = fx.tcx.normalize_erasing_late_bound_regions(
 399                         ParamEnv::reveal_all(),
 400                         &from_ty.fn_sig(fx.tcx),
 401                     );
 402                     let to_sig = fx.tcx.normalize_erasing_late_bound_regions(
 403                         ParamEnv::reveal_all(),
 404                         &to_ty.fn_sig(fx.tcx),
 405                     );
 406                     assert_eq!(
 407                         from_sig, to_sig,
 408                         "Can't write fn ptr with incompatible sig {:?} to place with sig {:?}\n\n{:#?}",
 409                         from_sig, to_sig, fx,
 410                     );
 411                     // fn(&T) -> for<'l> fn(&'l T) is allowed
 412                 }
 413                 (ty::Dynamic(from_traits, _), ty::Dynamic(to_traits, _)) => {
 414                     let from_traits = fx
 415                         .tcx
 416                         .normalize_erasing_late_bound_regions(ParamEnv::reveal_all(), from_traits);
 417                     let to_traits = fx
 418                         .tcx
 419                         .normalize_erasing_late_bound_regions(ParamEnv::reveal_all(), to_traits);
 420                     assert_eq!(
 421                         from_traits, to_traits,
 422                         "Can't write trait object of incompatible traits {:?} to place with traits {:?}\n\n{:#?}",
 423                         from_traits, to_traits, fx,
 424                     );
 425                     // dyn for<'r> Trait<'r> -> dyn Trait<'_> is allowed
 426                 }
 427                 _ => {
 428                     assert_eq!(
 429                         from_ty,
 430                         to_ty,
 431                         "Can't write value with incompatible type {:?} to place with type {:?}\n\n{:#?}",
 432                         from_ty,
 433                         to_ty,
 434                         fx,
 435                     );
 436                 }
 437             }
 438         }
 439
 440         assert_assignable(fx, from_ty, to_ty);
 441
 442         let dst_layout = self.layout();
 443         let to_ptr = match self.inner {
 444             CPlaceInner::Var(var) => {
 445                 let data = from.load_scalar(fx);
 446                 fx.bcx.set_val_label(data, cranelift_codegen::ir::ValueLabel::from_u32(var.as_u32()));
 447                 fx.bcx.def_var(mir_var(var), data);
 448                 return;
 449             }
 450             CPlaceInner::Addr(ptr, None) => ptr,
 451             CPlaceInner::NoPlace => {
 452                 if dst_layout.abi != Abi::Uninhabited {
 453                     assert_eq!(dst_layout.size.bytes(), 0, "{:?}", dst_layout);
 454                 }
 455                 return;
 456             }
 457             CPlaceInner::Addr(_, Some(_)) => bug!("Can't write value to unsized place {:?}", self),
 458         };
 459
 460         match self.layout().abi {
 461             // FIXME make Abi::Vector work too
 462             Abi::Scalar(_) => {
 463                 let val = from.load_scalar(fx);
 464                 to_ptr.store(fx, val, MemFlags::new());
 465                 return;
 466             }
 467             Abi::ScalarPair(ref a_scalar, ref b_scalar) => {
 468                 let (value, extra) = from.load_scalar_pair(fx);
 469                 let b_offset = scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
 470                 to_ptr.store(fx, value, MemFlags::new());
 471                 to_ptr.offset(fx, b_offset).store(fx, extra, MemFlags::new());
 472                 return;
 473             }
 474             _ => {}
 475         }
 476
 477         match from.0 {
 478             CValueInner::ByVal(val) => {
 479                 to_ptr.store(fx, val, MemFlags::new());
 480             }
 481             CValueInner::ByValPair(_, _) => {
 482                 bug!(
 483                     "Non ScalarPair abi {:?} for ByValPair CValue",
 484                     dst_layout.abi
 485                 );
 486             }
 487             CValueInner::ByRef(from_ptr) => {
 488                 let from_addr = from_ptr.get_addr(fx);
 489                 let to_addr = to_ptr.get_addr(fx);
 490                 let src_layout = from.1;
 491                 let size = dst_layout.size.bytes();
 492                 let src_align = src_layout.align.abi.bytes() as u8;
 493                 let dst_align = dst_layout.align.abi.bytes() as u8;
 494                 fx.bcx.emit_small_memcpy(
 495                     fx.module.target_config(),
 496                     to_addr,
 497                     from_addr,
 498                     size,
 499                     dst_align,
 500                     src_align,
 501                 );
 502             }
 503         }
 504     }
 505
 506     pub fn place_field(
 507         self,
 508         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 509         field: mir::Field,
 510     ) -> CPlace<'tcx> {
 511         let layout = self.layout();
 512         let (base, extra) = self.to_ptr_maybe_unsized(fx);
 513
 514         let (field_ptr, field_layout) = codegen_field(fx, base, extra, layout, field);
 515         if field_layout.is_unsized() {
 516             CPlace::for_ptr_with_extra(field_ptr, extra.unwrap(), field_layout)
 517         } else {
 518             CPlace::for_ptr(field_ptr, field_layout)
 519         }
 520     }
 521
 522     pub fn place_index(
 523         self,
 524         fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
 525         index: Value,
 526     ) -> CPlace<'tcx> {
 527         let (elem_layout, ptr) = match self.layout().ty.kind {
 528             ty::Array(elem_ty, _) => (fx.layout_of(elem_ty), self.to_ptr(fx)),
 529             ty::Slice(elem_ty) => (fx.layout_of(elem_ty), self.to_ptr_maybe_unsized(fx).0),
 530             _ => bug!("place_index({:?})", self.layout().ty),
 531         };
 532
 533         let offset = fx
 534             .bcx
 535             .ins()
 536             .imul_imm(index, elem_layout.size.bytes() as i64);
 537
 538         CPlace::for_ptr(ptr.offset_value(fx, offset), elem_layout)
 539     }
 540
 541     pub fn place_deref(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>) -> CPlace<'tcx> {
 542         let inner_layout = fx.layout_of(self.layout().ty.builtin_deref(true).unwrap().ty);
 543         if has_ptr_meta(fx.tcx, inner_layout.ty) {
 544             let (addr, extra) = self.to_cvalue(fx).load_scalar_pair(fx);
 545             CPlace::for_ptr_with_extra(Pointer::new(addr), extra, inner_layout)
 546         } else {
 547             CPlace::for_ptr(Pointer::new(self.to_cvalue(fx).load_scalar(fx)), inner_layout)
 548         }
 549     }
 550
 551     pub fn write_place_ref(self, fx: &mut FunctionCx<'_, 'tcx, impl Backend>, dest: CPlace<'tcx>) {
 552         if has_ptr_meta(fx.tcx, self.layout().ty) {
 553             let (ptr, extra) = self.to_ptr_maybe_unsized(fx);
 554             let ptr = CValue::by_val_pair(
 555                 ptr.get_addr(fx),
 556                 extra.expect("unsized type without metadata"),
 557                 dest.layout(),
 558             );
 559             dest.write_cvalue(fx, ptr);
 560         } else {
 561             let ptr = CValue::by_val(self.to_ptr(fx).get_addr(fx), dest.layout());
 562             dest.write_cvalue(fx, ptr);
 563         }
 564     }
 565
 566     pub fn unchecked_cast_to(self, layout: TyLayout<'tcx>) -> Self {
 567         assert!(!self.layout().is_unsized());
 568         match self.inner {
 569             CPlaceInner::NoPlace => {
 570                 assert!(layout.size.bytes() == 0);
 571             }
 572             _ => {}
 573         }
 574         CPlace {
 575             inner: self.inner,
 576             layout,
 577         }
 578     }
 579
 580     pub fn downcast_variant(
 581         self,
 582         fx: &FunctionCx<'_, 'tcx, impl Backend>,
 583         variant: VariantIdx,
 584     ) -> Self {
 585         let layout = self.layout().for_variant(fx, variant);
 586         self.unchecked_cast_to(layout)
 587     }
 588 }