compiler/rustc_mir/src/interpret/operand.rs

   1 //! Functions concerning immediate values and operands, and reading from operands.
   2 //! All high-level functions to read from memory work on operands as sources.
   3
   4 use std::convert::TryFrom;
   5 use std::fmt::Write;
   6
   7 use rustc_errors::ErrorReported;
   8 use rustc_hir::def::Namespace;
   9 use rustc_macros::HashStable;
  10 use rustc_middle::ty::layout::{PrimitiveExt, TyAndLayout};
  11 use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Printer};
  12 use rustc_middle::ty::{ConstInt, Ty};
  13 use rustc_middle::{mir, ty};
  14 use rustc_target::abi::{Abi, HasDataLayout, LayoutOf, Size, TagEncoding};
  15 use rustc_target::abi::{VariantIdx, Variants};
  16
  17 use super::{
  18     from_known_layout, mir_assign_valid_types, ConstValue, GlobalId, InterpCx, InterpResult,
  19     MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer, Scalar, ScalarMaybeUninit,
  20 };
  21
  22 /// An `Immediate` represents a single immediate self-contained Rust value.
  23 ///
  24 /// For optimization of a few very common cases, there is also a representation for a pair of
  25 /// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
  26 /// operations and wide pointers. This idea was taken from rustc's codegen.
  27 /// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
  28 /// defined on `Immediate`, and do not have to work with a `Place`.
  29 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
  30 pub enum Immediate<Tag = ()> {
  31     Scalar(ScalarMaybeUninit<Tag>),
  32     ScalarPair(ScalarMaybeUninit<Tag>, ScalarMaybeUninit<Tag>),
  33 }
  34
  35 impl<Tag> From<ScalarMaybeUninit<Tag>> for Immediate<Tag> {
  36     #[inline(always)]
  37     fn from(val: ScalarMaybeUninit<Tag>) -> Self {
  38         Immediate::Scalar(val)
  39     }
  40 }
  41
  42 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
  43     #[inline(always)]
  44     fn from(val: Scalar<Tag>) -> Self {
  45         Immediate::Scalar(val.into())
  46     }
  47 }
  48
  49 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
  50     #[inline(always)]
  51     fn from(val: Pointer<Tag>) -> Self {
  52         Immediate::Scalar(Scalar::from(val).into())
  53     }
  54 }
  55
  56 impl<'tcx, Tag> Immediate<Tag> {
  57     pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
  58         Immediate::ScalarPair(val.into(), Scalar::from_machine_usize(len, cx).into())
  59     }
  60
  61     pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
  62         Immediate::ScalarPair(val.into(), vtable.into())
  63     }
  64
  65     #[inline]
  66     pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Tag> {
  67         match self {
  68             Immediate::Scalar(val) => val,
  69             Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
  70         }
  71     }
  72
  73     #[inline]
  74     pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
  75         self.to_scalar_or_uninit().check_init()
  76     }
  77
  78     #[inline]
  79     pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Tag>, Scalar<Tag>)> {
  80         match self {
  81             Immediate::Scalar(..) => bug!("Got a thin pointer where a scalar pair was expected"),
  82             Immediate::ScalarPair(a, b) => Ok((a.check_init()?, b.check_init()?)),
  83         }
  84     }
  85 }
  86
  87 // ScalarPair needs a type to interpret, so we often have an immediate and a type together
  88 // as input for binary and cast operations.
  89 #[derive(Copy, Clone, Debug)]
  90 pub struct ImmTy<'tcx, Tag = ()> {
  91     imm: Immediate<Tag>,
  92     pub layout: TyAndLayout<'tcx>,
  93 }
  94
  95 impl<Tag: Copy> std::fmt::Display for ImmTy<'tcx, Tag> {
  96     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
  97         /// Helper function for printing a scalar to a FmtPrinter
  98         fn p<'a, 'tcx, F: std::fmt::Write, Tag>(
  99             cx: FmtPrinter<'a, 'tcx, F>,
 100             s: ScalarMaybeUninit<Tag>,
 101             ty: Ty<'tcx>,
 102         ) -> Result<FmtPrinter<'a, 'tcx, F>, std::fmt::Error> {
 103             match s {
 104                 ScalarMaybeUninit::Scalar(s) => {
 105                     cx.pretty_print_const_scalar(s.erase_tag(), ty, true)
 106                 }
 107                 ScalarMaybeUninit::Uninit => cx.typed_value(
 108                     |mut this| {
 109                         this.write_str("{uninit ")?;
 110                         Ok(this)
 111                     },
 112                     |this| this.print_type(ty),
 113                     " ",
 114                 ),
 115             }
 116         }
 117         ty::tls::with(|tcx| {
 118             match self.imm {
 119                 Immediate::Scalar(s) => {
 120                     if let Some(ty) = tcx.lift(&self.layout.ty) {
 121                         let cx = FmtPrinter::new(tcx, f, Namespace::ValueNS);
 122                         p(cx, s, ty)?;
 123                         return Ok(());
 124                     }
 125                     write!(f, "{}: {}", s.erase_tag(), self.layout.ty)
 126                 }
 127                 Immediate::ScalarPair(a, b) => {
 128                     // FIXME(oli-obk): at least print tuples and slices nicely
 129                     write!(f, "({}, {}): {}", a.erase_tag(), b.erase_tag(), self.layout.ty,)
 130                 }
 131             }
 132         })
 133     }
 134 }
 135
 136 impl<'tcx, Tag> ::std::ops::Deref for ImmTy<'tcx, Tag> {
 137     type Target = Immediate<Tag>;
 138     #[inline(always)]
 139     fn deref(&self) -> &Immediate<Tag> {
 140         &self.imm
 141     }
 142 }
 143
 144 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
 145 /// or still in memory. The latter is an optimization, to delay reading that chunk of
 146 /// memory and to avoid having to store arbitrary-sized data here.
 147 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
 148 pub enum Operand<Tag = ()> {
 149     Immediate(Immediate<Tag>),
 150     Indirect(MemPlace<Tag>),
 151 }
 152
 153 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 154 pub struct OpTy<'tcx, Tag = ()> {
 155     op: Operand<Tag>, // Keep this private; it helps enforce invariants.
 156     pub layout: TyAndLayout<'tcx>,
 157 }
 158
 159 impl<'tcx, Tag> ::std::ops::Deref for OpTy<'tcx, Tag> {
 160     type Target = Operand<Tag>;
 161     #[inline(always)]
 162     fn deref(&self) -> &Operand<Tag> {
 163         &self.op
 164     }
 165 }
 166
 167 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
 168     #[inline(always)]
 169     fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
 170         OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
 171     }
 172 }
 173
 174 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
 175     #[inline(always)]
 176     fn from(val: ImmTy<'tcx, Tag>) -> Self {
 177         OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
 178     }
 179 }
 180
 181 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
 182     #[inline]
 183     pub fn from_scalar(val: Scalar<Tag>, layout: TyAndLayout<'tcx>) -> Self {
 184         ImmTy { imm: val.into(), layout }
 185     }
 186
 187     #[inline]
 188     pub fn from_immediate(imm: Immediate<Tag>, layout: TyAndLayout<'tcx>) -> Self {
 189         ImmTy { imm, layout }
 190     }
 191
 192     #[inline]
 193     pub fn try_from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
 194         Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
 195     }
 196     #[inline]
 197     pub fn from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Self {
 198         Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
 199     }
 200
 201     #[inline]
 202     pub fn try_from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
 203         Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
 204     }
 205
 206     #[inline]
 207     pub fn from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Self {
 208         Self::from_scalar(Scalar::from_int(i, layout.size), layout)
 209     }
 210
 211     #[inline]
 212     pub fn to_const_int(self) -> ConstInt {
 213         assert!(self.layout.ty.is_integral());
 214         ConstInt::new(
 215             self.to_scalar()
 216                 .expect("to_const_int doesn't work on scalar pairs")
 217                 .assert_bits(self.layout.size),
 218             self.layout.size,
 219             self.layout.ty.is_signed(),
 220             self.layout.ty.is_ptr_sized_integral(),
 221         )
 222     }
 223 }
 224
 225 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 226     /// Normalice `place.ptr` to a `Pointer` if this is a place and not a ZST.
 227     /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
 228     #[inline]
 229     pub fn force_op_ptr(
 230         &self,
 231         op: OpTy<'tcx, M::PointerTag>,
 232     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 233         match op.try_as_mplace(self) {
 234             Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
 235             Err(imm) => Ok(imm.into()), // Nothing to cast/force
 236         }
 237     }
 238
 239     /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
 240     /// Returns `None` if the layout does not permit loading this as a value.
 241     fn try_read_immediate_from_mplace(
 242         &self,
 243         mplace: MPlaceTy<'tcx, M::PointerTag>,
 244     ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
 245         if mplace.layout.is_unsized() {
 246             // Don't touch unsized
 247             return Ok(None);
 248         }
 249
 250         let ptr = match self
 251             .check_mplace_access(mplace, None)
 252             .expect("places should be checked on creation")
 253         {
 254             Some(ptr) => ptr,
 255             None => {
 256                 if let Scalar::Ptr(ptr) = mplace.ptr {
 257                     // We may be reading from a static.
 258                     // In order to ensure that `static FOO: Type = FOO;` causes a cycle error
 259                     // instead of magically pulling *any* ZST value from the ether, we need to
 260                     // actually access the referenced allocation.
 261                     self.memory.get_raw(ptr.alloc_id)?;
 262                 }
 263                 return Ok(Some(ImmTy {
 264                     // zero-sized type
 265                     imm: Scalar::zst().into(),
 266                     layout: mplace.layout,
 267                 }));
 268             }
 269         };
 270
 271         let alloc = self.memory.get_raw(ptr.alloc_id)?;
 272
 273         match mplace.layout.abi {
 274             Abi::Scalar(..) => {
 275                 let scalar = alloc.read_scalar(self, ptr, mplace.layout.size)?;
 276                 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
 277             }
 278             Abi::ScalarPair(ref a, ref b) => {
 279                 // We checked `ptr_align` above, so all fields will have the alignment they need.
 280                 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
 281                 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
 282                 let (a, b) = (&a.value, &b.value);
 283                 let (a_size, b_size) = (a.size(self), b.size(self));
 284                 let a_ptr = ptr;
 285                 let b_offset = a_size.align_to(b.align(self).abi);
 286                 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
 287                 let b_ptr = ptr.offset(b_offset, self)?;
 288                 let a_val = alloc.read_scalar(self, a_ptr, a_size)?;
 289                 let b_val = alloc.read_scalar(self, b_ptr, b_size)?;
 290                 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
 291             }
 292             _ => Ok(None),
 293         }
 294     }
 295
 296     /// Try returning an immediate for the operand.
 297     /// If the layout does not permit loading this as an immediate, return where in memory
 298     /// we can find the data.
 299     /// Note that for a given layout, this operation will either always fail or always
 300     /// succeed!  Whether it succeeds depends on whether the layout can be represented
 301     /// in a `Immediate`, not on which data is stored there currently.
 302     pub(crate) fn try_read_immediate(
 303         &self,
 304         src: OpTy<'tcx, M::PointerTag>,
 305     ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
 306         Ok(match src.try_as_mplace(self) {
 307             Ok(mplace) => {
 308                 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
 309                     Ok(val)
 310                 } else {
 311                     Err(mplace)
 312                 }
 313             }
 314             Err(val) => Ok(val),
 315         })
 316     }
 317
 318     /// Read an immediate from a place, asserting that that is possible with the given layout.
 319     #[inline(always)]
 320     pub fn read_immediate(
 321         &self,
 322         op: OpTy<'tcx, M::PointerTag>,
 323     ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
 324         if let Ok(imm) = self.try_read_immediate(op)? {
 325             Ok(imm)
 326         } else {
 327             span_bug!(self.cur_span(), "primitive read failed for type: {:?}", op.layout.ty);
 328         }
 329     }
 330
 331     /// Read a scalar from a place
 332     pub fn read_scalar(
 333         &self,
 334         op: OpTy<'tcx, M::PointerTag>,
 335     ) -> InterpResult<'tcx, ScalarMaybeUninit<M::PointerTag>> {
 336         Ok(self.read_immediate(op)?.to_scalar_or_uninit())
 337     }
 338
 339     // Turn the wide MPlace into a string (must already be dereferenced!)
 340     pub fn read_str(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
 341         let len = mplace.len(self)?;
 342         let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len))?;
 343         let str = ::std::str::from_utf8(bytes).map_err(|err| err_ub!(InvalidStr(err)))?;
 344         Ok(str)
 345     }
 346
 347     /// Projection functions
 348     pub fn operand_field(
 349         &self,
 350         op: OpTy<'tcx, M::PointerTag>,
 351         field: usize,
 352     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 353         let base = match op.try_as_mplace(self) {
 354             Ok(mplace) => {
 355                 // We can reuse the mplace field computation logic for indirect operands.
 356                 let field = self.mplace_field(mplace, field)?;
 357                 return Ok(field.into());
 358             }
 359             Err(value) => value,
 360         };
 361
 362         let field_layout = op.layout.field(self, field)?;
 363         if field_layout.is_zst() {
 364             let immediate = Scalar::zst().into();
 365             return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
 366         }
 367         let offset = op.layout.fields.offset(field);
 368         let immediate = match *base {
 369             // the field covers the entire type
 370             _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
 371             // extract fields from types with `ScalarPair` ABI
 372             Immediate::ScalarPair(a, b) => {
 373                 let val = if offset.bytes() == 0 { a } else { b };
 374                 Immediate::from(val)
 375             }
 376             Immediate::Scalar(val) => span_bug!(
 377                 self.cur_span(),
 378                 "field access on non aggregate {:#?}, {:#?}",
 379                 val,
 380                 op.layout
 381             ),
 382         };
 383         Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
 384     }
 385
 386     pub fn operand_index(
 387         &self,
 388         op: OpTy<'tcx, M::PointerTag>,
 389         index: u64,
 390     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 391         if let Ok(index) = usize::try_from(index) {
 392             // We can just treat this as a field.
 393             self.operand_field(op, index)
 394         } else {
 395             // Indexing into a big array. This must be an mplace.
 396             let mplace = op.assert_mem_place(self);
 397             Ok(self.mplace_index(mplace, index)?.into())
 398         }
 399     }
 400
 401     pub fn operand_downcast(
 402         &self,
 403         op: OpTy<'tcx, M::PointerTag>,
 404         variant: VariantIdx,
 405     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 406         // Downcasts only change the layout
 407         Ok(match op.try_as_mplace(self) {
 408             Ok(mplace) => self.mplace_downcast(mplace, variant)?.into(),
 409             Err(..) => {
 410                 let layout = op.layout.for_variant(self, variant);
 411                 OpTy { layout, ..op }
 412             }
 413         })
 414     }
 415
 416     pub fn operand_projection(
 417         &self,
 418         base: OpTy<'tcx, M::PointerTag>,
 419         proj_elem: mir::PlaceElem<'tcx>,
 420     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 421         use rustc_middle::mir::ProjectionElem::*;
 422         Ok(match proj_elem {
 423             Field(field, _) => self.operand_field(base, field.index())?,
 424             Downcast(_, variant) => self.operand_downcast(base, variant)?,
 425             Deref => self.deref_operand(base)?.into(),
 426             Subslice { .. } | ConstantIndex { .. } | Index(_) => {
 427                 // The rest should only occur as mplace, we do not use Immediates for types
 428                 // allowing such operations.  This matches place_projection forcing an allocation.
 429                 let mplace = base.assert_mem_place(self);
 430                 self.mplace_projection(mplace, proj_elem)?.into()
 431             }
 432         })
 433     }
 434
 435     /// Read from a local. Will not actually access the local if reading from a ZST.
 436     /// Will not access memory, instead an indirect `Operand` is returned.
 437     ///
 438     /// This is public because it is used by [priroda](https://github.com/oli-obk/priroda) to get an
 439     /// OpTy from a local
 440     pub fn access_local(
 441         &self,
 442         frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
 443         local: mir::Local,
 444         layout: Option<TyAndLayout<'tcx>>,
 445     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 446         let layout = self.layout_of_local(frame, local, layout)?;
 447         let op = if layout.is_zst() {
 448             // Do not read from ZST, they might not be initialized
 449             Operand::Immediate(Scalar::zst().into())
 450         } else {
 451             M::access_local(&self, frame, local)?
 452         };
 453         Ok(OpTy { op, layout })
 454     }
 455
 456     /// Every place can be read from, so we can turn them into an operand.
 457     /// This will definitely return `Indirect` if the place is a `Ptr`, i.e., this
 458     /// will never actually read from memory.
 459     #[inline(always)]
 460     pub fn place_to_op(
 461         &self,
 462         place: PlaceTy<'tcx, M::PointerTag>,
 463     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 464         let op = match *place {
 465             Place::Ptr(mplace) => Operand::Indirect(mplace),
 466             Place::Local { frame, local } => {
 467                 *self.access_local(&self.stack()[frame], local, None)?
 468             }
 469         };
 470         Ok(OpTy { op, layout: place.layout })
 471     }
 472
 473     // Evaluate a place with the goal of reading from it.  This lets us sometimes
 474     // avoid allocations.
 475     pub fn eval_place_to_op(
 476         &self,
 477         place: mir::Place<'tcx>,
 478         layout: Option<TyAndLayout<'tcx>>,
 479     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 480         // Do not use the layout passed in as argument if the base we are looking at
 481         // here is not the entire place.
 482         let layout = if place.projection.is_empty() { layout } else { None };
 483
 484         let base_op = self.access_local(self.frame(), place.local, layout)?;
 485
 486         let op = place
 487             .projection
 488             .iter()
 489             .try_fold(base_op, |op, elem| self.operand_projection(op, elem))?;
 490
 491         trace!("eval_place_to_op: got {:?}", *op);
 492         // Sanity-check the type we ended up with.
 493         debug_assert!(mir_assign_valid_types(
 494             *self.tcx,
 495             self.param_env,
 496             self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
 497                 place.ty(&self.frame().body.local_decls, *self.tcx).ty
 498             ))?,
 499             op.layout,
 500         ));
 501         Ok(op)
 502     }
 503
 504     /// Evaluate the operand, returning a place where you can then find the data.
 505     /// If you already know the layout, you can save two table lookups
 506     /// by passing it in here.
 507     pub fn eval_operand(
 508         &self,
 509         mir_op: &mir::Operand<'tcx>,
 510         layout: Option<TyAndLayout<'tcx>>,
 511     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 512         use rustc_middle::mir::Operand::*;
 513         let op = match *mir_op {
 514             // FIXME: do some more logic on `move` to invalidate the old location
 515             Copy(place) | Move(place) => self.eval_place_to_op(place, layout)?,
 516
 517             Constant(ref constant) => {
 518                 let val =
 519                     self.subst_from_current_frame_and_normalize_erasing_regions(constant.literal);
 520                 self.const_to_op(val, layout)?
 521             }
 522         };
 523         trace!("{:?}: {:?}", mir_op, *op);
 524         Ok(op)
 525     }
 526
 527     /// Evaluate a bunch of operands at once
 528     pub(super) fn eval_operands(
 529         &self,
 530         ops: &[mir::Operand<'tcx>],
 531     ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
 532         ops.iter().map(|op| self.eval_operand(op, None)).collect()
 533     }
 534
 535     // Used when the miri-engine runs into a constant and for extracting information from constants
 536     // in patterns via the `const_eval` module
 537     /// The `val` and `layout` are assumed to already be in our interpreter
 538     /// "universe" (param_env).
 539     crate fn const_to_op(
 540         &self,
 541         val: &ty::Const<'tcx>,
 542         layout: Option<TyAndLayout<'tcx>>,
 543     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 544         let tag_scalar = |scalar| -> InterpResult<'tcx, _> {
 545             Ok(match scalar {
 546                 Scalar::Ptr(ptr) => Scalar::Ptr(self.global_base_pointer(ptr)?),
 547                 Scalar::Raw { data, size } => Scalar::Raw { data, size },
 548             })
 549         };
 550         // Early-return cases.
 551         let val_val = match val.val {
 552             ty::ConstKind::Param(_) | ty::ConstKind::Bound(..) => throw_inval!(TooGeneric),
 553             ty::ConstKind::Error(_) => throw_inval!(TypeckError(ErrorReported)),
 554             ty::ConstKind::Unevaluated(def, substs, promoted) => {
 555                 let instance = self.resolve(def, substs)?;
 556                 return Ok(self.eval_to_allocation(GlobalId { instance, promoted })?.into());
 557             }
 558             ty::ConstKind::Infer(..) | ty::ConstKind::Placeholder(..) => {
 559                 span_bug!(self.cur_span(), "const_to_op: Unexpected ConstKind {:?}", val)
 560             }
 561             ty::ConstKind::Value(val_val) => val_val,
 562         };
 563         // Other cases need layout.
 564         let layout =
 565             from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(val.ty))?;
 566         let op = match val_val {
 567             ConstValue::ByRef { alloc, offset } => {
 568                 let id = self.tcx.create_memory_alloc(alloc);
 569                 // We rely on mutability being set correctly in that allocation to prevent writes
 570                 // where none should happen.
 571                 let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
 572                 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
 573             }
 574             ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x)?.into()),
 575             ConstValue::Slice { data, start, end } => {
 576                 // We rely on mutability being set correctly in `data` to prevent writes
 577                 // where none should happen.
 578                 let ptr = Pointer::new(
 579                     self.tcx.create_memory_alloc(data),
 580                     Size::from_bytes(start), // offset: `start`
 581                 );
 582                 Operand::Immediate(Immediate::new_slice(
 583                     self.global_base_pointer(ptr)?.into(),
 584                     u64::try_from(end.checked_sub(start).unwrap()).unwrap(), // len: `end - start`
 585                     self,
 586                 ))
 587             }
 588         };
 589         Ok(OpTy { op, layout })
 590     }
 591
 592     /// Read discriminant, return the runtime value as well as the variant index.
 593     pub fn read_discriminant(
 594         &self,
 595         op: OpTy<'tcx, M::PointerTag>,
 596     ) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
 597         trace!("read_discriminant_value {:#?}", op.layout);
 598         // Get type and layout of the discriminant.
 599         let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
 600         trace!("discriminant type: {:?}", discr_layout.ty);
 601
 602         // We use "discriminant" to refer to the value associated with a particular enum variant.
 603         // This is not to be confused with its "variant index", which is just determining its position in the
 604         // declared list of variants -- they can differ with explicitly assigned discriminants.
 605         // We use "tag" to refer to how the discriminant is encoded in memory, which can be either
 606         // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
 607         let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
 608             Variants::Single { index } => {
 609                 let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
 610                     Some(discr) => {
 611                         // This type actually has discriminants.
 612                         assert_eq!(discr.ty, discr_layout.ty);
 613                         Scalar::from_uint(discr.val, discr_layout.size)
 614                     }
 615                     None => {
 616                         // On a type without actual discriminants, variant is 0.
 617                         assert_eq!(index.as_u32(), 0);
 618                         Scalar::from_uint(index.as_u32(), discr_layout.size)
 619                     }
 620                 };
 621                 return Ok((discr, index));
 622             }
 623             Variants::Multiple { ref tag, ref tag_encoding, tag_field, .. } => {
 624                 (tag, tag_encoding, tag_field)
 625             }
 626         };
 627
 628         // There are *three* layouts that come into play here:
 629         // - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
 630         //   the `Scalar` we return.
 631         // - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
 632         //   and used to interpret the value we read from the tag field.
 633         //   For the return value, a cast to `discr_layout` is performed.
 634         // - The field storing the tag has a layout, which is very similar to `tag_layout` but
 635         //   may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
 636
 637         // Get layout for tag.
 638         let tag_layout = self.layout_of(tag_scalar_layout.value.to_int_ty(*self.tcx))?;
 639
 640         // Read tag and sanity-check `tag_layout`.
 641         let tag_val = self.read_immediate(self.operand_field(op, tag_field)?)?;
 642         assert_eq!(tag_layout.size, tag_val.layout.size);
 643         assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
 644         let tag_val = tag_val.to_scalar()?;
 645         trace!("tag value: {:?}", tag_val);
 646
 647         // Figure out which discriminant and variant this corresponds to.
 648         Ok(match *tag_encoding {
 649             TagEncoding::Direct => {
 650                 let tag_bits = self
 651                     .force_bits(tag_val, tag_layout.size)
 652                     .map_err(|_| err_ub!(InvalidTag(tag_val.erase_tag())))?;
 653                 // Cast bits from tag layout to discriminant layout.
 654                 let discr_val = self.cast_from_scalar(tag_bits, tag_layout, discr_layout.ty);
 655                 let discr_bits = discr_val.assert_bits(discr_layout.size);
 656                 // Convert discriminant to variant index, and catch invalid discriminants.
 657                 let index = match *op.layout.ty.kind() {
 658                     ty::Adt(adt, _) => {
 659                         adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
 660                     }
 661                     ty::Generator(def_id, substs, _) => {
 662                         let substs = substs.as_generator();
 663                         substs
 664                             .discriminants(def_id, *self.tcx)
 665                             .find(|(_, var)| var.val == discr_bits)
 666                     }
 667                     _ => span_bug!(self.cur_span(), "tagged layout for non-adt non-generator"),
 668                 }
 669                 .ok_or_else(|| err_ub!(InvalidTag(tag_val.erase_tag())))?;
 670                 // Return the cast value, and the index.
 671                 (discr_val, index.0)
 672             }
 673             TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => {
 674                 // Compute the variant this niche value/"tag" corresponds to. With niche layout,
 675                 // discriminant (encoded in niche/tag) and variant index are the same.
 676                 let variants_start = niche_variants.start().as_u32();
 677                 let variants_end = niche_variants.end().as_u32();
 678                 let variant = match tag_val.to_bits_or_ptr(tag_layout.size, self) {
 679                     Err(ptr) => {
 680                         // The niche must be just 0 (which an inbounds pointer value never is)
 681                         let ptr_valid = niche_start == 0
 682                             && variants_start == variants_end
 683                             && !self.memory.ptr_may_be_null(ptr);
 684                         if !ptr_valid {
 685                             throw_ub!(InvalidTag(tag_val.erase_tag()))
 686                         }
 687                         dataful_variant
 688                     }
 689                     Ok(tag_bits) => {
 690                         // We need to use machine arithmetic to get the relative variant idx:
 691                         // variant_index_relative = tag_val - niche_start_val
 692                         let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
 693                         let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
 694                         let variant_index_relative_val =
 695                             self.binary_op(mir::BinOp::Sub, tag_val, niche_start_val)?;
 696                         let variant_index_relative = variant_index_relative_val
 697                             .to_scalar()?
 698                             .assert_bits(tag_val.layout.size);
 699                         // Check if this is in the range that indicates an actual discriminant.
 700                         if variant_index_relative <= u128::from(variants_end - variants_start) {
 701                             let variant_index_relative = u32::try_from(variant_index_relative)
 702                                 .expect("we checked that this fits into a u32");
 703                             // Then computing the absolute variant idx should not overflow any more.
 704                             let variant_index = variants_start
 705                                 .checked_add(variant_index_relative)
 706                                 .expect("overflow computing absolute variant idx");
 707                             let variants_len = op
 708                                 .layout
 709                                 .ty
 710                                 .ty_adt_def()
 711                                 .expect("tagged layout for non adt")
 712                                 .variants
 713                                 .len();
 714                             assert!(usize::try_from(variant_index).unwrap() < variants_len);
 715                             VariantIdx::from_u32(variant_index)
 716                         } else {
 717                             dataful_variant
 718                         }
 719                     }
 720                 };
 721                 // Compute the size of the scalar we need to return.
 722                 // No need to cast, because the variant index directly serves as discriminant and is
 723                 // encoded in the tag.
 724                 (Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)
 725             }
 726         })
 727     }
 728 }