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.
4 use std::convert::{TryFrom, TryInto};
6 use rustc::ty::layout::{
7 self, HasDataLayout, IntegerExt, LayoutOf, PrimitiveExt, Size, TyLayout, VariantIdx,
11 use super::{InterpCx, MPlaceTy, Machine, MemPlace, Place, PlaceTy};
12 pub use rustc::mir::interpret::ScalarMaybeUndef;
13 use rustc::mir::interpret::{
14 sign_extend, truncate, AllocId, ConstValue, GlobalId, InterpResult, Pointer, Scalar,
17 use rustc_macros::HashStable;
19 /// An `Immediate` represents a single immediate self-contained Rust value.
21 /// For optimization of a few very common cases, there is also a representation for a pair of
22 /// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
23 /// operations and wide pointers. This idea was taken from rustc's codegen.
24 /// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
25 /// defined on `Immediate`, and do not have to work with a `Place`.
26 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
27 pub enum Immediate<Tag = (), Id = AllocId> {
28 Scalar(ScalarMaybeUndef<Tag, Id>),
29 ScalarPair(ScalarMaybeUndef<Tag, Id>, ScalarMaybeUndef<Tag, Id>),
32 impl<Tag> From<ScalarMaybeUndef<Tag>> for Immediate<Tag> {
34 fn from(val: ScalarMaybeUndef<Tag>) -> Self {
35 Immediate::Scalar(val)
39 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
41 fn from(val: Scalar<Tag>) -> Self {
42 Immediate::Scalar(val.into())
46 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
48 fn from(val: Pointer<Tag>) -> Self {
49 Immediate::Scalar(Scalar::from(val).into())
53 impl<'tcx, Tag> Immediate<Tag> {
54 pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
55 Immediate::ScalarPair(
57 Scalar::from_uint(len, cx.data_layout().pointer_size).into(),
61 pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
62 Immediate::ScalarPair(val.into(), vtable.into())
66 pub fn to_scalar_or_undef(self) -> ScalarMaybeUndef<Tag> {
68 Immediate::Scalar(val) => val,
69 Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
74 pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
75 self.to_scalar_or_undef().not_undef()
79 pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Tag>, Scalar<Tag>)> {
81 Immediate::Scalar(..) => bug!("Got a thin pointer where a scalar pair was expected"),
82 Immediate::ScalarPair(a, b) => Ok((a.not_undef()?, b.not_undef()?)),
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 pub(crate) imm: Immediate<Tag>,
92 pub layout: TyLayout<'tcx>,
95 // `Tag: Copy` because some methods on `Scalar` consume them by value
96 impl<Tag: Copy> std::fmt::Display for ImmTy<'tcx, Tag> {
97 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
99 // We cannot use `to_bits_or_ptr` as we do not have a `tcx`.
100 // So we use `is_bits` and circumvent a bunch of sanity checking -- but
101 // this is anyway only for printing.
102 Immediate::Scalar(ScalarMaybeUndef::Scalar(s)) if s.is_ptr() => {
103 fmt.write_str("{pointer}")
105 Immediate::Scalar(ScalarMaybeUndef::Scalar(s)) => {
106 let s = s.assert_bits(self.layout.size);
107 match self.layout.ty.kind {
109 return write!(fmt, "{}", super::sign_extend(s, self.layout.size) as i128,);
111 ty::Uint(_) => return write!(fmt, "{}", s),
112 ty::Bool if s == 0 => return fmt.write_str("false"),
113 ty::Bool if s == 1 => return fmt.write_str("true"),
115 if let Some(c) = u32::try_from(s).ok().and_then(std::char::from_u32) {
116 return write!(fmt, "{}", c);
119 ty::Float(ast::FloatTy::F32) => {
120 if let Ok(u) = u32::try_from(s) {
121 return write!(fmt, "{}", f32::from_bits(u));
124 ty::Float(ast::FloatTy::F64) => {
125 if let Ok(u) = u64::try_from(s) {
126 return write!(fmt, "{}", f64::from_bits(u));
131 write!(fmt, "{:x}", s)
133 Immediate::Scalar(ScalarMaybeUndef::Undef) => fmt.write_str("{undef}"),
134 Immediate::ScalarPair(..) => fmt.write_str("{wide pointer or tuple}"),
139 impl<'tcx, Tag> ::std::ops::Deref for ImmTy<'tcx, Tag> {
140 type Target = Immediate<Tag>;
142 fn deref(&self) -> &Immediate<Tag> {
147 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
148 /// or still in memory. The latter is an optimization, to delay reading that chunk of
149 /// memory and to avoid having to store arbitrary-sized data here.
150 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
151 pub enum Operand<Tag = (), Id = AllocId> {
152 Immediate(Immediate<Tag, Id>),
153 Indirect(MemPlace<Tag, Id>),
156 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
157 pub struct OpTy<'tcx, Tag = ()> {
158 op: Operand<Tag>, // Keep this private; it helps enforce invariants.
159 pub layout: TyLayout<'tcx>,
162 impl<'tcx, Tag> ::std::ops::Deref for OpTy<'tcx, Tag> {
163 type Target = Operand<Tag>;
165 fn deref(&self) -> &Operand<Tag> {
170 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
172 fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
173 OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
177 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
179 fn from(val: ImmTy<'tcx, Tag>) -> Self {
180 OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
184 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
186 pub fn from_scalar(val: Scalar<Tag>, layout: TyLayout<'tcx>) -> Self {
187 ImmTy { imm: val.into(), layout }
191 pub fn try_from_uint(i: impl Into<u128>, layout: TyLayout<'tcx>) -> Option<Self> {
192 Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
195 pub fn from_uint(i: impl Into<u128>, layout: TyLayout<'tcx>) -> Self {
196 Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
200 pub fn try_from_int(i: impl Into<i128>, layout: TyLayout<'tcx>) -> Option<Self> {
201 Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
205 pub fn from_int(i: impl Into<i128>, layout: TyLayout<'tcx>) -> Self {
206 Self::from_scalar(Scalar::from_int(i, layout.size), layout)
210 // Use the existing layout if given (but sanity check in debug mode),
211 // or compute the layout.
213 pub(super) fn from_known_layout<'tcx>(
214 layout: Option<TyLayout<'tcx>>,
215 compute: impl FnOnce() -> InterpResult<'tcx, TyLayout<'tcx>>,
216 ) -> InterpResult<'tcx, TyLayout<'tcx>> {
220 if cfg!(debug_assertions) {
221 let layout2 = compute()?;
223 layout.details, layout2.details,
224 "mismatch in layout of supposedly equal-layout types {:?} and {:?}",
225 layout.ty, layout2.ty
233 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
234 /// Normalice `place.ptr` to a `Pointer` if this is a place and not a ZST.
235 /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
239 op: OpTy<'tcx, M::PointerTag>,
240 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
241 match op.try_as_mplace(self) {
242 Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
243 Err(imm) => Ok(imm.into()), // Nothing to cast/force
247 /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
248 /// Returns `None` if the layout does not permit loading this as a value.
249 fn try_read_immediate_from_mplace(
251 mplace: MPlaceTy<'tcx, M::PointerTag>,
252 ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
253 if mplace.layout.is_unsized() {
254 // Don't touch unsized
259 .check_mplace_access(mplace, None)
260 .expect("places should be checked on creation")
264 return Ok(Some(ImmTy {
266 imm: Scalar::zst().into(),
267 layout: mplace.layout,
272 match mplace.layout.abi {
273 layout::Abi::Scalar(..) => {
274 let scalar = self.memory.get_raw(ptr.alloc_id)?.read_scalar(
279 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
281 layout::Abi::ScalarPair(ref a, ref b) => {
282 // We checked `ptr_align` above, so all fields will have the alignment they need.
283 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
284 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
285 let (a, b) = (&a.value, &b.value);
286 let (a_size, b_size) = (a.size(self), b.size(self));
288 let b_offset = a_size.align_to(b.align(self).abi);
289 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
290 let b_ptr = ptr.offset(b_offset, self)?;
291 let a_val = self.memory.get_raw(ptr.alloc_id)?.read_scalar(self, a_ptr, a_size)?;
292 let b_val = self.memory.get_raw(ptr.alloc_id)?.read_scalar(self, b_ptr, b_size)?;
293 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
299 /// Try returning an immediate for the operand.
300 /// If the layout does not permit loading this as an immediate, return where in memory
301 /// we can find the data.
302 /// Note that for a given layout, this operation will either always fail or always
303 /// succeed! Whether it succeeds depends on whether the layout can be represented
304 /// in a `Immediate`, not on which data is stored there currently.
305 pub(crate) fn try_read_immediate(
307 src: OpTy<'tcx, M::PointerTag>,
308 ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
309 Ok(match src.try_as_mplace(self) {
311 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
321 /// Read an immediate from a place, asserting that that is possible with the given layout.
323 pub fn read_immediate(
325 op: OpTy<'tcx, M::PointerTag>,
326 ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
327 if let Ok(imm) = self.try_read_immediate(op)? {
330 bug!("primitive read failed for type: {:?}", op.layout.ty);
334 /// Read a scalar from a place
337 op: OpTy<'tcx, M::PointerTag>,
338 ) -> InterpResult<'tcx, ScalarMaybeUndef<M::PointerTag>> {
339 Ok(self.read_immediate(op)?.to_scalar_or_undef())
342 // Turn the wide MPlace into a string (must already be dereferenced!)
343 pub fn read_str(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
344 let len = mplace.len(self)?;
345 let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len as u64))?;
346 let str = ::std::str::from_utf8(bytes)
347 .map_err(|err| err_unsup!(ValidationFailure(err.to_string())))?;
351 /// Projection functions
352 pub fn operand_field(
354 op: OpTy<'tcx, M::PointerTag>,
356 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
357 let base = match op.try_as_mplace(self) {
360 let field = self.mplace_field(mplace, field)?;
361 return Ok(field.into());
366 let field = field.try_into().unwrap();
367 let field_layout = op.layout.field(self, field)?;
368 if field_layout.is_zst() {
369 let immediate = Scalar::zst().into();
370 return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
372 let offset = op.layout.fields.offset(field);
373 let immediate = match *base {
374 // the field covers the entire type
375 _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
376 // extract fields from types with `ScalarPair` ABI
377 Immediate::ScalarPair(a, b) => {
378 let val = if offset.bytes() == 0 { a } else { b };
381 Immediate::Scalar(val) => {
382 bug!("field access on non aggregate {:#?}, {:#?}", val, op.layout)
385 Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
388 pub fn operand_downcast(
390 op: OpTy<'tcx, M::PointerTag>,
392 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
393 // Downcasts only change the layout
394 Ok(match op.try_as_mplace(self) {
395 Ok(mplace) => self.mplace_downcast(mplace, variant)?.into(),
397 let layout = op.layout.for_variant(self, variant);
398 OpTy { layout, ..op }
403 pub fn operand_projection(
405 base: OpTy<'tcx, M::PointerTag>,
406 proj_elem: &mir::PlaceElem<'tcx>,
407 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
408 use rustc::mir::ProjectionElem::*;
409 Ok(match *proj_elem {
410 Field(field, _) => self.operand_field(base, field.index() as u64)?,
411 Downcast(_, variant) => self.operand_downcast(base, variant)?,
412 Deref => self.deref_operand(base)?.into(),
413 Subslice { .. } | ConstantIndex { .. } | Index(_) => {
414 // The rest should only occur as mplace, we do not use Immediates for types
415 // allowing such operations. This matches place_projection forcing an allocation.
416 let mplace = base.assert_mem_place(self);
417 self.mplace_projection(mplace, proj_elem)?.into()
422 /// This is used by [priroda](https://github.com/oli-obk/priroda) to get an OpTy from a local
425 frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
427 layout: Option<TyLayout<'tcx>>,
428 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
429 assert_ne!(local, mir::RETURN_PLACE);
430 let layout = self.layout_of_local(frame, local, layout)?;
431 let op = if layout.is_zst() {
432 // Do not read from ZST, they might not be initialized
433 Operand::Immediate(Scalar::zst().into())
435 M::access_local(&self, frame, local)?
437 Ok(OpTy { op, layout })
440 /// Every place can be read from, so we can turn them into an operand
444 place: PlaceTy<'tcx, M::PointerTag>,
445 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
446 let op = match *place {
447 Place::Ptr(mplace) => Operand::Indirect(mplace),
448 Place::Local { frame, local } => *self.access_local(&self.stack[frame], local, None)?,
450 Ok(OpTy { op, layout: place.layout })
453 // Evaluate a place with the goal of reading from it. This lets us sometimes
454 // avoid allocations.
455 pub fn eval_place_to_op(
457 place: &mir::Place<'tcx>,
458 layout: Option<TyLayout<'tcx>>,
459 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
460 let base_op = match place.local {
461 mir::RETURN_PLACE => throw_unsup!(ReadFromReturnPointer),
463 // Do not use the layout passed in as argument if the base we are looking at
464 // here is not the entire place.
465 let layout = if place.projection.is_empty() { layout } else { None };
467 self.access_local(self.frame(), local, layout)?
474 .try_fold(base_op, |op, elem| self.operand_projection(op, elem))?;
476 trace!("eval_place_to_op: got {:?}", *op);
480 /// Evaluate the operand, returning a place where you can then find the data.
481 /// If you already know the layout, you can save two table lookups
482 /// by passing it in here.
485 mir_op: &mir::Operand<'tcx>,
486 layout: Option<TyLayout<'tcx>>,
487 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
488 use rustc::mir::Operand::*;
489 let op = match *mir_op {
490 // FIXME: do some more logic on `move` to invalidate the old location
491 Copy(ref place) | Move(ref place) => self.eval_place_to_op(place, layout)?,
493 Constant(ref constant) => {
494 let val = self.subst_from_frame_and_normalize_erasing_regions(constant.literal);
495 self.eval_const_to_op(val, layout)?
498 trace!("{:?}: {:?}", mir_op, *op);
502 /// Evaluate a bunch of operands at once
503 pub(super) fn eval_operands(
505 ops: &[mir::Operand<'tcx>],
506 ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
507 ops.iter().map(|op| self.eval_operand(op, None)).collect()
510 // Used when the miri-engine runs into a constant and for extracting information from constants
511 // in patterns via the `const_eval` module
512 /// The `val` and `layout` are assumed to already be in our interpreter
513 /// "universe" (param_env).
514 crate fn eval_const_to_op(
516 val: &ty::Const<'tcx>,
517 layout: Option<TyLayout<'tcx>>,
518 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
519 let tag_scalar = |scalar| match scalar {
520 Scalar::Ptr(ptr) => Scalar::Ptr(self.tag_static_base_pointer(ptr)),
521 Scalar::Raw { data, size } => Scalar::Raw { data, size },
523 // Early-return cases.
524 let val_val = match val.val {
525 ty::ConstKind::Param(_) => throw_inval!(TooGeneric),
526 ty::ConstKind::Unevaluated(def_id, substs, promoted) => {
527 let instance = self.resolve(def_id, substs)?;
528 // We use `const_eval` here and `const_eval_raw` elsewhere in mir interpretation.
529 // The reason we use `const_eval_raw` everywhere else is to prevent cycles during
530 // validation, because validation automatically reads through any references, thus
531 // potentially requiring the current static to be evaluated again. This is not a
532 // problem here, because we are building an operand which means an actual read is
534 return Ok(self.const_eval(GlobalId { instance, promoted }, val.ty)?);
536 ty::ConstKind::Infer(..)
537 | ty::ConstKind::Bound(..)
538 | ty::ConstKind::Placeholder(..) => {
539 bug!("eval_const_to_op: Unexpected ConstKind {:?}", val)
541 ty::ConstKind::Value(val_val) => val_val,
543 // Other cases need layout.
544 let layout = from_known_layout(layout, || self.layout_of(val.ty))?;
545 let op = match val_val {
546 ConstValue::ByRef { alloc, offset } => {
547 let id = self.tcx.alloc_map.lock().create_memory_alloc(alloc);
548 // We rely on mutability being set correctly in that allocation to prevent writes
549 // where none should happen.
550 let ptr = self.tag_static_base_pointer(Pointer::new(id, offset));
551 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
553 ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x).into()),
554 ConstValue::Slice { data, start, end } => {
555 // We rely on mutability being set correctly in `data` to prevent writes
556 // where none should happen.
557 let ptr = Pointer::new(
558 self.tcx.alloc_map.lock().create_memory_alloc(data),
559 Size::from_bytes(start as u64), // offset: `start`
561 Operand::Immediate(Immediate::new_slice(
562 self.tag_static_base_pointer(ptr).into(),
563 (end - start) as u64, // len: `end - start`
568 Ok(OpTy { op, layout })
571 /// Read discriminant, return the runtime value as well as the variant index.
572 pub fn read_discriminant(
574 rval: OpTy<'tcx, M::PointerTag>,
575 ) -> InterpResult<'tcx, (u128, VariantIdx)> {
576 trace!("read_discriminant_value {:#?}", rval.layout);
578 let (discr_layout, discr_kind, discr_index) = match rval.layout.variants {
579 layout::Variants::Single { index } => {
583 .discriminant_for_variant(*self.tcx, index)
584 .map_or(index.as_u32() as u128, |discr| discr.val);
585 return Ok((discr_val, index));
587 layout::Variants::Multiple {
588 discr: ref discr_layout,
592 } => (discr_layout, discr_kind, discr_index),
595 // read raw discriminant value
596 let discr_op = self.operand_field(rval, discr_index as u64)?;
597 let discr_val = self.read_immediate(discr_op)?;
598 let raw_discr = discr_val.to_scalar_or_undef();
599 trace!("discr value: {:?}", raw_discr);
601 Ok(match *discr_kind {
602 layout::DiscriminantKind::Tag => {
603 let bits_discr = raw_discr
605 .and_then(|raw_discr| self.force_bits(raw_discr, discr_val.layout.size))
606 .map_err(|_| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
607 let real_discr = if discr_val.layout.ty.is_signed() {
608 // going from layout tag type to typeck discriminant type
609 // requires first sign extending with the discriminant layout
610 let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;
611 // and then zeroing with the typeck discriminant type
616 .expect("tagged layout corresponds to adt")
619 let size = layout::Integer::from_attr(self, discr_ty).size();
620 let truncatee = sexted as u128;
621 truncate(truncatee, size)
625 // Make sure we catch invalid discriminants
626 let index = match rval.layout.ty.kind {
628 adt.discriminants(self.tcx.tcx).find(|(_, var)| var.val == real_discr)
630 ty::Generator(def_id, substs, _) => {
631 let substs = substs.as_generator();
633 .discriminants(def_id, self.tcx.tcx)
634 .find(|(_, var)| var.val == real_discr)
636 _ => bug!("tagged layout for non-adt non-generator"),
638 .ok_or_else(|| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
639 (real_discr, index.0)
641 layout::DiscriminantKind::Niche {
646 let variants_start = niche_variants.start().as_u32();
647 let variants_end = niche_variants.end().as_u32();
648 let raw_discr = raw_discr
650 .map_err(|_| err_ub!(InvalidDiscriminant(ScalarMaybeUndef::Undef)))?;
651 match raw_discr.to_bits_or_ptr(discr_val.layout.size, self) {
653 // The niche must be just 0 (which an inbounds pointer value never is)
654 let ptr_valid = niche_start == 0
655 && variants_start == variants_end
656 && !self.memory.ptr_may_be_null(ptr);
658 throw_ub!(InvalidDiscriminant(raw_discr.erase_tag().into()))
660 (dataful_variant.as_u32() as u128, dataful_variant)
663 // We need to use machine arithmetic to get the relative variant idx:
664 // variant_index_relative = discr_val - niche_start_val
666 self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
667 let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
668 let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
669 let variant_index_relative_val =
670 self.binary_op(mir::BinOp::Sub, discr_val, niche_start_val)?;
671 let variant_index_relative = variant_index_relative_val
673 .assert_bits(discr_val.layout.size);
674 // Check if this is in the range that indicates an actual discriminant.
675 if variant_index_relative <= u128::from(variants_end - variants_start) {
676 let variant_index_relative = u32::try_from(variant_index_relative)
677 .expect("we checked that this fits into a u32");
678 // Then computing the absolute variant idx should not overflow any more.
679 let variant_index = variants_start
680 .checked_add(variant_index_relative)
681 .expect("oveflow computing absolute variant idx");
682 let variants_len = rval
686 .expect("tagged layout for non adt")
689 assert!((variant_index as usize) < variants_len);
690 (u128::from(variant_index), VariantIdx::from_u32(variant_index))
692 (u128::from(dataful_variant.as_u32()), dataful_variant)