bjoernager/rust
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interpret: rename Tag/PointerTag to Prov/Provenance

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Let's avoid using two different terms for the same thing -- let's just call it "provenance" everywhere.
In Miri, provenance consists of an AllocId and an SbTag (Stacked Borrows tag), which made this even more confusing.
This commit is contained in:
Ralf Jung 2022-07-18 18:47:31 -04:00
parent 29c5a028b0
commit 0ec3269db8
24 changed files with 606 additions and 601 deletions
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146
compiler/rustc_const_eval/src/interpret/operand.rs
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@ -25,14 +25,14 @@ use super::{
/// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
/// defined on `Immediate`, and do not have to work with a `Place`.
#[derive(Copy, Clone, Debug)]
pub enum Immediate<Tag: Provenance = AllocId> {
pub enum Immediate<Prov: Provenance = AllocId> {
/// A single scalar value (must have *initialized* `Scalar` ABI).
/// FIXME: we also currently often use this for ZST.
/// `ScalarMaybeUninit` should reject ZST, and we should use `Uninit` for them instead.
Scalar(ScalarMaybeUninit<Tag>),
Scalar(ScalarMaybeUninit<Prov>),
/// A pair of two scalar value (must have `ScalarPair` ABI where both fields are
/// `Scalar::Initialized`).
ScalarPair(ScalarMaybeUninit<Tag>, ScalarMaybeUninit<Tag>),
ScalarPair(ScalarMaybeUninit<Prov>, ScalarMaybeUninit<Prov>),
/// A value of fully uninitialized memory. Can have and size and layout.
Uninit,
}
@ -40,36 +40,36 @@ pub enum Immediate<Tag: Provenance = AllocId> {
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
rustc_data_structures::static_assert_size!(Immediate, 56);
impl<Tag: Provenance> From<ScalarMaybeUninit<Tag>> for Immediate<Tag> {
impl<Prov: Provenance> From<ScalarMaybeUninit<Prov>> for Immediate<Prov> {
#[inline(always)]
fn from(val: ScalarMaybeUninit<Tag>) -> Self {
fn from(val: ScalarMaybeUninit<Prov>) -> Self {
Immediate::Scalar(val)
}
}
impl<Tag: Provenance> From<Scalar<Tag>> for Immediate<Tag> {
impl<Prov: Provenance> From<Scalar<Prov>> for Immediate<Prov> {
#[inline(always)]
fn from(val: Scalar<Tag>) -> Self {
fn from(val: Scalar<Prov>) -> Self {
Immediate::Scalar(val.into())
}
}
impl<'tcx, Tag: Provenance> Immediate<Tag> {
pub fn from_pointer(p: Pointer<Tag>, cx: &impl HasDataLayout) -> Self {
impl<'tcx, Prov: Provenance> Immediate<Prov> {
pub fn from_pointer(p: Pointer<Prov>, cx: &impl HasDataLayout) -> Self {
Immediate::Scalar(ScalarMaybeUninit::from_pointer(p, cx))
}
pub fn from_maybe_pointer(p: Pointer<Option<Tag>>, cx: &impl HasDataLayout) -> Self {
pub fn from_maybe_pointer(p: Pointer<Option<Prov>>, cx: &impl HasDataLayout) -> Self {
Immediate::Scalar(ScalarMaybeUninit::from_maybe_pointer(p, cx))
}
pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
pub fn new_slice(val: Scalar<Prov>, len: u64, cx: &impl HasDataLayout) -> Self {
Immediate::ScalarPair(val.into(), Scalar::from_machine_usize(len, cx).into())
}
pub fn new_dyn_trait(
val: Scalar<Tag>,
vtable: Pointer<Option<Tag>>,
val: Scalar<Prov>,
vtable: Pointer<Option<Prov>>,
cx: &impl HasDataLayout,
) -> Self {
Immediate::ScalarPair(val.into(), ScalarMaybeUninit::from_maybe_pointer(vtable, cx))
@ -77,7 +77,7 @@ impl<'tcx, Tag: Provenance> Immediate<Tag> {
#[inline]
#[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Tag> {
pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Prov> {
match self {
Immediate::Scalar(val) => val,
Immediate::ScalarPair(..) => bug!("Got a scalar pair where a scalar was expected"),
@ -87,13 +87,13 @@ impl<'tcx, Tag: Provenance> Immediate<Tag> {
#[inline]
#[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Prov>> {
self.to_scalar_or_uninit().check_init()
}
#[inline]
#[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
pub fn to_scalar_or_uninit_pair(self) -> (ScalarMaybeUninit<Tag>, ScalarMaybeUninit<Tag>) {
pub fn to_scalar_or_uninit_pair(self) -> (ScalarMaybeUninit<Prov>, ScalarMaybeUninit<Prov>) {
match self {
Immediate::ScalarPair(val1, val2) => (val1, val2),
Immediate::Scalar(..) => bug!("Got a scalar where a scalar pair was expected"),
@ -103,7 +103,7 @@ impl<'tcx, Tag: Provenance> Immediate<Tag> {
#[inline]
#[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Tag>, Scalar<Tag>)> {
pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Prov>, Scalar<Prov>)> {
let (val1, val2) = self.to_scalar_or_uninit_pair();
Ok((val1.check_init()?, val2.check_init()?))
}
@ -112,20 +112,20 @@ impl<'tcx, Tag: Provenance> Immediate<Tag> {
// ScalarPair needs a type to interpret, so we often have an immediate and a type together
// as input for binary and cast operations.
#[derive(Clone, Debug)]
pub struct ImmTy<'tcx, Tag: Provenance = AllocId> {
imm: Immediate<Tag>,
pub struct ImmTy<'tcx, Prov: Provenance = AllocId> {
imm: Immediate<Prov>,
pub layout: TyAndLayout<'tcx>,
}
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
rustc_data_structures::static_assert_size!(ImmTy<'_>, 72);
impl<Tag: Provenance> std::fmt::Display for ImmTy<'_, Tag> {
impl<Prov: Provenance> std::fmt::Display for ImmTy<'_, Prov> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// Helper function for printing a scalar to a FmtPrinter
fn p<'a, 'tcx, Tag: Provenance>(
fn p<'a, 'tcx, Prov: Provenance>(
cx: FmtPrinter<'a, 'tcx>,
s: ScalarMaybeUninit<Tag>,
s: ScalarMaybeUninit<Prov>,
ty: Ty<'tcx>,
) -> Result<FmtPrinter<'a, 'tcx>, std::fmt::Error> {
match s {
@ -170,10 +170,10 @@ impl<Tag: Provenance> std::fmt::Display for ImmTy<'_, Tag> {
}
}
impl<'tcx, Tag: Provenance> std::ops::Deref for ImmTy<'tcx, Tag> {
type Target = Immediate<Tag>;
impl<'tcx, Prov: Provenance> std::ops::Deref for ImmTy<'tcx, Prov> {
type Target = Immediate<Prov>;
#[inline(always)]
fn deref(&self) -> &Immediate<Tag> {
fn deref(&self) -> &Immediate<Prov> {
&self.imm
}
}
@ -182,17 +182,17 @@ impl<'tcx, Tag: Provenance> std::ops::Deref for ImmTy<'tcx, Tag> {
/// or still in memory. The latter is an optimization, to delay reading that chunk of
/// memory and to avoid having to store arbitrary-sized data here.
#[derive(Copy, Clone, Debug)]
pub enum Operand<Tag: Provenance = AllocId> {
Immediate(Immediate<Tag>),
Indirect(MemPlace<Tag>),
pub enum Operand<Prov: Provenance = AllocId> {
Immediate(Immediate<Prov>),
Indirect(MemPlace<Prov>),
}
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
rustc_data_structures::static_assert_size!(Operand, 64);
#[derive(Clone, Debug)]
pub struct OpTy<'tcx, Tag: Provenance = AllocId> {
op: Operand<Tag>, // Keep this private; it helps enforce invariants.
pub struct OpTy<'tcx, Prov: Provenance = AllocId> {
op: Operand<Prov>, // Keep this private; it helps enforce invariants.
pub layout: TyAndLayout<'tcx>,
/// rustc does not have a proper way to represent the type of a field of a `repr(packed)` struct:
/// it needs to have a different alignment than the field type would usually have.
@ -207,50 +207,50 @@ pub struct OpTy<'tcx, Tag: Provenance = AllocId> {
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
rustc_data_structures::static_assert_size!(OpTy<'_>, 88);
impl<'tcx, Tag: Provenance> std::ops::Deref for OpTy<'tcx, Tag> {
type Target = Operand<Tag>;
impl<'tcx, Prov: Provenance> std::ops::Deref for OpTy<'tcx, Prov> {
type Target = Operand<Prov>;
#[inline(always)]
fn deref(&self) -> &Operand<Tag> {
fn deref(&self) -> &Operand<Prov> {
&self.op
}
}
impl<'tcx, Tag: Provenance> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> From<MPlaceTy<'tcx, Prov>> for OpTy<'tcx, Prov> {
#[inline(always)]
fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
fn from(mplace: MPlaceTy<'tcx, Prov>) -> Self {
OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout, align: Some(mplace.align) }
}
}
impl<'tcx, Tag: Provenance> From<&'_ MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> From<&'_ MPlaceTy<'tcx, Prov>> for OpTy<'tcx, Prov> {
#[inline(always)]
fn from(mplace: &MPlaceTy<'tcx, Tag>) -> Self {
fn from(mplace: &MPlaceTy<'tcx, Prov>) -> Self {
OpTy { op: Operand::Indirect(**mplace), layout: mplace.layout, align: Some(mplace.align) }
}
}
impl<'tcx, Tag: Provenance> From<&'_ mut MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> From<&'_ mut MPlaceTy<'tcx, Prov>> for OpTy<'tcx, Prov> {
#[inline(always)]
fn from(mplace: &mut MPlaceTy<'tcx, Tag>) -> Self {
fn from(mplace: &mut MPlaceTy<'tcx, Prov>) -> Self {
OpTy { op: Operand::Indirect(**mplace), layout: mplace.layout, align: Some(mplace.align) }
}
}
impl<'tcx, Tag: Provenance> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> From<ImmTy<'tcx, Prov>> for OpTy<'tcx, Prov> {
#[inline(always)]
fn from(val: ImmTy<'tcx, Tag>) -> Self {
fn from(val: ImmTy<'tcx, Prov>) -> Self {
OpTy { op: Operand::Immediate(val.imm), layout: val.layout, align: None }
}
}
impl<'tcx, Tag: Provenance> ImmTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
#[inline]
pub fn from_scalar(val: Scalar<Tag>, layout: TyAndLayout<'tcx>) -> Self {
pub fn from_scalar(val: Scalar<Prov>, layout: TyAndLayout<'tcx>) -> Self {
ImmTy { imm: val.into(), layout }
}
#[inline]
pub fn from_immediate(imm: Immediate<Tag>, layout: TyAndLayout<'tcx>) -> Self {
pub fn from_immediate(imm: Immediate<Prov>, layout: TyAndLayout<'tcx>) -> Self {
ImmTy { imm, layout }
}
@ -286,7 +286,7 @@ impl<'tcx, Tag: Provenance> ImmTy<'tcx, Tag> {
}
}
impl<'tcx, Tag: Provenance> OpTy<'tcx, Tag> {
impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
pub fn len(&self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
if self.layout.is_unsized() {
// There are no unsized immediates.
@ -302,7 +302,7 @@ impl<'tcx, Tag: Provenance> OpTy<'tcx, Tag> {
pub fn offset_with_meta(
&self,
offset: Size,
meta: MemPlaceMeta<Tag>,
meta: MemPlaceMeta<Prov>,
layout: TyAndLayout<'tcx>,
cx: &impl HasDataLayout,
) -> InterpResult<'tcx, Self> {
@ -338,9 +338,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// This is an internal function; call `read_immediate` instead.
fn read_immediate_from_mplace_raw(
&self,
mplace: &MPlaceTy<'tcx, M::PointerTag>,
mplace: &MPlaceTy<'tcx, M::Provenance>,
force: bool,
) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::Provenance>>> {
if mplace.layout.is_unsized() {
// Don't touch unsized
return Ok(None);
@ -418,9 +418,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// ConstProp needs it, though.
pub fn read_immediate_raw(
&self,
src: &OpTy<'tcx, M::PointerTag>,
src: &OpTy<'tcx, M::Provenance>,
force: bool,
) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::Provenance>, MPlaceTy<'tcx, M::Provenance>>> {
Ok(match src.try_as_mplace() {
Ok(ref mplace) => {
if let Some(val) = self.read_immediate_from_mplace_raw(mplace, force)? {
@ -437,8 +437,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
#[inline(always)]
pub fn read_immediate(
&self,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
if let Ok(imm) = self.read_immediate_raw(op, /*force*/ false)? {
Ok(imm)
} else {
@ -449,21 +449,21 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// Read a scalar from a place
pub fn read_scalar(
&self,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, ScalarMaybeUninit<M::PointerTag>> {
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, ScalarMaybeUninit<M::Provenance>> {
Ok(self.read_immediate(op)?.to_scalar_or_uninit())
}
/// Read a pointer from a place.
pub fn read_pointer(
&self,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, Pointer<Option<M::PointerTag>>> {
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, Pointer<Option<M::Provenance>>> {
self.scalar_to_ptr(self.read_scalar(op)?.check_init()?)
}
/// Turn the wide MPlace into a string (must already be dereferenced!)
pub fn read_str(&self, mplace: &MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
pub fn read_str(&self, mplace: &MPlaceTy<'tcx, M::Provenance>) -> InterpResult<'tcx, &str> {
let len = mplace.len(self)?;
let bytes = self.read_bytes_ptr(mplace.ptr, Size::from_bytes(len))?;
let str = std::str::from_utf8(bytes).map_err(|err| err_ub!(InvalidStr(err)))?;
@ -476,8 +476,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// Can (but does not always) trigger UB if `op` is uninitialized.
pub fn operand_to_simd(
&self,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::PointerTag>, u64)> {
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, u64)> {
// Basically we just transmute this place into an array following simd_size_and_type.
// This only works in memory, but repr(simd) types should never be immediates anyway.
assert!(op.layout.ty.is_simd());
@ -501,10 +501,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// OpTy from a local.
pub fn local_to_op(
&self,
frame: &Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
frame: &Frame<'mir, 'tcx, M::Provenance, M::FrameExtra>,
local: mir::Local,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
let layout = self.layout_of_local(frame, local, layout)?;
let op = if layout.is_zst() {
// Bypass `access_local` (helps in ConstProp)
@ -521,8 +521,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
#[inline(always)]
pub fn place_to_op(
&self,
place: &PlaceTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
place: &PlaceTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
let op = match **place {
Place::Ptr(mplace) => Operand::Indirect(mplace),
Place::Local { frame, local } => {
@ -538,7 +538,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
&self,
mir_place: mir::Place<'tcx>,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
// Do not use the layout passed in as argument if the base we are looking at
// here is not the entire place.
let layout = if mir_place.projection.is_empty() { layout } else { None };
@ -575,7 +575,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
&self,
mir_op: &mir::Operand<'tcx>,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
use rustc_middle::mir::Operand::*;
let op = match *mir_op {
// FIXME: do some more logic on `move` to invalidate the old location
@ -600,7 +600,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
pub(super) fn eval_operands(
&self,
ops: &[mir::Operand<'tcx>],
) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::Provenance>>> {
ops.iter().map(|op| self.eval_operand(op, None)).collect()
}
@ -612,7 +612,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
&self,
c: ty::Const<'tcx>,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
match c.kind() {
ty::ConstKind::Param(_) | ty::ConstKind::Bound(..) => throw_inval!(TooGeneric),
ty::ConstKind::Error(DelaySpanBugEmitted { reported, .. }) => {
@ -637,7 +637,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
&self,
val: &mir::ConstantKind<'tcx>,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
match val {
mir::ConstantKind::Ty(ct) => self.const_to_op(*ct, layout),
mir::ConstantKind::Val(val, ty) => self.const_val_to_op(*val, *ty, layout),
@ -649,9 +649,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
val_val: ConstValue<'tcx>,
ty: Ty<'tcx>,
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
// Other cases need layout.
let tag_scalar = |scalar| -> InterpResult<'tcx, _> {
let adjust_scalar = |scalar| -> InterpResult<'tcx, _> {
Ok(match scalar {
Scalar::Ptr(ptr, size) => Scalar::Ptr(self.global_base_pointer(ptr)?, size),
Scalar::Int(int) => Scalar::Int(int),
@ -666,7 +666,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
Operand::Indirect(MemPlace::from_ptr(ptr.into()))
}
ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x)?.into()),
ConstValue::Scalar(x) => Operand::Immediate(adjust_scalar(x)?.into()),
ConstValue::ZeroSized => Operand::Immediate(Immediate::Uninit),
ConstValue::Slice { data, start, end } => {
// We rely on mutability being set correctly in `data` to prevent writes
@ -689,8 +689,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// Can also legally be called on non-enums (e.g. through the discriminant_value intrinsic)!
pub fn read_discriminant(
&self,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, (Scalar<M::Provenance>, VariantIdx)> {
trace!("read_discriminant_value {:#?}", op.layout);
// Get type and layout of the discriminant.
let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;