1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306
//! Functions for reading and writing discriminants of multi-variant layouts (enums and coroutines).
use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt};
use rustc_middle::ty::{self, CoroutineArgsExt, ScalarInt, Ty};
use rustc_middle::{mir, span_bug};
use rustc_target::abi::{self, TagEncoding, VariantIdx, Variants};
use tracing::{instrument, trace};
use super::{
err_ub, throw_ub, ImmTy, InterpCx, InterpResult, Machine, Readable, Scalar, Writeable,
};
impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
/// Writes the discriminant of the given variant.
///
/// If the variant is uninhabited, this is UB.
#[instrument(skip(self), level = "trace")]
pub fn write_discriminant(
&mut self,
variant_index: VariantIdx,
dest: &impl Writeable<'tcx, M::Provenance>,
) -> InterpResult<'tcx> {
// Layout computation excludes uninhabited variants from consideration
// therefore there's no way to represent those variants in the given layout.
// Essentially, uninhabited variants do not have a tag that corresponds to their
// discriminant, so we cannot do anything here.
// When evaluating we will always error before even getting here, but ConstProp 'executes'
// dead code, so we cannot ICE here.
if dest.layout().for_variant(self, variant_index).abi.is_uninhabited() {
throw_ub!(UninhabitedEnumVariantWritten(variant_index))
}
match self.tag_for_variant(dest.layout().ty, variant_index)? {
Some((tag, tag_field)) => {
// No need to validate that the discriminant here because the
// `TyAndLayout::for_variant()` call earlier already checks the
// variant is valid.
let tag_dest = self.project_field(dest, tag_field)?;
self.write_scalar(tag, &tag_dest)
}
None => {
// No need to write the tag here, because an untagged variant is
// implicitly encoded. For `Niche`-optimized enums, this works by
// simply by having a value that is outside the niche variants.
// But what if the data stored here does not actually encode
// this variant? That would be bad! So let's double-check...
let actual_variant = self.read_discriminant(&dest.to_op(self)?)?;
if actual_variant != variant_index {
throw_ub!(InvalidNichedEnumVariantWritten { enum_ty: dest.layout().ty });
}
Ok(())
}
}
}
/// Read discriminant, return the runtime value as well as the variant index.
/// Can also legally be called on non-enums (e.g. through the discriminant_value intrinsic)!
///
/// Will never return an uninhabited variant.
#[instrument(skip(self), level = "trace")]
pub fn read_discriminant(
&self,
op: &impl Readable<'tcx, M::Provenance>,
) -> InterpResult<'tcx, VariantIdx> {
let ty = op.layout().ty;
trace!("read_discriminant_value {:#?}", op.layout());
// Get type and layout of the discriminant.
let discr_layout = self.layout_of(ty.discriminant_ty(*self.tcx))?;
trace!("discriminant type: {:?}", discr_layout.ty);
// We use "discriminant" to refer to the value associated with a particular enum variant.
// This is not to be confused with its "variant index", which is just determining its position in the
// declared list of variants -- they can differ with explicitly assigned discriminants.
// We use "tag" to refer to how the discriminant is encoded in memory, which can be either
// straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout().variants {
Variants::Single { index } => {
// Do some extra checks on enums.
if ty.is_enum() {
// Hilariously, `Single` is used even for 0-variant enums.
// (See https://github.com/rust-lang/rust/issues/89765).
if matches!(ty.kind(), ty::Adt(def, ..) if def.variants().is_empty()) {
throw_ub!(UninhabitedEnumVariantRead(index))
}
// For consistency with `write_discriminant`, and to make sure that
// `project_downcast` cannot fail due to strange layouts, we declare immediate UB
// for uninhabited variants.
if op.layout().for_variant(self, index).abi.is_uninhabited() {
throw_ub!(UninhabitedEnumVariantRead(index))
}
}
return Ok(index);
}
Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => {
(tag, tag_encoding, tag_field)
}
};
// There are *three* layouts that come into play here:
// - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
// the `Scalar` we return.
// - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
// and used to interpret the value we read from the tag field.
// For the return value, a cast to `discr_layout` is performed.
// - The field storing the tag has a layout, which is very similar to `tag_layout` but
// may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
// Get layout for tag.
let tag_layout = self.layout_of(tag_scalar_layout.primitive().to_int_ty(*self.tcx))?;
// Read tag and sanity-check `tag_layout`.
let tag_val = self.read_immediate(&self.project_field(op, tag_field)?)?;
assert_eq!(tag_layout.size, tag_val.layout.size);
assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
trace!("tag value: {}", tag_val);
// Figure out which discriminant and variant this corresponds to.
let index = match *tag_encoding {
TagEncoding::Direct => {
// Generate a specific error if `tag_val` is not an integer.
// (`tag_bits` itself is only used for error messages below.)
let tag_bits = tag_val
.to_scalar()
.try_to_scalar_int()
.map_err(|dbg_val| err_ub!(InvalidTag(dbg_val)))?
.to_bits(tag_layout.size);
// Cast bits from tag layout to discriminant layout.
// After the checks we did above, this cannot fail, as
// discriminants are int-like.
let discr_val = self.int_to_int_or_float(&tag_val, discr_layout).unwrap();
let discr_bits = discr_val.to_scalar().to_bits(discr_layout.size)?;
// Convert discriminant to variant index, and catch invalid discriminants.
let index = match *ty.kind() {
ty::Adt(adt, _) => {
adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
}
ty::Coroutine(def_id, args) => {
let args = args.as_coroutine();
args.discriminants(def_id, *self.tcx).find(|(_, var)| var.val == discr_bits)
}
_ => span_bug!(self.cur_span(), "tagged layout for non-adt non-coroutine"),
}
.ok_or_else(|| err_ub!(InvalidTag(Scalar::from_uint(tag_bits, tag_layout.size))))?;
// Return the cast value, and the index.
index.0
}
TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => {
let tag_val = tag_val.to_scalar();
// Compute the variant this niche value/"tag" corresponds to. With niche layout,
// discriminant (encoded in niche/tag) and variant index are the same.
let variants_start = niche_variants.start().as_u32();
let variants_end = niche_variants.end().as_u32();
let variant = match tag_val.try_to_scalar_int() {
Err(dbg_val) => {
// So this is a pointer then, and casting to an int failed.
// Can only happen during CTFE.
// The niche must be just 0, and the ptr not null, then we know this is
// okay. Everything else, we conservatively reject.
let ptr_valid = niche_start == 0
&& variants_start == variants_end
&& !self.scalar_may_be_null(tag_val)?;
if !ptr_valid {
throw_ub!(InvalidTag(dbg_val))
}
untagged_variant
}
Ok(tag_bits) => {
let tag_bits = tag_bits.to_bits(tag_layout.size);
// We need to use machine arithmetic to get the relative variant idx:
// variant_index_relative = tag_val - niche_start_val
let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
let variant_index_relative_val =
self.binary_op(mir::BinOp::Sub, &tag_val, &niche_start_val)?;
let variant_index_relative =
variant_index_relative_val.to_scalar().to_bits(tag_val.layout.size)?;
// Check if this is in the range that indicates an actual discriminant.
if variant_index_relative <= u128::from(variants_end - variants_start) {
let variant_index_relative = u32::try_from(variant_index_relative)
.expect("we checked that this fits into a u32");
// Then computing the absolute variant idx should not overflow any more.
let variant_index = VariantIdx::from_u32(
variants_start
.checked_add(variant_index_relative)
.expect("overflow computing absolute variant idx"),
);
let variants =
ty.ty_adt_def().expect("tagged layout for non adt").variants();
assert!(variant_index < variants.next_index());
variant_index
} else {
untagged_variant
}
}
};
// Compute the size of the scalar we need to return.
// No need to cast, because the variant index directly serves as discriminant and is
// encoded in the tag.
variant
}
};
// Reading the discriminant of an uninhabited variant is UB. This is the basis for the
// `uninhabited_enum_branching` MIR pass. It also ensures consistency with
// `write_discriminant`.
if op.layout().for_variant(self, index).abi.is_uninhabited() {
throw_ub!(UninhabitedEnumVariantRead(index))
}
Ok(index)
}
pub fn discriminant_for_variant(
&self,
ty: Ty<'tcx>,
variant: VariantIdx,
) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
let discr_layout = self.layout_of(ty.discriminant_ty(*self.tcx))?;
let discr_value = match ty.discriminant_for_variant(*self.tcx, variant) {
Some(discr) => {
// This type actually has discriminants.
assert_eq!(discr.ty, discr_layout.ty);
Scalar::from_uint(discr.val, discr_layout.size)
}
None => {
// On a type without actual discriminants, variant is 0.
assert_eq!(variant.as_u32(), 0);
Scalar::from_uint(variant.as_u32(), discr_layout.size)
}
};
Ok(ImmTy::from_scalar(discr_value, discr_layout))
}
/// Computes how to write the tag of a given variant of enum `ty`:
/// - `None` means that nothing needs to be done as the variant is encoded implicitly
/// - `Some((val, field_idx))` means that the given integer value needs to be stored at the
/// given field index.
pub(crate) fn tag_for_variant(
&self,
ty: Ty<'tcx>,
variant_index: VariantIdx,
) -> InterpResult<'tcx, Option<(ScalarInt, usize)>> {
match self.layout_of(ty)?.variants {
abi::Variants::Single { .. } => {
// The tag of a `Single` enum is like the tag of the niched
// variant: there's no tag as the discriminant is encoded
// entirely implicitly. If `write_discriminant` ever hits this
// case, we do a "validation read" to ensure the right
// discriminant is encoded implicitly, so any attempt to write
// the wrong discriminant for a `Single` enum will reliably
// result in UB.
Ok(None)
}
abi::Variants::Multiple {
tag_encoding: TagEncoding::Direct,
tag: tag_layout,
tag_field,
..
} => {
// raw discriminants for enums are isize or bigger during
// their computation, but the in-memory tag is the smallest possible
// representation
let discr = self.discriminant_for_variant(ty, variant_index)?;
let discr_size = discr.layout.size;
let discr_val = discr.to_scalar().to_bits(discr_size)?;
let tag_size = tag_layout.size(self);
let tag_val = tag_size.truncate(discr_val);
let tag = ScalarInt::try_from_uint(tag_val, tag_size).unwrap();
Ok(Some((tag, tag_field)))
}
abi::Variants::Multiple {
tag_encoding: TagEncoding::Niche { untagged_variant, .. },
..
} if untagged_variant == variant_index => {
// The untagged variant is implicitly encoded simply by having a
// value that is outside the niche variants.
Ok(None)
}
abi::Variants::Multiple {
tag_encoding:
TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start },
tag: tag_layout,
tag_field,
..
} => {
assert!(variant_index != untagged_variant);
let variants_start = niche_variants.start().as_u32();
let variant_index_relative = variant_index
.as_u32()
.checked_sub(variants_start)
.expect("overflow computing relative variant idx");
// We need to use machine arithmetic when taking into account `niche_start`:
// tag_val = variant_index_relative + niche_start_val
let tag_layout = self.layout_of(tag_layout.primitive().to_int_ty(*self.tcx))?;
let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
let variant_index_relative_val =
ImmTy::from_uint(variant_index_relative, tag_layout);
let tag = self
.binary_op(mir::BinOp::Add, &variant_index_relative_val, &niche_start_val)?
.to_scalar_int()?;
Ok(Some((tag, tag_field)))
}
}
}
}