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interpret: update comment about read_discriminant on uninhabited variants #122183

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Mar 8, 2024
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interpret: update comment about read_discriminant on uninhabited variants #122183

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8 changes: 7 additions & 1 deletion compiler/rustc_const_eval/src/interpret/discriminant.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,8 @@ use super::{ImmTy, InterpCx, InterpResult, Machine, Readable, Scalar, Writeable}

impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, '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,
Expand Down Expand Up @@ -102,6 +104,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {

/// 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,
Expand Down Expand Up @@ -244,7 +248,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
variant
}
};
// 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.
// 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))
}
Expand Down