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Rollup merge of #96599 - tmiasko:discriminant-docs, r=estebank

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Update `RValue::Discriminant` documentation

`RValue::Discriminant` returns zero for types without discriminant.
This guarantee is already documented for `discriminant_value`
intrinsics which is implemented in terms of `RValue::Discriminant`.
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Yuki Okushi 2022-05-03 14:58:59 +09:00 committed by GitHub
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compiler/rustc_middle/src/mir/mod.rs
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@ -2561,16 +2561,12 @@ pub enum Rvalue<'tcx> {
UnaryOp(UnOp, Operand<'tcx>),
/// Computes the discriminant of the place, returning it as an integer of type
/// [`discriminant_ty`].
/// [`discriminant_ty`]. Returns zero for types without discriminant.
///
/// The validity requirements for the underlying value are undecided for this rvalue, see
/// [#91095]. Note too that the value of the discriminant is not the same thing as the
/// variant index; use [`discriminant_for_variant`] to convert.
///
/// For types defined in the source code as enums, this is well behaved. This is also well
/// formed for other types, but yields no particular value - there is no reason it couldn't be
/// defined to yield eg zero though.
///
/// [`discriminant_ty`]: crate::ty::Ty::discriminant_ty
/// [#91095]: https://github.com/rust-lang/rust/issues/91095
/// [`discriminant_for_variant`]: crate::ty::Ty::discriminant_for_variant