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Rollup merge of #118935 - RalfJung:interpret-downcast, r=saethlin

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interpret: extend comment on the inhabitedness check in downcast

Cc https://github.com/rust-lang/rust/issues/115145
r? ``@saethlin``
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Matthias Krüger 2023-12-14 20:33:11 +01:00 • committed by GitHub
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18
compiler/rustc_const_eval/src/interpret/projection.rs
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@ -208,6 +208,24 @@ where
if layout.abi.is_uninhabited() { if layout.abi.is_uninhabited() {
// `read_discriminant` should have excluded uninhabited variants... but ConstProp calls // `read_discriminant` should have excluded uninhabited variants... but ConstProp calls
// us on dead code. // us on dead code.
// In the future we might want to allow this to permit code like this:
// (this is a Rust/MIR pseudocode mix)
// ```
// enum Option2 {
// Some(i32, !),
// None,
// }
//
// fn panic() -> ! { panic!() }
//
// let x: Option2;
// x.Some.0 = 42;
// x.Some.1 = panic();
// SetDiscriminant(x, Some);
// ```
// However, for now we don't generate such MIR, and this check here *has* found real
// bugs (see https://github.com/rust-lang/rust/issues/115145), so we will keep rejecting
// it.
throw_inval!(ConstPropNonsense) throw_inval!(ConstPropNonsense)
} }
// This cannot be `transmute` as variants *can* have a smaller size than the entire enum. // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.