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Handle discriminant in DataflowConstProp cc ``@jachris`` r? ``@JakobDegen`` This PR attempts to extend the DataflowConstProp pass to handle propagation of discriminants. We handle this by adding 2 new variants to `TrackElem`: `TrackElem::Variant` for enum variants and `TrackElem::Discriminant` for the enum discriminant pseudo-place. The difficulty is that the enum discriminant and enum variants may alias each another. This is the issue of the `Option<NonZeroUsize>` test, which is the equivalent of https://github.com/rust-lang/unsafe-code-guidelines/issues/84 with a direct write. To handle that, we generalize the flood process to flood all the potentially aliasing places. In particular: - any write to `(PLACE as Variant)`, either direct or through a projection, floods `(PLACE as OtherVariant)` for all other variants and `discriminant(PLACE)`; - `SetDiscriminant(PLACE)` floods `(PLACE as Variant)` for each variant. This implies that flooding is not hierarchical any more, and that an assignment to a non-tracked place may need to flood a tracked place. This is handled by `for_each_aliasing_place` which generalizes `preorder_invoke`. As we deaggregate enums by putting `SetDiscriminant` last, this allows to propagate the value of the discriminant. This refactor will allow to make https://github.com/rust-lang/rust/pull/107009 able to handle discriminants too. |
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