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safe transmute: support Variants::Single enums

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Previously, the implementation of `Tree::from_enum` incorrectly
treated enums with `Variants::Single` and `Variants::Multiple`
identically. This is incorrect for `Variants::Single` enums,
which delegate their layout to that of a variant with a particular
index (or no variant at all if the enum is empty).

This flaw manifested first as an ICE. `Tree::from_enum` attempted
to compute the tag of variants other than the one at
`Variants::Single`'s `index`, and fell afoul of a sanity-checking
assertion in `compiler/rustc_const_eval/src/interpret/discriminant.rs`.
This assertion is non-load-bearing, and can be removed; the routine
its in is well-behaved even without it.

With the assertion removed, the proximate issue becomes apparent:
calling `Tree::from_variant` on a variant that does not exist is
ill-defined. A sanity check the given variant has
`FieldShapes::Arbitrary` fails, and the analysis is (correctly)
aborted with `Err::NotYetSupported`.

This commit corrects this chain of failures by ensuring that
`Tree::from_variant` is not called on variants that are, as far as
layout is concerned, nonexistent. Specifically, the implementation
of `Tree::from_enum` is now partitioned into three cases:

  1. enums that are uninhabited
  2. enums for which all but one variant is uninhabited
  3. enums with multiple inhabited variants

`Tree::from_variant` is now only invoked in the third case. In the
first case, `Tree::uninhabited()` is produced. In the second case,
the layout is delegated to `Variants::Single`'s index.

Fixes #125811
This commit is contained in:
Jack Wrenn 2024-06-12 22:07:37 +00:00
parent 1d43fbbc73
commit fb662f2126
4 changed files with 81 additions and 59 deletions
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5
compiler/rustc_const_eval/src/interpret/discriminant.rs
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@ -241,10 +241,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
variant_index: VariantIdx,
) -> InterpResult<'tcx, Option<(ScalarInt, usize)>> {
match self.layout_of(ty)?.variants {
abi::Variants::Single { index } => {
assert_eq!(index, variant_index);
Ok(None)
}
abi::Variants::Single { .. } => Ok(None),
abi::Variants::Multiple {
tag_encoding: TagEncoding::Direct,