Commits on Jun 14, 2024

1. safe transmute: support non-ZST, variantful, uninhabited enums …

   Previously, `Tree::from_enum`'s implementation branched into three disjoint
   cases:
   
    1. enums that uninhabited
    2. enums for which all but one variant is uninhabited
    3. enums with multiple inhabited variants
   
   This branching (incorrectly) did not differentiate between variantful and
   variantless uninhabited enums. In both cases, we assumed (and asserted) that
   uninhabited enums are zero-sized types. This assumption is false for enums like:
   
       enum Uninhabited { A(!, u128) }
   
   ...which, currently, has the same size as `u128`. This faulty assumption
   manifested as the ICE reported in rust-lang#126460.
   
   In this PR, we revise the first case of `Tree::from_enum` to consider only the
   narrow category of "enums that are uninhabited ZSTs". These enums, whose layouts
   are described with `Variants::Single { index }`, are special in their layouts
   otherwise resemble the `!` type and cannot be descended into like typical enums.
   This first case captures uninhabited enums like:
   
       enum Uninhabited { A(!, !), B(!) }
   
   The second case is revised to consider the broader category of "enums that defer
   their layout to one of their variants"; i.e., enums whose layouts are described
   with `Variants::Single { index }` and that do have a variant at `index`. This
   second case captures uninhabited enums that are not ZSTs, like:
   
       enum Uninhabited { A(!, u128) }
   
   ...which represent their variants with `Variants::Single`.
   
   Finally, the third case is revised to cover the broader category of "enums with
   multiple variants", which captures uninhabited, non-ZST enums like:
   
       enum Uninhabited { A(u8, !), B(!, u32) }
   
   ...which represent their variants with `Variants::Multiple`.
   
   This PR also adds a comment requested by RalfJung in his review of rust-lang#126358 to
   `compiler/rustc_const_eval/src/interpret/discriminant.rs`.
   
   Fixes rust-lang#126460

   

   jswrenn committed Jun 14, 2024
   Configuration menu

   • View commit details
   • Copy full SHA for df1d616
   • Browse repository at this point

   Copy the full SHA

   df1d616 View commit details

   Browse the repository at this point in the history