`Layout` is another type that is sometimes interned, sometimes not, and
we always use references to refer to it so we can't take any advantage
of the uniqueness properties for hashing or equality checks.
This commit renames `Layout` as `LayoutS`, and then introduces a new
`Layout` that is a newtype around an `Interned<LayoutS>`. It also
interns more layouts than before. Previously layouts within layouts
(via the `variants` field) were never interned, but now they are. Hence
the lifetime on the new `Layout` type.
Unlike other interned types, these ones are in `rustc_target` instead of
`rustc_middle`. This reflects the existing structure of the code, which
does layout-specific stuff in `rustc_target` while `TyAndLayout` is
generic over the `Ty`, allowing the type-specific stuff to occur in
`rustc_middle`.
The commit also adds a `HashStable` impl for `Interned`, which was
needed. It hashes the contents, unlike the `Hash` impl which hashes the
pointer.
The `uninhabited_enum_branch` miropt now also checks whether the fallthrough
case is inhabited, and if not will ensure that it points to an unreachable
block.
Fix variant index / discriminant confusion in uninhabited enum branching
Fix confusion between variant index and variant discriminant. The pass
incorrectly assumed that for `Variants::Single` variant index is the same as
variant discriminant.
r? `@wesleywiser`
Previously for enums using the `Variants::Single` layout, the variant
index was being confused with its discriminant. For example, in the case
of `enum E { A = 1 }`.
Use `discriminant_for_variant` to avoid the issue.
The change is limited to the iteration over indices instead of using
`basic_blocks_mut()` directly, in the case the previous implementation
intentionally avoided invalidating the caches stored in MIR body.
This performs a substitution of code following the pattern:
let <id> = if let <pat> = ... { identity } else { ... : ! };
To simplify it to:
let <pat> = ... { identity } else { ... : ! };
By adopting the let_else feature.
