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.
