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fix issue #78496

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This commit is contained in:
Chenguang Wang 2020-12-09 18:56:27 -08:00
parent 8080f54677
commit c6f2d49ff8
2 changed files with 44 additions and 0 deletions
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28
compiler/rustc_mir/src/transform/early_otherwise_branch.rs
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@ -283,6 +283,34 @@ impl<'a, 'tcx> Helper<'a, 'tcx> {
return None;
}
// when one place is the projection of the other, it's not safe to calculate their discriminant values sequentially.
// for example, this should not be optimized:
//
// ```rust
// enum E<'a> { Empty, Some(&'a E<'a>), }
// let Some(Some(_)) = e;
// ```
//
// ```mir
// bb0: {
// _2 = discriminant(*_1)
// switchInt(move _2) -> [...]
// }
// bb1: {
// _3 = discriminant(*(((*_1) as Some).0: &E))
// switchInt(move _3) -> [...]
// }
// ```
let discr_place = discr_info.place_of_adt_discr_read;
let this_discr_place = this_bb_discr_info.place_of_adt_discr_read;
if discr_place.local == this_discr_place.local
&& (discr_place.projection.starts_with(this_discr_place.projection)
|| this_discr_place.projection.starts_with(discr_place.projection))
{
trace!("NO: one target is the projection of another");
return None;
}
// if we reach this point, the optimization applies, and we should be able to optimize this case
// store the info that is needed to apply the optimization