From c6f2d49ff8be6b42bba94e7dc96e0fff8b5ca580 Mon Sep 17 00:00:00 2001
From: Chenguang Wang <w3cing@gmail.com>
Date: Wed, 9 Dec 2020 18:56:27 -0800
Subject: [PATCH] fix issue #78496
---
.../src/transform/early_otherwise_branch.rs | 28 +++++++++++++++++++
src/test/ui/mir/issue-78496.rs | 16 +++++++++++
2 files changed, 44 insertions(+)
create mode 100644 src/test/ui/mir/issue-78496.rs
diff --git a/compiler/rustc_mir/src/transform/early_otherwise_branch.rs b/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
index f91477911a4..5829c522935 100644
--- a/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
+++ b/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
@@ -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
diff --git a/src/test/ui/mir/issue-78496.rs b/src/test/ui/mir/issue-78496.rs
new file mode 100644
index 00000000000..cc45945a2b8
--- /dev/null
+++ b/src/test/ui/mir/issue-78496.rs
@@ -0,0 +1,16 @@
+// run-pass
+// compile-flags: -Z mir-opt-level=2 -C opt-level=0
+
+// example from #68867
+pub enum E<'a> {
+ Empty,
+ Some(&'a E<'a>),
+}
+
+fn f(e: &E) -> u32 {
+ if let E::Some(E::Some(_)) = e { 1 } else { 2 }
+}
+
+fn main() {
+ assert_eq!(f(&E::Empty), 2);
+}