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 1/3] 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 f91477911a489..5829c52293530 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 0000000000000..cc45945a2b8d5
--- /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);
+}

From 78c0680b3febef5aaa2330d8b0a7879fd44e9eba Mon Sep 17 00:00:00 2001
From: Chenguang Wang <w3cing@gmail.com>
Date: Wed, 9 Dec 2020 19:50:11 -0800
Subject: [PATCH 2/3] update comments

---
 .../rustc_mir/src/transform/early_otherwise_branch.rs    | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_mir/src/transform/early_otherwise_branch.rs b/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
index 5829c52293530..f28a4b4671571 100644
--- a/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
+++ b/compiler/rustc_mir/src/transform/early_otherwise_branch.rs
@@ -283,7 +283,7 @@ 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.
+            // when the second place is a projection of the first one, it's not safe to calculate their discriminant values sequentially.
             // for example, this should not be optimized:
             //
             // ```rust
@@ -294,18 +294,17 @@ impl<'a, 'tcx> Helper<'a, 'tcx> {
             // ```mir
             // bb0: {
             //   _2 = discriminant(*_1)
-            //   switchInt(move _2) -> [...]
+            //   switchInt(_2) -> [...]
             // }
             // bb1: {
             //   _3 = discriminant(*(((*_1) as Some).0: &E))
-            //   switchInt(move _3) -> [...]
+            //   switchInt(_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))
+                && this_discr_place.projection.starts_with(discr_place.projection)
             {
                 trace!("NO: one target is the projection of another");
                 return None;

From 3812f70355132b53092e826c9d1a753dfd8a1874 Mon Sep 17 00:00:00 2001
From: Chenguang Wang <w3cing@gmail.com>
Date: Wed, 9 Dec 2020 20:11:32 -0800
Subject: [PATCH 3/3] fix test case issue ref

---
 src/test/ui/mir/issue-78496.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/test/ui/mir/issue-78496.rs b/src/test/ui/mir/issue-78496.rs
index cc45945a2b8d5..1b0687cfac3f6 100644
--- a/src/test/ui/mir/issue-78496.rs
+++ b/src/test/ui/mir/issue-78496.rs
@@ -1,7 +1,7 @@
 // run-pass
 // compile-flags: -Z mir-opt-level=2 -C opt-level=0
 
-// example from #68867
+// example from #78496
 pub enum E<'a> {
     Empty,
     Some(&'a E<'a>),