read_via_copy: don't prematurely optimize away the read

1 files changed, 11 insertions, 12 deletions

diff --git a/compiler/rustc_mir_transform/src/lower_intrinsics.rs b/compiler/rustc_mir_transform/src/lower_intrinsics.rs
index fc36c6e4124..a8dc91ca439 100644
--- a/compiler/rustc_mir_transform/src/lower_intrinsics.rs
+++ b/compiler/rustc_mir_transform/src/lower_intrinsics.rs
@@ -176,23 +176,22 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
                             } else {
                                 span_bug!(terminator.source_info.span, "Only passing a local is supported");
                             };
+                        // Add new statement at the end of the block that does the read, and patch
+                        // up the terminator.
+                        block.statements.push(Statement {
+                            source_info: terminator.source_info,
+                            kind: StatementKind::Assign(Box::new((
+                                *destination,
+                                Rvalue::Use(Operand::Copy(derefed_place)),
+                            ))),
+                        });
                         terminator.kind = match *target {
                             None => {
                                 // No target means this read something uninhabited,
-                                // so it must be unreachable, and we don't need to
-                                // preserve the assignment either.
+                                // so it must be unreachable.
                                 TerminatorKind::Unreachable
                             }
-                            Some(target) => {
-                                block.statements.push(Statement {
-                                    source_info: terminator.source_info,
-                                    kind: StatementKind::Assign(Box::new((
-                                        *destination,
-                                        Rvalue::Use(Operand::Copy(derefed_place)),
-                                    ))),
-                                });
-                                TerminatorKind::Goto { target }
-                            }
+                            Some(target) => TerminatorKind::Goto { target },
                         }
                     }
                     sym::write_via_move => {