Actually keep PlaceMention if requested.

compiler/rustc_mir_transform/src/remove_place_mention.rs

@@ -0,0 +1,23 @@
+//! This pass removes `PlaceMention` statement, which has no effect at codegen.
+
+use crate::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct RemovePlaceMention;
+
+impl<'tcx> MirPass<'tcx> for RemovePlaceMention {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        !sess.opts.unstable_opts.mir_keep_place_mention
+    }
+
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running RemovePlaceMention on {:?}", body.source);
+        for data in body.basic_blocks.as_mut_preserves_cfg() {
+            data.statements.retain(|statement| match statement.kind {
+                StatementKind::PlaceMention(..) | StatementKind::Nop => false,
+                _ => true,
+            })
+        }
+    }
+}