Remove StorageDead and StorageLive statements using MIR locals of…

… type `()`

src/librustc_mir/transform/mod.rs

@@ -31,6 +31,7 @@ pub mod no_landing_pads;
 pub mod promote_consts;
 pub mod qualify_min_const_fn;
 pub mod remove_noop_landing_pads;
+pub mod remove_unit_storage;
 pub mod rustc_peek;
 pub mod simplify;
 pub mod simplify_branches;
@@ -300,6 +301,7 @@ fn run_optimization_passes<'tcx>(
             // From here on out, regions are gone.
             &erase_regions::EraseRegions,
             // Optimizations begin.
+            &remove_unit_storage::RemoveUnitStorage,
             &unreachable_prop::UnreachablePropagation,
             &uninhabited_enum_branching::UninhabitedEnumBranching,
             &simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),

src/librustc_mir/transform/remove_unit_storage.rs

@@ -0,0 +1,29 @@
+//! The `RemoveUnitStorage` pass removes `StorageLive` and `StorageDead` statements
+//! which operates on locals of type `()`.
+
+use crate::transform::{MirPass, MirSource};
+use rustc::mir::*;
+use rustc::ty::TyCtxt;
+use smallvec::SmallVec;
+
+pub struct RemoveUnitStorage;
+
+impl<'tcx> MirPass<'tcx> for RemoveUnitStorage {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
+        let unit_locals: SmallVec<[_; 32]> =
+            body.local_decls.iter().map(|local| local.ty == tcx.types.unit).collect();
+
+        for block in body.basic_blocks_mut() {
+            for stmt in &mut block.statements {
+                match &stmt.kind {
+                    StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
+                        if unit_locals[l.as_usize()] {
+                            stmt.make_nop();
+                        }
+                    }
+                    _ => (),
+                }
+            }
+        }
+    }
+}