unit rvalue: use constant () instead of tuple

src/librustc_mir/transform/promote_consts.rs

@@ -835,7 +835,11 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                     if self.keep_original {
                         rhs.clone()
                     } else {
-                        let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
+                        let unit = Rvalue::Use(Operand::Constant(box Constant {
+                            span: statement.source_info.span,
+                            user_ty: None,
+                            literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit),
+                        }));
                         mem::replace(rhs, unit)
                     },
                     statement.source_info,

src/librustc_mir_build/build/block.rs

@@ -187,7 +187,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             if destination_ty.is_unit() {
                 // We only want to assign an implicit `()` as the return value of the block if the
                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
-                this.cfg.push_assign_unit(block, source_info, destination);
+                this.cfg.push_assign_unit(block, source_info, destination, this.hir.tcx());
             }
         }
         // Finally, we pop all the let scopes before exiting out from the scope of block

src/librustc_mir_build/build/cfg.rs

@@ -2,6 +2,7 @@
 
 use crate::build::CFG;
 use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
 
 impl<'tcx> CFG<'tcx> {
     crate fn block_data(&self, blk: BasicBlock) -> &BasicBlockData<'tcx> {
@@ -58,12 +59,17 @@ impl<'tcx> CFG<'tcx> {
         block: BasicBlock,
         source_info: SourceInfo,
         place: Place<'tcx>,
+        tcx: TyCtxt<'tcx>,
     ) {
         self.push_assign(
             block,
             source_info,
             place,
-            Rvalue::Aggregate(box AggregateKind::Tuple, vec![]),
+            Rvalue::Use(Operand::Constant(box Constant {
+                span: source_info.span,
+                user_ty: None,
+                literal: ty::Const::zero_sized(tcx, tcx.types.unit),
+            })),
         );
     }
 

src/librustc_mir_build/build/expr/as_rvalue.rs

@@ -225,7 +225,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
                 block = unpack!(this.stmt_expr(block, expr, None));
-                block.and(this.unit_rvalue())
+                block.and(Rvalue::Use(Operand::Constant(box Constant {
+                    span: expr_span,
+                    user_ty: None,
+                    literal: ty::Const::zero_sized(this.hir.tcx(), this.hir.tcx().types.unit),
+                })))
             }
             ExprKind::Yield { .. }
             | ExprKind::Literal { .. }

src/librustc_mir_build/build/expr/into.rs

@@ -331,7 +331,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::LlvmInlineAsm { .. }
             | ExprKind::Return { .. } => {
                 unpack!(block = this.stmt_expr(block, expr, None));
-                this.cfg.push_assign_unit(block, source_info, destination);
+                this.cfg.push_assign_unit(block, source_info, destination, this.hir.tcx());
                 block.unit()
             }
 

src/librustc_mir_build/build/misc.rs

@@ -32,10 +32,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         Operand::Constant(constant)
     }
 
-    crate fn unit_rvalue(&mut self) -> Rvalue<'tcx> {
-        Rvalue::Aggregate(box AggregateKind::Tuple, vec![])
-    }
-
     // Returns a zero literal operand for the appropriate type, works for
     // bool, char and integers.
     crate fn zero_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx> {

src/librustc_mir_build/build/scope.rs

@@ -523,7 +523,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 unpack!(block = self.into(destination, block, value));
                 self.block_context.pop();
             } else {
-                self.cfg.push_assign_unit(block, source_info, destination)
+                self.cfg.push_assign_unit(block, source_info, destination, self.hir.tcx())
             }
         } else {
             assert!(value.is_none(), "`return` and `break` should have a destination");