MatchBranchSimplification: copy discriminant instead of moving it

It might be necessary to use its value more than once.

Author: tmiasko

src/librustc_mir/transform/match_branches.rs

@@ -78,7 +78,7 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
                         );
                         if let Some(c) = c.literal.try_eval_bool(tcx, param_env) {
                             let op = if c { BinOp::Eq } else { BinOp::Ne };
-                            *rhs = Rvalue::BinaryOp(op, Operand::Move(discr), const_cmp);
+                            *rhs = Rvalue::BinaryOp(op, Operand::Copy(discr), const_cmp);
                         }
                     }
                 }

src/test/mir-opt/matches_reduce_branches.foo.MatchBranchSimplification.diff.32bit

@@ -11,7 +11,7 @@
           StorageLive(_2);                 // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           _3 = discriminant(_1);           // scope 0 at $DIR/matches_reduce_branches.rs:5:22: 5:26
 -         switchInt(move _3) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/matches_reduce_branches.rs:5:22: 5:26
-+         _2 = Eq(move _3, const 0_isize); // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
++         _2 = Eq(_3, const 0_isize);      // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
 +                                          // ty::Const
 +                                          // + ty: isize
 +                                          // + val: Value(Scalar(0x00000000))

src/test/mir-opt/matches_reduce_branches.foo.MatchBranchSimplification.diff.64bit

@@ -11,7 +11,7 @@
           StorageLive(_2);                 // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           _3 = discriminant(_1);           // scope 0 at $DIR/matches_reduce_branches.rs:5:22: 5:26
 -         switchInt(move _3) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/matches_reduce_branches.rs:5:22: 5:26
-+         _2 = Eq(move _3, const 0_isize); // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
++         _2 = Eq(_3, const 0_isize);      // scope 0 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
 +                                          // ty::Const
 +                                          // + ty: isize
 +                                          // + val: Value(Scalar(0x0000000000000000))