[const-prop] Support propagating into SwitchInt's discr Operand

Author: wesleywiser

src/librustc_mir/transform/const_prop.rs

@@ -725,6 +725,13 @@ impl<'b, 'a, 'tcx> MutVisitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
                     }
                 }
             },
+            TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
+                if let Some(value) = self.eval_operand(&discr, source_info) {
+                    if let ScalarMaybeUndef::Scalar(scalar) = self.ecx.read_scalar(value).unwrap() {
+                        *discr = self.operand_from_scalar(scalar, switch_ty);
+                    }
+                }
+            },
             _ => {}
         }
     }

src/test/mir-opt/const_prop/switch_int.rs

@@ -0,0 +1,21 @@
+fn main() {
+    match 1 {
+        1 => std::process::exit(0),
+        _ => std::process::exit(-1),
+    }
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _1 = const 1i32;
+//      switchInt(_1) -> [1i32: bb1, otherwise: bb2];
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      switchInt(const 1i32) -> [1i32: bb1, otherwise: bb2];
+//  }
+// END rustc.main.ConstProp.after.mir