[const-prop] Handle MIR Rvalue::Discriminant

Author: wesleywiser

src/librustc_mir/transform/const_prop.rs

@@ -436,13 +436,13 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
         // if this isn't a supported operation, then return None
         match rvalue {
-            Rvalue::NullaryOp(NullOp::Box, _) |
-            Rvalue::Discriminant(..) => return None,
+            Rvalue::NullaryOp(NullOp::Box, _) => return None,
 
             Rvalue::Use(_) |
             Rvalue::Len(_) |
             Rvalue::Repeat(..) |
             Rvalue::Aggregate(..) |
+            Rvalue::Discriminant(..) |
             Rvalue::Cast(..) |
             Rvalue::NullaryOp(..) |
             Rvalue::CheckedBinaryOp(..) |

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

@@ -0,0 +1,53 @@
+// compile-flags: -O
+
+fn main() {
+    let x = (if let Some(true) = Some(true) { 42 } else { 10 }) + 0;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _3 = std::option::Option::<bool>::Some(const true,);
+//      _4 = discriminant(_3);
+//      switchInt(move _4) -> [1isize: bb3, otherwise: bb2];
+//  }
+//  bb1: {
+//      _2 = const 42i32;
+//      goto -> bb4;
+//  }
+//  bb2: {
+//      _2 = const 10i32;
+//      goto -> bb4;
+//  }
+//  bb3: {
+//      switchInt(((_3 as Some).0: bool)) -> [false: bb2, otherwise: bb1];
+//  }
+//  bb4: {
+//      _1 = Add(move _2, const 0i32);
+//      ...
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _3 = const Scalar(0x01) : std::option::Option<bool>;
+//      _4 = const 1isize;
+//      switchInt(const 1isize) -> [1isize: bb3, otherwise: bb2];
+//  }
+//  bb1: {
+//      _2 = const 42i32;
+//      goto -> bb4;
+//  }
+//  bb2: {
+//      _2 = const 10i32;
+//      goto -> bb4;
+//  }
+//  bb3: {
+//      switchInt(const true) -> [false: bb2, otherwise: bb1];
+//  }
+//  bb4: {
+//      _1 = Add(move _2, const 0i32);
+//      ...
+//  }
+// END rustc.main.ConstProp.after.mir