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

Author: wesleywiser

src/librustc_mir/transform/const_prop.rs

@@ -736,6 +736,13 @@ 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, source_info.span);
+                    }
+                }
+            },
             _ => {}
         }
     }

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

@@ -0,0 +1,38 @@
+#[inline(never)]
+fn foo(_: i32) { }
+
+fn main() {
+    match 1 {
+        1 => foo(0),
+        _ => foo(-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
+// START rustc.main.SimplifyBranches-after-const-prop.before.mir
+//  bb0: {
+//      ...
+//      _1 = const 1i32;
+//      switchInt(const 1i32) -> [1i32: bb1, otherwise: bb2];
+//  }
+// END rustc.main.SimplifyBranches-after-const-prop.before.mir
+// START rustc.main.SimplifyBranches-after-const-prop.after.mir
+//  bb0: {
+//      ...
+//      _1 = const 1i32;
+//      goto -> bb1;
+//  }
+// END rustc.main.SimplifyBranches-after-const-prop.after.mir

src/test/mir-opt/simplify_if.rs

@@ -5,15 +5,15 @@ fn main() {
 }
 
 // END RUST SOURCE
-// START rustc.main.SimplifyBranches-after-copy-prop.before.mir
+// START rustc.main.SimplifyBranches-after-const-prop.before.mir
 // bb0: {
 //     ...
 //     switchInt(const false) -> [false: bb3, otherwise: bb1];
 // }
-// END rustc.main.SimplifyBranches-after-copy-prop.before.mir
-// START rustc.main.SimplifyBranches-after-copy-prop.after.mir
+// END rustc.main.SimplifyBranches-after-const-prop.before.mir
+// START rustc.main.SimplifyBranches-after-const-prop.after.mir
 // bb0: {
 //     ...
 //     goto -> bb3;
 // }
-// END rustc.main.SimplifyBranches-after-copy-prop.after.mir
+// END rustc.main.SimplifyBranches-after-const-prop.after.mir