[const-prop] Handle MIR Rvalue::Box

Author: wesleywiser

src/librustc_mir/transform/const_prop.rs

@@ -8,7 +8,7 @@ use rustc::hir::def::DefKind;
 use rustc::hir::def_id::DefId;
 use rustc::mir::{
     AggregateKind, Constant, Location, Place, PlaceBase, Body, Operand, Rvalue,
-    Local, NullOp, UnOp, StatementKind, Statement, LocalKind,
+    Local, UnOp, StatementKind, Statement, LocalKind,
     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp,
     SourceScope, SourceScopeLocalData, LocalDecl, BasicBlock,
 };
@@ -434,23 +434,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     ) -> Option<Const<'tcx>> {
         let span = source_info.span;
 
-        // if this isn't a supported operation, then return None
-        match rvalue {
-            Rvalue::NullaryOp(NullOp::Box, _) => return None,
-
-            Rvalue::Use(_) |
-            Rvalue::Len(_) |
-            Rvalue::Repeat(..) |
-            Rvalue::Aggregate(..) |
-            Rvalue::Discriminant(..) |
-            Rvalue::Cast(..) |
-            Rvalue::NullaryOp(..) |
-            Rvalue::CheckedBinaryOp(..) |
-            Rvalue::Ref(..) |
-            Rvalue::UnaryOp(..) |
-            Rvalue::BinaryOp(..) => { }
-        }
-
         // perform any special checking for specific Rvalue types
         if let Rvalue::UnaryOp(op, arg) = rvalue {
             trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);

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

@@ -0,0 +1,56 @@
+// compile-flags: -O
+// ignore-emscripten compiled with panic=abort by default
+// ignore-wasm32
+// ignore-wasm64
+
+#![feature(box_syntax)]
+
+// Note: this test verifies that we, in fact, do not const prop `box`
+
+fn main() {
+    let x = *(box 42) + 0;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _4 = Box(i32);
+//      (*_4) = const 42i32;
+//      _3 = move _4;
+//      ...
+//      _2 = (*_3);
+//      _1 = Add(move _2, const 0i32);
+//      ...
+//      drop(_3) -> [return: bb2, unwind: bb1];
+//  }
+//  bb1 (cleanup): {
+//      resume;
+//  }
+//  bb2: {
+//      ...
+//      _0 = ();
+//      ...
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _4 = Box(i32);
+//      (*_4) = const 42i32;
+//      _3 = move _4;
+//      ...
+//      _2 = (*_3);
+//      _1 = Add(move _2, const 0i32);
+//      ...
+//      drop(_3) -> [return: bb2, unwind: bb1];
+//  }
+//  bb1 (cleanup): {
+//      resume;
+//  }
+//  bb2: {
+//      ...
+//      _0 = ();
+//      ...
+//  }
+// END rustc.main.ConstProp.after.mir