[const-prop] Handle MIR Rvalue::Repeat

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::Repeat(..) |
             Rvalue::Aggregate(..) |
             Rvalue::NullaryOp(NullOp::Box, _) |
             Rvalue::Discriminant(..) => return None,
 
             Rvalue::Use(_) |
             Rvalue::Len(_) |
+            Rvalue::Repeat(..) |
             Rvalue::Cast(..) |
             Rvalue::NullaryOp(..) |
             Rvalue::CheckedBinaryOp(..) |

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

@@ -0,0 +1,37 @@
+// compile-flags: -O
+
+fn main() {
+    let x: u32 = [42; 8][2] + 0;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _3 = [const 42u32; 8];
+//      ...
+//      _4 = const 2usize;
+//      _5 = const 8usize;
+//      _6 = Lt(_4, _5);
+//      assert(move _6, "index out of bounds: the len is move _5 but the index is _4") -> bb1;
+//  }
+//  bb1: {
+//      _2 = _3[_4];
+//      _1 = Add(move _2, const 0u32);
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _6 = const true;
+//      assert(const true, "index out of bounds: the len is move _5 but the index is _4") -> bb1;
+//  }
+//  bb1: {
+//      _2 = const 42u32;
+//      _1 = Add(move _2, const 0u32);
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.after.mir

src/test/ui/consts/const-prop-ice.rs

@@ -1,3 +1,4 @@
 fn main() {
     [0; 3][3u64 as usize]; //~ ERROR the len is 3 but the index is 3
+    //~| ERROR this expression will panic at runtime
 }

src/test/ui/consts/const-prop-ice.stderr

@@ -6,5 +6,11 @@ LL |     [0; 3][3u64 as usize];
    |
    = note: `#[deny(const_err)]` on by default
 
-error: aborting due to previous error
+error: this expression will panic at runtime
+  --> $DIR/const-prop-ice.rs:2:5
+   |
+LL |     [0; 3][3u64 as usize];
+   |     ^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the len is 3 but the index is 3
+
+error: aborting due to 2 previous errors