MIR: terminate unreachable blocks in construct_const

src/librustc_mir/build/mod.rs

@@ -472,6 +472,13 @@ fn construct_const<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
     // Constants can't `return` so a return block should not be created.
     assert_eq!(builder.cached_return_block, None);
 
+    // Constants may be match expressions in which case an unreachable block may
+    // be created, so terminate it properly.
+    if let Some(unreachable_block) = builder.cached_unreachable_block {
+        builder.cfg.terminate(unreachable_block, source_info,
+                              TerminatorKind::Unreachable);
+    }
+
     builder.finish(vec![], None)
 }
 

src/test/compile-fail/issue-46843.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+enum Thing { This, That }
+
+fn non_const() -> Thing {
+    Thing::This
+}
+
+pub const Q: i32 = match non_const() { //~ ERROR E0015
+    Thing::This => 1, //~ ERROR unimplemented expression type
+    Thing::That => 0
+};
+
+fn main() {}