Allow casting integers to unsafe pointers in constant expressions.

src/librustc/middle/trans/consts.rs

@@ -345,6 +345,9 @@ fn const_expr_unchecked(cx: @CrateContext, e: @ast::expr) -> ValueRef {
               (expr::cast_pointer, expr::cast_pointer) => {
                 llvm::LLVMConstPointerCast(v, llty)
               }
+              (expr::cast_integral, expr::cast_pointer) => {
+                llvm::LLVMConstIntToPtr(v, llty)
+              }
               _ => {
                 cx.sess.impossible_case(e.span,
                                         ~"bad combination of types for cast")

src/test/run-pass/const-cast-ptr-int.rs

@@ -0,0 +1,15 @@
+// Copyright 2013 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.
+
+const a: *u8 = 0 as *u8;
+
+fn main() {
+    assert a == ptr::null();
+}