Fix span typo in the arithmetic overflow for array length reporting.

Added pair of regression tests that I should have made when I thought
of doing this in the first place.

src/librustc_typeck/astconv.rs

@@ -1399,7 +1399,7 @@ pub fn ast_ty_to_ty<'tcx>(this: &AstConv<'tcx>,
                 Err(ref r) => {
                     let subspan  =
                         ast_ty.span.lo <= r.span.lo && r.span.hi <= ast_ty.span.hi;
-                    span_err!(tcx.sess, ast_ty.span, E0250,
+                    span_err!(tcx.sess, r.span, E0250,
                               "array length constant evaluation error: {}",
                               r.description().as_slice());
                     if !subspan {

src/test/compile-fail/const-len-underflow-separate-spans.rs

@@ -0,0 +1,23 @@
+// Copyright 2012-2013 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.
+
+// Check that an constant-evaluation underflow highlights the correct
+// spot (where the underflow occurred), while also providing the
+// overall context for what caused the evaluation.
+
+const ONE: usize = 1;
+const TWO: usize = 2;
+const LEN: usize = ONE - TWO;
+//~^ ERROR array length constant evaluation error: attempted to sub with overflow [E0250]
+
+fn main() {
+    let a: [i8; LEN] = unimplemented!();
+    //~^ NOTE for array length here
+}

src/test/compile-fail/const-len-underflow-subspans.rs

@@ -0,0 +1,20 @@
+// Copyright 2012-2013 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.
+
+// Check that an constant-evaluation underflow highlights the correct
+// spot (where the underflow occurred).
+
+const ONE: usize = 1;
+const TWO: usize = 2;
+
+fn main() {
+    let a: [i8; ONE - TWO] = unimplemented!();
+    //~^ ERROR array length constant evaluation error: attempted to sub with overflow [E0250]
+}