Fix an ICE with error types in a vec

Closes #16783

Author: nrc

src/librustc/middle/ty.rs

@@ -3266,6 +3266,11 @@ pub fn adjust_ty(cx: &ctxt,
                  -> ty::t {
     /*! See `expr_ty_adjusted` */
 
+    match get(unadjusted_ty).sty {
+        ty_err => return unadjusted_ty,
+        _ => {}
+    }
+
     return match adjustment {
         Some(adjustment) => {
             match *adjustment {

src/test/compile-fail/issue-16783.rs

@@ -0,0 +1,15 @@
+// Copyright 2014 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.
+
+pub fn main() {
+    let x = [1, 2, 3];
+    //~^ ERROR cannot determine a type for this local variable: cannot determine the type of this
+    let y = x.as_slice();
+}