librustc: Make sure lifetimes in for loop heads outlive the for loop

itself.

This breaks code like:

    for &x in my_vector.iter() {
        my_vector[2] = "wibble";
        ...
    }

Change this code to not invalidate iterators. For example:

    for i in range(0, my_vector.len()) {
        my_vector[2] = "wibble";
        ...
    }

The `for-loop-does-not-borrow-iterators` test for #8372 was incorrect
and has been removed.

Closes #16820.

[breaking-change]

Author: pcwalton

src/librustc/middle/typeck/check/regionck.rs

@@ -769,6 +769,10 @@ fn visit_expr(rcx: &mut Rcx, expr: &ast::Expr) {
             }
 
             rcx.visit_expr(&**head, ());
+            type_of_node_must_outlive(rcx,
+                                      infer::AddrOf(expr.span),
+                                      head.id,
+                                      ty::ReScope(expr.id));
 
             let repeating_scope = rcx.set_repeating_scope(body.id);
             rcx.visit_block(&**body, ());

src/test/compile-fail/borrowck-for-loop-head-linkage.rs

@@ -0,0 +1,19 @@
+// 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.
+
+fn main() {
+    let mut vector = vec![1u, 2];
+    for &x in vector.iter() {
+        let cap = vector.capacity();
+        vector.grow(cap, &0u);      //~ ERROR cannot borrow
+        *vector.get_mut(1u) = 5u;   //~ ERROR cannot borrow
+    }
+}
+