librustc: Remove cross-borrowing of `Box<T>` to `&T` from the language,

except where trait objects are involved.

Part of issue #15349, though I'm leaving it open for trait objects.
Cross borrowing for trait objects remains because it is needed until we
have DST.

This will break code like:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(a);

Change this code to:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(&*a);

[breaking-change]

1 files changed, 1 insertions, 1 deletions

diff --git a/src/doc/guide-pointers.md b/src/doc/guide-pointers.md
index 08652822097..17a1114be55 100644
--- a/src/doc/guide-pointers.md
+++ b/src/doc/guide-pointers.md
@@ -279,7 +279,7 @@ fn main() {
     let origin =    &Point { x: 0.0, y: 0.0 };
     let p1     = box Point { x: 5.0, y: 3.0 };
 
-    println!("{}", compute_distance(origin, p1));
+    println!("{}", compute_distance(origin, &*p1));
 }
 ~~~