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, 3 insertions, 3 deletions

diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 6760d7a3932..cd953607ea2 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -215,7 +215,7 @@ pub fn walk_explicit_self<E: Clone, V: Visitor<E>>(visitor: &mut V,
         SelfRegion(ref lifetime, _, _) => {
             visitor.visit_opt_lifetime_ref(explicit_self.span, lifetime, env)
         }
-        SelfExplicit(ref typ, _) => visitor.visit_ty(*typ, env.clone()),
+        SelfExplicit(ref typ, _) => visitor.visit_ty(&**typ, env.clone()),
     }
 }
 
@@ -565,8 +565,8 @@ pub fn walk_method_helper<E: Clone, V: Visitor<E>>(visitor: &mut V,
         MethDecl(ident, ref generics, _, _, decl, body, _) => {
             visitor.visit_ident(method.span, ident, env.clone());
             visitor.visit_fn(&FkMethod(ident, generics, method),
-                             decl,
-                             body,
+                             &*decl,
+                             &*body,
                              method.span,
                              method.id,
                              env.clone());