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]

src/libsyntax/ext/deriving/generic/mod.rs

@@ -954,7 +954,9 @@ impl<'a> MethodDef<'a> {
                 // expressions for referencing every field of every
                 // Self arg, assuming all are instances of VariantK.
                 // Build up code associated with such a case.
-                let substructure = EnumMatching(index, variant, field_tuples);
+                let substructure = EnumMatching(index,
+                                                &*variant,
+                                                field_tuples);
                 let arm_expr = self.call_substructure_method(
                     cx, trait_, type_ident, self_args, nonself_args,
                     &substructure);