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, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index bdfd928cfbc..a77f24f98f8 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -502,7 +502,9 @@ impl<'a> Parser<'a> {
                        inedible: &[token::Token]) {
         debug!("commit_stmt {:?}", s);
         let _s = s; // unused, but future checks might want to inspect `s`.
-        if self.last_token.as_ref().map_or(false, |t| is_ident_or_path(*t)) {
+        if self.last_token
+               .as_ref()
+               .map_or(false, |t| is_ident_or_path(&**t)) {
             let expected = edible.iter().map(|x| (*x).clone()).collect::<Vec<_>>()
                            .append(inedible.as_slice());
             self.check_for_erroneous_unit_struct_expecting(