Add AutoBorrowMutability; its like hir::Mutability but w/ two-phase borrow info too.

Namely, the mutable borrows also carries a flag indicating whether
they should support two-phase borrows.

This allows us to thread down, from the point of the borrow's
introduction, whether the particular adjustment that created it is one
that yields two-phase mutable borrows.

src/librustc/ich/impls_ty.rs

@@ -163,6 +163,20 @@ impl_stable_hash_for!(struct ty::adjustment::Adjustment<'tcx> { kind, target });
 impl_stable_hash_for!(struct ty::adjustment::OverloadedDeref<'tcx> { region, mutbl });
 impl_stable_hash_for!(struct ty::UpvarBorrow<'tcx> { kind, region });
 
+impl<'gcx> HashStable<StableHashingContext<'gcx>> for ty::adjustment::AutoBorrowMutability {
+    fn hash_stable<W: StableHasherResult>(&self,
+                                          hcx: &mut StableHashingContext<'gcx>,
+                                          hasher: &mut StableHasher<W>) {
+        mem::discriminant(self).hash_stable(hcx, hasher);
+        match *self {
+            ty::adjustment::AutoBorrowMutability::Mutable { ref allow_two_phase_borrow } => {
+                allow_two_phase_borrow.hash_stable(hcx, hasher);
+            }
+            ty::adjustment::AutoBorrowMutability::Immutable => {}
+        }
+    }
+}
+
 impl_stable_hash_for!(struct ty::UpvarId { var_id, closure_expr_id });
 
 impl_stable_hash_for!(enum ty::BorrowKind {

src/librustc/middle/expr_use_visitor.rs

@@ -760,7 +760,7 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                                      expr.span,
                                      cmt_base,
                                      r,
-                                     ty::BorrowKind::from_mutbl(m),
+                                     ty::BorrowKind::from_mutbl(m.into()),
                                      AutoRef);
             }
 

src/librustc/ty/adjustment.rs

@@ -119,10 +119,25 @@ impl<'a, 'gcx, 'tcx> OverloadedDeref<'tcx> {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable)]
+pub enum AutoBorrowMutability {
+    Mutable { allow_two_phase_borrow: bool },
+    Immutable,
+}
+
+impl From<AutoBorrowMutability> for hir::Mutability {
+    fn from(m: AutoBorrowMutability) -> Self {
+        match m {
+            AutoBorrowMutability::Mutable { .. } => hir::MutMutable,
+            AutoBorrowMutability::Immutable => hir::MutImmutable,
+        }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable)]
 pub enum AutoBorrow<'tcx> {
     /// Convert from T to &T.
-    Ref(ty::Region<'tcx>, hir::Mutability),
+    Ref(ty::Region<'tcx>, AutoBorrowMutability),
 
     /// Convert from T to *T.
     RawPtr(hir::Mutability),