Extend closure special-casing for generators.

This commit extends existing special-casing of closures to highlight the
use of variables within generators that are causing the generator to
borrow them.

Author: davidtwco

src/librustc_borrowck/borrowck/check_loans.rs

@@ -609,12 +609,12 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                         new_loan.span, &nl, old_loan.span, previous_end_span, Origin::Ast),
                 (ty::UniqueImmBorrow, _) =>
                     self.bccx.cannot_uniquely_borrow_by_one_closure(
-                        new_loan.span, &nl, &new_loan_msg,
+                        new_loan.span, "closure", &nl, &new_loan_msg,
                         old_loan.span, &ol_pronoun, &old_loan_msg, previous_end_span, Origin::Ast),
                 (_, ty::UniqueImmBorrow) => {
                     let new_loan_str = &new_loan.kind.to_user_str();
                     self.bccx.cannot_reborrow_already_uniquely_borrowed(
-                        new_loan.span, &nl, &new_loan_msg, new_loan_str,
+                        new_loan.span, "closure", &nl, &new_loan_msg, new_loan_str,
                         old_loan.span, &old_loan_msg, previous_end_span, Origin::Ast)
                 }
                 (..) =>