Lint for unused borrows as part of UNUSED_MUST_USE

compiler/rustc_lint/src/unused.rs

@@ -154,6 +154,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 | hir::BinOpKind::Shl
                 | hir::BinOpKind::Shr => Some("bitwise operation"),
             },
+            hir::ExprKind::AddrOf(..) => Some("borrow"),
             hir::ExprKind::Unary(..) => Some("unary operation"),
             _ => None,
         };