error[E0596]: cannot borrow `*self.s` as mutable, as it is behind a `&` reference --> $DIR/issue-38147-1.rs:17:9 | LL | fn f(&self) { | ----- help: consider changing this to be a mutable reference: `&mut self` LL | self.s.push('x'); | ^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable error: aborting due to previous error For more information about this error, try `rustc --explain E0596`.