error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference --> $DIR/mut-arg-hint.rs:3:9 | LL | a.push_str("bar"); | ^^^^^^^^^^^^^^^^^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable | help: consider changing this to be a mutable reference | LL | fn foo(mut a: &mut String) { | +++ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference --> $DIR/mut-arg-hint.rs:8:5 | LL | a.push_str("foo"); | ^^^^^^^^^^^^^^^^^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable | help: consider changing this to be a mutable reference | LL | pub fn foo<'a>(mut a: &'a mut String) { | +++ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference --> $DIR/mut-arg-hint.rs:15:9 | LL | a.push_str("foo"); | ^^^^^^^^^^^^^^^^^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable | help: consider changing this to be a mutable reference | LL | pub fn foo(mut a: &mut String) { | +++ error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0596`.