error[E0502]: cannot borrow `foo` as mutable because it is also borrowed as immutable --> $DIR/borrowck-issue-49631.rs:20:9 | LL | while let Some(Ok(string)) = foo.get() { | --- immutable borrow occurs here LL | foo.mutate(); | ^^^^^^^^^^^^ mutable borrow occurs here LL | LL | println!("foo={:?}", *string); | ------- immutable borrow later used here error: aborting due to previous error For more information about this error, try `rustc --explain E0502`.