immutable_mutable_scope_cell.rs

#[derive(Clone, Copy)]
pub struct MyStruct<'a> {
  name: &'a str,
}

impl<'a> MyStruct<'a> {
  fn new() -> Self {
    MyStruct { name: "Before" }
  }

  fn read(&self) {
    print!("Value is {}\n", self.name);
  }

  fn write(&mut self) {
    self.name = "After";
    print!("Value is now {}\n", self.name);
  }
}

fn error_fn() {
  let mut ms = MyStruct::new();
  let reader = &ms;
  reader.read();
  // let mut _writer = &mut ms; // Error: cannot borrow `ms` as mutable because it is also borrowed as immutable
}

fn good_fn() {
  let mut ms = MyStruct::new();
  {
    let reader = &ms;
    reader.read();
  }
  {
    let mut _writer = &mut ms;
    _writer.write();
  }
  {
    let reader = &ms;
    reader.read();
  }
}

fn cell_fn() {
  let mut c = std::cell::Cell::new(MyStruct::new());
  c.get().read();
  c.get_mut().write();
  c.get().read();
  c.get_mut().write();
}


fn main() {
  error_fn();
  cell_fn();
}