E4090

Tuples are not mutable. You cannot change the value of a field in a tuple using assignment.

错误示例

fn main {
  let a = (1, 2, 3)
  a.2 = 4 // Error: tuples are not mutable
  println(a.2)
}

建议

If you need to change the value of a field in a tuple, you should use a struct instead.

pub struct MyStruct {
  a: Int
  b: Int
  mut c: Int
}

fn main {
  let a : MyStruct = { a : 1, b : 2, c : 3 }
  a.c = 4
  println(a.c)
}