E0017

The usage of identifier is ambiguous. If it refers to the loop variable, please use as <id> to bind it in the pattern. If it refers to the original value of the variable before entering the loop, please bind it to a new binder outside the loop.

错误示例

fn main {
  let a : @string.View = "asdf"
  loop a {
    [_, .. d] => {
      println(a)
      //      ^
      // Warning: The usage of 'a' is ambiguous. If it refers to the loop
      // variable, please use `as a` to bind it in the pattern. If it refers to
      // the original value of 'a' before entering the loop, please bind it to a
      // new binder outside the loop.
      continue d
    }
    [] => ()
  }
}

The output is:

asdf
asdf
asdf
asdf

Because a refers to the value of a before entering the loop, therefore the value is always the same.

建议

It is often the case that you may want to refer to the loop variable that changes with loop iterations. If so, use as <id> to bind it in the pattern.

fn main {
  let a : @string.View = "asdf"
  loop a {
    [_, .. d] as a => {
      println(a)
      continue d
    }
    [] => ()
  }
}

Output:

asdf
sdf
df
f

Or, if you want to refer to the original value of the variable before entering the loop, explicitly bind it to another name outside the loop.

fn main {
  let a : @string.View = "asdf"
  let b = a
  loop a {
    [_, .. d] => {
      println(b)
      continue d
    }
    [] => ()
  }
}

Output:

asdf
asdf
asdf
asdf