E4121

The attribute (!, ?, !! mark) cannot be used on this application.

This error occurs when:

• The attribute (!, ?, !! mark) is used on a constructor.

• The attribute (!, ?, !! mark) is used on a function that does fit.

  • ! and ? must be annotated on a function that may raise errors.

  • !! must be annotated on an async function (this will be reported as E4150 though).

错误示例

enum Err {
  IntErr(Int)
}

fn square(x: Int) -> Int {
  x * x
}

fn main {
  IntErr!(1) |> ignore() // Error: The attribute `!` cannot be used on constructors.
  IntErr?(2) |> ignore() // Error: The attribute `?` cannot be used on constructors.
  IntErr!!(3) |> ignore() // Error: The attribute `!!` cannot be used on constructors.
  square!(1) |> ignore() // Error: The attribute `!` cannot be used on application that does not raise errors
  square?(2) |> ignore() // Error: The attribute `?` cannot be used on application that does not raise errors
}

建议

Remove the attribute (!, ?, !! mark) from the application.

fn main {
  IntErr(1) |> ignore()
  IntErr(2) |> ignore()
  IntErr(3) |> ignore()
  square(1) |> ignore()
  square(2) |> ignore()
}