E3019

Inclusive range pattern a..=b cannot have _ as upper bound.

Writing a..=_ does not make sense, because _ is a wildcard that matches any value. It is unclear what does "equals to any value" mean. If you want to express the meaning of "greater than or equal to a", you can use a..<_ instead.

Erroneous example

fn main {
  match 0 {
    0..=_ => println("Big than or equal to 0")
    //  ^
    // Error: Inclusive range pattern `a..=b` cannot have `_` as upper bound
    _ => println("Less than 0")
  }
}

Suggestion

Replace = with <:

fn main {
  match 0 {
    0..<_ => println("Big than or equal to 0")
    _ => println("Less than 0")
  }
}