E4211

Compiler diagnostic name: control_in_list_comprehension.

return, break, continue, and error-producing calls are not allowed inside a list comprehension whose result type is Iter.

An Iter comprehension is lazy: its body runs later, when the iterator is consumed. Control-flow operations that jump out of the surrounding function or loop, or calls that need immediate error propagation, cannot be preserved safely across that lazy boundary.

Erroneous example

fn values() -> Iter[Int] {
  let xs : Iter[Int] = [ for _ in 0..<3 => { return Iter::empty() } ]
  xs
}

Suggestion

Keep the comprehension body as a normal value-producing expression. If you need early exit, use an eager collection target or write an explicit loop around the iterator consumption.

fn values() -> Iter[Int] {
  [ for i in 0..<3 => i ]
}

test {
  let mut sum = 0
  values().each(x => sum = sum + x)
  inspect(sum, content="3")
}