Credence.Rule.NoListAppendInLoop (credence v0.2.0)

Performance rule: Detects the use of ++ inside looping constructs (Enum.reduce, for comprehensions) and inside recursive functions.

Appending to a list with ++ is O(n) because it must copy the entire left-hand list. Inside a loop or recursion this compounds to O(n²). Prefer prepending with [item | acc] and calling Enum.reverse/1 after the loop completes.

Bad — inside Enum.reduce

Enum.reduce(list, [], fn item, acc ->
  acc ++ [item * 2]
end)

Bad — inside a recursive function

defp slide([next | rest], window, current, max) do
  new_window = window ++ [next]
  slide(rest, new_window, current, max)
end

Good

Enum.reduce(list, [], fn item, acc ->
  [item * 2 | acc]
end)
|> Enum.reverse()