Credence.Rule.NoLengthInGuard (credence v0.3.2)

Performance rule: Detects the use of length/1 inside guard clauses (when) in cases that cannot be automatically rewritten as pattern matches.

length/1 traverses the entire list to compute its size, making it O(n). When placed in a guard, this cost is paid on every function call attempt, and the list is almost always traversed again inside the function body.

Note: simple non-empty checks (length(list) > 0) and exact-size checks (length(list) == N for N in 1..5) are handled by the LengthGuardToPattern rule, which can auto-fix them into pattern matches.

Bad

def kth_largest(nums, k) when k <= length(nums) do
  Enum.sort(nums, :desc) |> Enum.at(k - 1)
end

Good

def kth_largest(nums, k) do
  if k > length(nums), do: raise(ArgumentError, "k out of bounds")
  Enum.sort(nums, :desc) |> Enum.at(k - 1)
end

For bounds checks, moving the validation into the function body avoids redundant traversals when the guard fails and another clause is tried.