Credence.Pattern.NoParamRebinding (credence v0.4.0)

Style & correctness rule: Detects rebinding of parameter names inside anonymous function (fn) bodies. When a variable from the parameter destructure is rebound inside the body, readers lose track of which binding is "live" at each point. This is a common source of subtle bugs, especially in Enum.reduce callbacks where the accumulator is destructured.

Bad

Enum.reduce(arr, {0, :queue.new()}, fn x, {count, q} ->
  q = :queue.in(x, q)       # rebinds `q` from the parameter
  count = count + 1          # rebinds `count` from the parameter
  {count, q}
end)

Good

Enum.reduce(arr, {0, :queue.new()}, fn x, {count, q} ->
  new_q = :queue.in(x, q)
  new_count = count + 1
  {new_count, new_q}
end)