Credence.Pattern.NoCaseDestructureInPipe (credence v0.7.0)

Readability rule: Detects a single-clause case used inside a pipeline whose clause head is an irrefutable variable pattern (a bare variable or _, with no guard). That shape is exactly equivalent to then/1 and is non-idiomatic.

LLMs often use |> case do pattern -> body end as a workaround when |> (fn ... end).() is caught by no_anon_fn_application_in_pipe. Both are non-idiomatic; then/1 is the idiomatic alternative.

Bad

result
|> compute()
|> case do
  value -> value + 1
end

Good

result
|> compute()
|> then(fn value -> value + 1 end)

Why only irrefutable patterns

A refutable pattern (e.g. {x, acc}, {:ok, v}, a pin, or any clause with a guard) is not safe to rewrite. When the piped value does not match, a single-clause case raises CaseClauseError while then(fn pattern -> ... end) raises FunctionClauseError. Those are different exceptions, so a caller that rescues CaseClauseError would behave differently after the rewrite. An irrefutable variable pattern always matches, so there is no divergent error path and the rewrite is answer-preserving for every input.