Muex.Mutator.ReturnValue (Muex v0.8.0)

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Mutator that replaces function return values with type-appropriate zero values.

Targets def and defp function definitions. For multi-statement bodies (__block__), replaces the last expression. For single-expression bodies, replaces the entire body.

The replacement value is inferred from the shape of the original return expression to preserve the caller's type expectations and avoid spurious MatchError / FunctionClauseError crashes that would mask real mutation-testing signal:

  • {:ok, _} → {:error, :mutated}
  • {:error, _} → {:ok, :mutated}
  • :ok → :error
  • :error → :ok
  • true → false
  • false → true
  • integer → 0
  • float → 0.0
  • string → ""
  • list → []
  • map/struct → %{}
  • tuple → {}
  • everything else → nil

Skips functions whose return expression already matches the zero value.

Complements StatementDeletion: that mutator tests whether intermediate statements matter, this one tests whether the return value matters.