Credence.Rule.NoStringConcatInLoop (credence v0.2.0)

Performance rule: Detects string concatenation with <> inside looping constructs (Enum.reduce, Enum.reduce_while, for comprehensions) or recursive functions.

Each <> concatenation copies the entire accumulated binary, making character-by-character string building O(n²). This is the string equivalent of list ++ [element].

Bad

Enum.reduce(graphemes, "", fn char, acc ->
  acc <> char
end)

Enum.reduce_while(chars, "", fn char, prefix ->
  candidate = prefix <> char
  ...
end)

Good

graphemes
|> Enum.reduce([], fn char, acc -> [char | acc] end)
|> Enum.reverse()
|> IO.iodata_to_binary()

# Or simply:
Enum.join(graphemes)