rule candidate: spanmat — always materialize a (src,n) span before a read

[noah-emp]

Category

suspicious

Shape

A (src, n) span is always turned into an owned String (span.str / take/drop) before a consumer that only needs to read those n characters (equality, quoting when plain, hashing). Prefer span-aware helpers that walk src for n steps.

Bad

def span.str(src: String, +n: U32, z: Bool) -> String:
  match src z:
    case s True{}:
      ""
    case SCon{h, t} False{}:
      SCon{h, span.str(t, U32.sub(n, 1), U32.is_zero(U32.sub(n, 1)))}
    case _ _:
      ""

def quote_span(src: String, +n: U32) -> String:
  key.basic(span.str(src, n, U32.is_zero(n)))

Good

def quote_span(src: String, +n: U32) -> String:
  # walk n chars; if all plain, cons quotes; else fall back to escaping
  span.quote(src, n)