Credence.Rule.NoEnumCountForLength (credence v0.3.0)

Detects Enum.count/1 (without a predicate) and suggests length/1 or a more specific size function.

Why this matters

Enum.count/1 goes through the Enumerable protocol, adding dispatch overhead. When the argument is a list — which it almost always is in LLM-generated code — length/1 is a BIF that does the same traversal without protocol dispatch:

# Flagged — protocol dispatch overhead
total = Enum.count(chars)

# Idiomatic — BIF, no protocol overhead
total = length(chars)

For other collection types, more specific functions exist: map_size/1 for maps, MapSet.size/1 for sets, tuple_size/1 for tuples.

Flagged patterns

Only the single-argument form Enum.count(x) is flagged. The two-argument form Enum.count(x, predicate) is not flagged because it filters and counts in one pass — there is no simpler replacement.