Behaviour for pattern-level rules that detect and auto-fix anti-patterns.

Every Pattern rule fixes the issue it detects — there is no "warn-only" mode. Rules that could only detect but not fix were archived to docs/unfixable_rules/ and removed from compilation.

Interface

Three callbacks, all mandatory (only priority/0 has a default):

• priority() :: integer() — fire order, lower runs first. Default 500.
• check(ast, opts) :: [Issue.t()] — detect issues in the AST.
• fix_patches(ast, opts) :: [patch] — emit byte-range patches that, when applied, resolve the issues check/2 reported. Empty list = no change.

Both callbacks receive Sourceror AST (Sourceror.parse_string!/1) and opts containing :source for rules that need raw source bytes.

Implementation choices for fix_patches/2

All rules return [patch], but the way they compute those patches varies with what the transformation needs:

• AST-walking — Credence.RuleHelpers.patches_from_postwalk/2 handles rules whose fix is a single Macro.postwalk/2 matcher.
• AST-with-restructuring — Credence.RuleHelpers.patches_from_ast_transform/3 for fixes that prune, reorder, or insert siblings (the rendered result is re-parsed for clean range diffing).
• Direct patch emission — a rule walks the AST itself and builds [%{range: Sourceror.Range, change: String.t()}] manually. Useful when the kept subtree's source bytes must be preserved verbatim (e.g. parens metadata Sourceror's renderer would drop) — slice the original source bytes for the kept range instead of re-rendering.

See each helper's docstring for the rule-side calling convention.