The behaviour a Grammar.IR.Custom @native("Module", "function", ...) node dispatches to -- the escape hatch for grammar constructs no static PEG grammar can express: a Prolog-style mutable operator-precedence table, C's typedef-vs-expression ambiguity, and similar "what matches here depends on something read earlier" problems.

match/4 receives:

• stream/pos -- the same token stream and position any compiled rule matcher already works over; advance pos however far was consumed, same contract as everywhere else.
• context -- read-only. Whatever Ichor.Actions.evaluate/5 last produced from a previous top-level form (via run_sequence/4), or whatever run/4's initial_context was outside a sequence. Matching never produces a new context; only evaluating a previous form does.
• rule_matchers -- a map restricted to exactly the dependencies declared in @native(...)'s own argument list (never the whole compiled grammar), each a (stream, pos -> {:ok, new_pos, Ichor.Capture.node_t()} | :fail) function delegating back into the grammar's own compiled rule of that name.

The return shape is the same Ichor.Capture.node_t() every other rule produces, so whatever the callback returns plugs straight into ordinary Ichor.Actions.handle_rule/handle_token dispatch downstream, with no special-casing.

@native("Module", "function", ...) names its own callback function explicitly, so it doesn't have to be called match -- @behaviour Ichor.CustomRule + @impl true is a convenience for a module that happens to name its callback match/4 (the common case), not a requirement; a differently-named function of the same shape works exactly the same way, it just implements this informally.