Macro batch_impl_open 

batch_impl_open!() { /* proc-macro */ }

Documentation placeholder for the open-extension protocol.

A #name(args){body} whose name is not a built-in directive expands to a call of a user-defined function-like macro of the same name, handed the args, body and trait definition: #my_ext(x){y} → { my_ext!{ (x) {y} trait_def } }.

The protocol is top-level only: codegen prepends the spec body, making the four-segment input {spec}(args){body} trait, and your macro emits arbitrary items (typically its own impl). The legacy in-impl form T {m!{...}} (no !, the call lands in the impl body as associated items) is deprecated since 0.7.2 and kept only for compatibility — write new extensions against the top-level protocol.

macro_rules! my_ext { ($($rest:tt)*) => {}; }
#[batch_impl(Box<u32> #my_ext(x){y})]
trait O {}

Documentation marker only — never call this function.