Macro batch_preview 

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

The DSL-aware expansion preview: wrap the exact attribute-macro form you would feed #[batch_impl] and the preview reports every generated impl through a compile_error! message (the only stable terminal channel a proc macro has). The message is the expansion verbatim — the trait plus the impls, exactly what #[batch_impl] emits — followed by preview-only guidance.

The preview exists because the DSL is a type-matrix description: the generated impls are token-equivalent to hand-written code, and seeing them is the fastest way to check a matrix. Copy the #[batch_impl(...)] line back unchanged once you are happy with the output; DSL errors surface exactly as they would under the real attribute macro.

Examples

batch_preview! {
    #[batch_impl(<T> Sortable<T> [Box, Rc]^Vec<T> where T: Ord {
        fn is_sorted(&self) -> bool { self.windows(2).all(|w| w[0] <= w[1]) }
    })]
    trait Sortable<T> { fn is_sorted(&self) -> bool; }
}

The preview’s diagnostic channel makes this block fail to compile by design (the message IS the output); it reports one message containing the trait and both generated impls. It also teaches the ^/- associativity identity: a known 1-arity container rendered with 2+ args (Box<Vec, u32>) is the shape of Box^Vec-u32 (= Box-Vec-u32, since A^B-C = A-B-C = A<B, C>), and the note suggests the nesting rewrite (Box^Vec^u32). This guidance is preview-only: the compiler path never guesses, and a user type shadowing a known container name costs a wrong note, never a wrong build.