Macro grammar 

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

Generates a zero-cost grammar with compile-time dispatch.

Produces a grammar struct, a symbol enum, and a GrammarDef implementation with all dispatch resolved via match arms (no allocations, no HashMap lookups).

Syntax

grammar! {
    grammar MyGrammar;
    symbol MySymbol;
    start Expr;

    Expr => [Expr, Expr, BinOp] | [Val];
    BinOp => [Add] | [Sub] | [Mul];
    Val => [X] | [One];
}

• grammar <name> — name of the generated struct implementing GrammarDef
• symbol <name> — name of the generated enum with all grammar symbols
• start <rule> — the start rule
• Rules: <name> => [symbols...] | [symbols...];

Symbols appearing on the left side of => are non-terminals. All other symbols are terminals.