Macro op 

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

preprocessor::op!(...) — Expression-level macro for compile-time evaluation.

Parses the input expression, evaluates it at compile time, and replaces it with the literal result.

Important: If the expression cannot be fully evaluated at compile time (e.g., contains free variables, unknown functions, etc.), the macro will generate a compile-time error instead of passing through unchanged.

Example

let result = preprocessor::op!(1 + 2 * 3);  // → let result = 7;
let x = preprocessor::op!(a + 1);           // → COMPILE ERROR: cannot evaluate at compile time