Macro method_validator 

macro_rules! method_validator {
    ($(#[$meta:meta])* $fn_name:ident, $method:ident, $code:literal, $message:literal) => { ... };
    ($(#[$meta:meta])* $fn_name:ident, $method:ident, $code:literal) => { ... };
    ($(#[$meta:meta])* $fn_name:ident, $code:literal, $message:literal) => { ... };
    ($(#[$meta:meta])* $fn_name:ident, $code:literal) => { ... };
}

Generate a validator function that delegates to a method on the input value

Creates a public function fn(value: &str) -> Result<(), ValidationError> that calls the given method on the input and validates the boolean result.

Syntax

// Full form: separate function name, method name, error code, and message
method_validator!(
    /// Doc comment
    function_name,
    method_name,
    "error_code",
    "Error message"
);

// Without message: generates "Provide a valid {code}"
method_validator!(
    /// Doc comment
    function_name,
    method_name,
    "error_code"
);

// Shorthand with message: uses function name as method name
method_validator!(
    /// Doc comment
    function_name,
    "error_code",
    "Error message"
);

// Minimal: uses function name as method name, generates default message
method_validator!(
    /// Doc comment
    function_name,
    "error_code"
);