Declarative Macro Factory: Building Custom Code Generators

Declarative macros in Rust offer a powerful way to generate code based on input tokens. This challenge focuses on creating a macro factory – a macro that generates other macros. You'll design a system that allows users to define a set of parameters and a template, and the factory macro will produce a new macro tailored to those specifications. This is useful for creating repetitive code patterns with slight variations, reducing boilerplate and improving code maintainability.

Problem Description

You are tasked with creating a macro named macro_factory! that takes a macro name, a set of parameters (name, type), and a code template as input. The macro_factory! macro will then generate a new macro with the specified name, accepting the defined parameters, and expanding to the provided code template, substituting the parameters into the template. The generated macro should behave as a standard Rust macro.

Key Requirements:

• Macro Name: The user must provide a string literal representing the name of the macro to be generated.
• Parameters: The user must define a list of parameters, each consisting of a name (string literal) and a type (token tree representing a type).
• Code Template: The user must provide a code template as a token stream. This template will contain $parameter_name placeholders for each defined parameter.
• Generated Macro: The macro_factory! macro must generate a new macro with the specified name and parameters, expanding to the provided template with parameter substitution.
• Error Handling: The macro factory should provide helpful error messages if the input is invalid (e.g., missing parameters, invalid types, incorrect placeholder syntax).

Expected Behavior:

When invoked, macro_factory! should parse the input, validate it, and generate a new macro definition. This new macro should then be usable in the same compilation unit as the macro_factory! macro.

Edge Cases to Consider:

• Empty parameter lists.
• Parameter names that are Rust keywords.
• Code templates with invalid syntax.
• Duplicate parameter names.
• Incorrect placeholder syntax in the template (e.g., $param instead of $parameter_name).
• Type mismatches between declared parameter types and usage in the template.

Examples

Example 1:

macro_rules! macro_factory {
    ($macro_name:ident, $( $param_name:ident : $param_type:ty ),*, $template:tt *) => {
        #[macro_export]
        macro_rules! $macro_name {
            $( $param_name: $param_type, )* => {
                $template
            }
        }
    };
}

macro_factory!(MyMacro, a: i32, b: f64, "println!(\"a = {}, b = {}\", a, b);");

fn main() {
    MyMacro!(10, 3.14); // Prints "a = 10, b = 3.14"
}

Example 2:

macro_rules! macro_factory {
    ($macro_name:ident, $( $param_name:ident : $param_type:ty ),*, $template:tt *) => {
        #[macro_export]
        macro_rules! $macro_name {
            $( $param_name: $param_type, )* => {
                $template
            }
        }
    };
}

macro_factory!(PrintValue, value: i32, "println!(\"Value: {}\", value);");

fn main() {
    PrintValue!(42); // Prints "Value: 42"
}

Example 3: (Edge Case - Empty Parameter List)

macro_rules! macro_factory {
    ($macro_name:ident, $( $param_name:ident : $param_type:ty ),*, $template:tt *) => {
        #[macro_export]
        macro_rules! $macro_name {
            $( $param_name: $param_type, )* => {
                $template
            }
        }
    };
}

macro_factory!(NoParams, , "println!(\"Hello, world!\");");

fn main() {
    NoParams!(); // Prints "Hello, world!"
}

Constraints

• The generated macro must be a macro_rules! macro.
• The generated macro must accept a variable number of parameters based on the input to macro_factory!.
• The code template must be a valid Rust code snippet.
• The macro factory must handle errors gracefully and provide informative error messages.
• The generated macro must be usable within the same compilation unit.
• The input macro name must be a valid Rust identifier.

Notes

• This is a challenging problem that requires a good understanding of Rust's macro system, particularly macro_rules!.
• Consider using syn and quote crates for parsing and generating Rust code, respectively. While not strictly required, they can significantly simplify the process.
• Focus on creating a robust and error-resistant macro factory. Think about how to handle various edge cases and provide helpful error messages to the user.
• The $( ... )* syntax is crucial for handling a variable number of parameters.
• The $template token stream represents the code that will be generated for the new macro. It's important to ensure that the template is valid Rust code and that the parameter placeholders are correctly substituted.
• The #[macro_export] attribute is necessary to make the generated macro visible outside the current module.