Building a Custom println! with Macros

Macros in Rust are powerful tools for code generation and abstraction. This challenge will guide you through creating a custom macro that mimics some of the functionality of println!, allowing you to format and print strings with variable arguments. This exercise is fundamental to understanding Rust's metaprogramming capabilities.

Problem Description

Your task is to implement a Rust macro named my_println that behaves similarly to the standard println! macro. This macro should accept a format string as its first argument and then a variable number of subsequent arguments that will be substituted into the format string.

Key requirements:

• The macro should be named my_println.
• It must accept at least one argument: a string literal representing the format string.
• It should support placeholders within the format string, represented by {}. Each {} should be replaced by the corresponding argument passed to the macro.
• The macro should print the resulting formatted string to standard output, followed by a newline character.
• The macro should handle a variable number of arguments.

Consider the following:

• What happens if the number of placeholders doesn't match the number of arguments?
• How will you handle different data types for the arguments?

Examples

Example 1:

fn main() {
    let name = "Alice";
    let age = 30;
    my_println!("Hello, my name is {} and I am {} years old.", name, age);
}

Hello, my name is Alice and I am 30 years old.

Explanation: The macro substitutes "Alice" for the first {} and 30 for the second {} in the format string and prints the result.

Example 2:

fn main() {
    my_println!("This is a simple message.");
}

This is a simple message.

Explanation: When no arguments are provided beyond the format string, the macro simply prints the format string as is.

Example 3:

fn main() {
    let count = 5;
    my_println!("You have {} items.", count);
    my_println!("There are {} red apples and {} green apples.", 2, 3);
}

You have 5 items.
There are 2 red apples and 3 green apples.

Explanation: Demonstrates handling multiple calls to the macro with different numbers of arguments.

Constraints

• The format string must be a string literal.
• The macro should only handle the simple {} placeholder. No complex formatting specifiers like :? or :x are required.
• The provided arguments must implement the Display trait.
• The macro should not incur significant performance overhead compared to direct println! calls.

Notes

Think about how macros work in Rust, specifically declarative macros (macro_rules!). You'll need to define rules for matching different patterns of input. Consider how you can iterate over the arguments and substitute them into the format string. The format! macro might be a useful tool to help construct the final string internally within your macro definition.