Implement a Custom format! Macro in Rust

Rust's built-in format! macro is a powerful tool for constructing strings. This challenge asks you to implement a simplified version of this macro, allowing you to understand the underlying mechanisms of macro programming and string formatting in Rust. You'll learn how to process macro arguments and generate code dynamically.

Problem Description

Your task is to create a procedural macro in Rust named my_format. This macro should accept a format string literal and a variable number of arguments, similar to the standard format! macro. The macro should then generate a String by interpolating the provided arguments into the format string.

Key Requirements:

1. Accept a format string literal: The first argument to my_format! must be a string literal.
2. Accept variable arguments: The macro should support an arbitrary number of arguments after the format string.
3. Argument interpolation: Placeholder in the format string (e.g., {}) should be replaced by the corresponding argument's string representation.
4. No complex formatting specifiers: For this challenge, you only need to handle the simplest form of interpolation: {}. You do not need to implement width, precision, alignment, or other formatting specifiers.
5. Generate a String: The macro should expand to code that returns a std::string::String.
6. Error handling (basic): If the number of placeholders does not match the number of arguments, the macro should ideally produce a compilation error or, for simplicity in this challenge, it's acceptable if it panics at runtime (though a compile-time error is preferred).

Expected Behavior:

The my_format! macro will behave like a subset of std::format!:

• my_format!("Hello, {}!", "World") should produce "Hello, World!".
• my_format!("The answer is: {}", 42) should produce "The answer is: 42".
• my_format!("{} {} {}", 1, "two", 3.0) should produce "1 two 3".

Edge Cases to Consider:

• No arguments: my_format!("Just a string") should return "Just a string".
• Mismatch in placeholders and arguments: Consider how to handle cases where the count of {} in the format string doesn't equal the number of provided arguments.

Examples

Example 1:

let name = "Alice";
let greeting = my_format!("Hello, {}!", name);
// greeting will be "Hello, Alice!"

Explanation: The placeholder {} is replaced by the value of the name variable.

Example 2:

let num = 123;
let text = "apples";
let message = my_format!("I have {} {}", num, text);
// message will be "I have 123 apples"

Explanation: Two placeholders are replaced by two separate arguments.

Example 3:

let message = my_format!("This is a literal string.");
// message will be "This is a literal string."

Explanation: No placeholders, so the format string is returned as is.

Constraints

• The format string must be a string literal (e.g., "..."). You do not need to handle dynamic format strings passed as variables.
• The arguments provided must implement the std::fmt::Display trait, as this is how they will be converted to strings.
• The solution must be implemented using Rust's procedural macros.
• The generated code should be efficient and avoid unnecessary string allocations if possible.

Notes

This challenge requires creating a procedural macro, specifically a "declarative macro that generates code" or a "procedural macro crate." You will need to define a #[proc_macro] or #[proc_macro_derive] function.

A good starting point for parsing macro arguments and generating code is the syn and quote crates. syn helps parse Rust code into abstract syntax trees (AST), and quote helps generate Rust code from ASTs.

Think about how you can iterate through the format string and the arguments simultaneously. You might need to split the format string by the {} placeholders.

Consider the case where the format string might contain characters that are special to Rust strings, like backslashes or quotes, and how quote handles these.