Creating a Function-Like Macro in Rust

Macros in Rust offer a powerful way to generate code at compile time. This challenge asks you to implement a function-like macro that takes a variable number of arguments and generates a specific code snippet based on those arguments. This is useful for reducing boilerplate code and creating more expressive APIs.

Problem Description

You need to create a macro named generate_greeting that takes a variable number of string arguments and generates a formatted greeting string. The macro should accept any number of string literals as arguments. It should then construct a string that starts with "Greetings, " followed by the concatenated arguments, and ends with an exclamation mark.

Key Requirements:

• The macro must be function-like, meaning it can be called like a function with arguments enclosed in parentheses.
• It must handle a variable number of string arguments.
• The generated string should be correctly formatted as described above.
• The macro should expand to valid Rust code.

Expected Behavior:

When called with multiple string arguments, the macro should concatenate them into a single string with the specified prefix and suffix. When called with no arguments, it should produce a default greeting.

Edge Cases to Consider:

• No arguments provided.
• A single argument provided.
• Multiple arguments provided.
• Arguments containing special characters (e.g., spaces, quotes). The macro should handle these correctly without causing compilation errors.

Examples

Example 1:

Input: generate_greeting!("Alice", "Bob", "Charlie")
Output: "Greetings, AliceBobCharlie!"
Explanation: The macro concatenates "Alice", "Bob", and "Charlie" into "AliceBobCharlie" and prepends "Greetings, " and appends "!".

Example 2:

Input: generate_greeting!("World")
Output: "Greetings, World!"
Explanation: The macro concatenates "World" and prepends "Greetings, " and appends "!".

Example 3:

Input: generate_greeting!()
Output: "Greetings, !"
Explanation: When no arguments are provided, the macro should default to a greeting with just the exclamation mark.

Constraints

• The macro must be implemented using Rust's macro system.
• The generated string must be a valid &str (string slice).
• The macro should compile without warnings.
• The macro should be efficient in terms of compile time (avoiding unnecessary complexity).

Notes

• Consider using the format! macro or string concatenation within the macro expansion to build the final greeting string.
• The stringify! macro might be useful, but is not strictly required for this problem.
• Think about how to handle the case where no arguments are provided. A default value or an empty string might be appropriate.
• Rust's macro system can be tricky. Start with a simple case (e.g., a single argument) and gradually add complexity. Pay close attention to the syntax and how the macro expands.