Inline Expansion with Macro Rules

This challenge asks you to implement a simplified version of inline expansion using Rust's macro rules. The goal is to define a macro that takes a sequence of expressions and expands them into a comma-separated list within a specified context (e.g., a function argument list or a tuple initialization). This is useful for reducing boilerplate and improving code readability when dealing with repetitive expressions.

Problem Description

You need to create a macro named expand that takes a sequence of expressions as input and expands them into a comma-separated list enclosed in parentheses. The macro should accept a variable number of expressions. The expanded output should be a valid Rust expression suitable for use in contexts like function arguments or tuple initialization.

Key Requirements:

• Variable Arguments: The macro must accept a variable number of expressions.
• Comma Separation: The expanded expressions must be separated by commas.
• Parentheses: The entire expanded list must be enclosed in parentheses.
• Correct Syntax: The generated code must be valid Rust syntax.
• No Evaluation: The macro should not evaluate the expressions at compile time. It should simply generate the comma-separated list.

Expected Behavior:

When the macro is invoked with a set of expressions, it should produce a string that represents a comma-separated list of those expressions enclosed in parentheses.

Edge Cases to Consider:

• Zero Arguments: What should happen if the macro is called with no arguments? Return an empty tuple ()
• Empty Expressions: Handle cases where an expression might be empty (e.g., ()).
• Complex Expressions: The expressions can be complex, including function calls, variable references, and operators. The macro should simply output them as they are provided.

Examples

Example 1:

Input: expand(1, "hello", 3.14)
Output: (1, "hello", 3.14)
Explanation: The macro takes three expressions (1, "hello", and 3.14) and expands them into a comma-separated list enclosed in parentheses.

Example 2:

Input: expand()
Output: ()
Explanation: The macro is called with no arguments. It should return an empty tuple.

Example 3:

Input: expand(x + y, z * 2, ())
Output: (x + y, z * 2, ())
Explanation: The macro handles more complex expressions and an empty expression `()`.

Constraints

• The macro must be defined using Rust's declarative macro system (macro_rules!).
• The generated code must be valid Rust syntax.
• The macro should not perform any runtime evaluation of the expressions.
• The input expressions can be any valid Rust expressions.
• The macro should handle zero arguments gracefully.

Notes

• Think about how to pattern match on the number of arguments passed to the macro.
• Consider using string concatenation to build the expanded expression.
• The goal is to generate the text of the expression, not to evaluate it. Focus on the syntax of the macro rule.
• This is a simplified version of inline expansion. Real-world macro systems are much more powerful and complex. This exercise focuses on the core concept of generating code based on input.