Resource Management with RAII in Rust

Rust strongly encourages safe and predictable resource management through its ownership system and the RAII (Resource Acquisition Is Initialization) pattern. This challenge will have you implement RAII for a custom resource, ensuring it's properly acquired and cleaned up even in the face of errors or unexpected program flow.

Problem Description

Your task is to create a Rust struct that manages a simulated external resource. This resource needs to be "acquired" when the struct is created and "released" when the struct goes out of scope. This is a classic use case for RAII.

Key Requirements:

1. Resource Acquisition: When an instance of your struct is created, it should simulate acquiring a resource. This could be represented by printing a message to the console like "Acquiring resource...".
2. Resource Release: When an instance of your struct goes out of scope (i.e., is dropped), it must simulate releasing the resource. This should be represented by printing a message like "Releasing resource...".
3. Error Handling Robustness: The resource release must happen regardless of whether the program exits normally, panics, or returns early from a function.
4. No Manual Cleanup: The user of your struct should not have to explicitly call a release() or cleanup() method. The RAII pattern should handle this automatically.

Expected Behavior:

• When a struct instance is created, an acquisition message is printed.
• When that struct instance goes out of scope, a release message is printed.
• This release behavior should be consistent, even if the scope is exited prematurely.

Edge Cases:

• Consider scenarios where multiple instances of your struct are created and go out of scope in nested or sequential ways. The order of release should be deterministic and correspond to the reverse order of acquisition.
• Think about how panics within the scope where the resource is managed would affect the release mechanism.

Examples

Example 1: Basic Scope

fn main() {
    println!("Starting main function.");
    let my_resource = MyResource::new(); // Acquire resource
    println!("Resource acquired.");
    // my_resource goes out of scope here
    println!("Ending main function.");
}

Expected Output:

Starting main function.
Acquiring resource...
Resource acquired.
Releasing resource...
Ending main function.

Explanation:

The MyResource is created, triggering its acquisition message. It lives within the main function's scope. When main finishes, my_resource goes out of scope, triggering its release message before "Ending main function." is printed.

Example 2: Nested Scopes

fn process_data() {
    println!("Entering process_data.");
    {
        let inner_resource = MyResource::new(); // Acquire inner resource
        println!("Inner resource acquired.");
        // inner_resource goes out of scope here
    }
    println!("Exiting process_data.");
}

fn main() {
    println!("Starting main function.");
    let outer_resource = MyResource::new(); // Acquire outer resource
    println!("Outer resource acquired.");
    process_data();
    // outer_resource goes out of scope here
    println!("Ending main function.");
}

Expected Output:

Starting main function.
Acquiring resource...
Outer resource acquired.
Entering process_data.
Acquiring resource...
Inner resource acquired.
Releasing resource...
Exiting process_data.
Releasing resource...
Ending main function.

Explanation:

outer_resource is acquired first. Then process_data is called, where inner_resource is acquired. inner_resource goes out of scope and is released. Control returns to main, and finally outer_resource goes out of scope and is released. Notice the LIFO (Last-In, First-Out) nature of the releases.

Example 3: Panic Scenario

fn main() {
    println!("Starting main function.");
    let my_resource = MyResource::new(); // Acquire resource
    println!("Resource acquired. About to panic...");
    panic!("Something went terribly wrong!");
    // my_resource *will* be dropped here due to unwind
    println!("This will never be printed.");
}

Expected Output (will vary slightly due to panic message, but the key is the release):

Starting main function.
Acquiring resource...
Resource acquired. About to panic...
Releasing resource...
thread 'main' panicked at 'Something went terribly wrong!', src/main.rs:XX:YY

Explanation:

Even though the program panics, Rust's unwinding mechanism ensures that destructors (drop trait implementation) are called for variables that are still in scope when the panic occurs. This guarantees the resource is released.

Constraints

• Your MyResource struct must not require any external crates.
• The solution should be entirely contained within a single Rust file (main.rs).
• The primary goal is correctness of the RAII pattern, not extreme performance optimization.
• Input will be the code you write to define MyResource and demonstrate its usage.

Notes

• In Rust, the RAII pattern is implemented using the Drop trait. You'll need to implement this trait for your struct.
• The drop method is automatically called by the Rust runtime when a value goes out of scope.
• Focus on the core RAII mechanism. You don't need to simulate a complex resource; simple print statements are sufficient to demonstrate acquisition and release.
• Think about how you would structure the MyResource struct and what its new function (or similar constructor) would do.