Implementing Strong Exception Guarantee with Result and Drop in Rust

Rust's error handling primarily relies on the Result enum, which is powerful for propagating errors. However, ensuring a strong exception guarantee (or no-throw guarantee in Rust terms) requires careful management of resources and state when operations can fail. This challenge focuses on building a data structure that maintains its integrity even when operations that modify it encounter errors.

Problem Description

You are tasked with implementing a SafeVector data structure that wraps a Vec<T>. This SafeVector should provide methods for adding and removing elements, but crucially, these operations must guarantee a strong exception guarantee. This means that if any operation fails (returns an Err), the SafeVector must be left in a valid, consistent state as if the operation never happened. If an operation succeeds, the SafeVector should be in the state as if the operation completed successfully.

Key requirements:

• push(item: T): Adds an item to the end of the vector. If an error occurs during allocation or any internal process, the SafeVector should remain unchanged.
• pop(): Removes and returns the last item from the vector. If an error occurs (e.g., empty vector), the SafeVector should remain unchanged.
• clear(): Removes all elements from the vector. This operation should ideally always succeed, but if it were to fail internally (e.g., due to dropping elements that panic), the SafeVector should be empty.
• Drop Implementation: The SafeVector itself must implement Drop to ensure that all its contained resources are properly deallocated. If any T within the vector panics during its own drop, this panic should not leave the SafeVector in an inconsistent state (though the panic will propagate).

Expected behavior:

• When push or pop encounters an error (simulated or actual), the SafeVector should revert to its state before the operation was attempted.
• clear should result in an empty SafeVector.
• The SafeVector should not leak memory or leave partially updated internal state.

Examples

Example 1: Successful push

// Assume SafeVector<i32> is initialized as empty
let mut sv = SafeVector::new();
assert_eq!(sv.len(), 0);

// Successful push
let result = sv.push(10);
assert!(result.is_ok());
assert_eq!(sv.len(), 1);
assert_eq!(sv.get(0), Some(&10));

Example 2: Failing push (simulated)

Let's assume push can fail if T implements a special trait FailablePush and returns Err when a certain condition is met. For this example, we'll abstract away the specific failure mechanism and focus on the guarantee.

struct Item(i32);

impl Item {
    // Simulate a condition that might cause push to fail
    fn should_fail(&self) -> bool {
        self.0 == 0
    }
}

// We need a way to simulate a failing push without actually panicking the test.
// For demonstration, imagine a scenario where allocating memory for the new item fails,
// or the item itself signals an unrecoverable issue during insertion.

// Let's represent a hypothetical failure during push
enum PushError {
    AllocationFailed,
    InvalidItemState,
}

struct SafeVector<T> {
    data: Vec<T>,
}

impl<T> SafeVector<T> {
    fn new() -> Self {
        SafeVector { data: Vec::new() }
    }

    fn len(&self) -> usize {
        self.data.len()
    }

    fn get(&self, index: usize) -> Option<&T> {
        self.data.get(index)
    }

    // Simplified push for demonstration:
    // In a real scenario, this would involve careful memory management and rollback.
    fn push(&mut self, item: T) -> Result<(), PushError> {
        // Simulate a failure condition
        // In a real implementation, this would be more complex.
        // For this challenge, we'll manually trigger failure for testing.
        // Let's assume `item` has a method to indicate if it would cause failure.
        // For testing purposes, we'll just return an Err on a specific item.
        if std::mem::size_of::<T>() > 0 && std::any::type_name::<T>() == "Item" && unsafe { std::mem::transmute::<T, Item>(std::ptr::read(&item as *const T as *mut T)).0 == 0 } {
             // Important: If an error occurs *after* some internal state has been modified,
             // we must revert it. This is the core of the strong guarantee.
             // In this simplified example, if we detect the failure *before* modifying `self.data`,
             // we simply return Err. If we had already done something, we'd need to undo it.

             // To make this testable, let's assume the `push` implementation
             // internally does something that might fail, and we need to roll back.
             // A common pattern is to use a temporary buffer or a "commit" step.

             // For the purpose of this challenge, let's simulate a failure
             // that happens *after* `Vec::push` potentially allocates, but before
             // the item is fully "committed". If this simulated push were to fail,
             // the vector *must not* contain the partially added element.

             // In this simplified test, if we were to encounter an item that signifies
             // failure, we would return `Err` and ensure `self.data` is unchanged.
             // The actual Rust `Vec::push` *itself* provides a strong guarantee
             // for its own allocation. The challenge is about what happens
             // if the *item* itself or a process involving the item fails.

             // For the sake of demonstration and testability:
             // Assume `item` contains a value that signals an intentional failure
             // for this test case.
             if std::mem::size_of::<T>() > 0 && std::any::type_name::<T>() == "Item" && unsafe { std::mem::transmute::<&T, &Item>(std::ptr::read(&item as *const T)).0 == 0 } {
                return Err(PushError::InvalidItemState);
             }
        }

        // Actual push: `Vec::push` itself provides a strong guarantee regarding its own state.
        self.data.push(item);
        Ok(())
    }

    // Simplified pop for demonstration:
    fn pop(&mut self) -> Option<T> {
        // `Vec::pop` itself provides a strong guarantee.
        self.data.pop()
    }

    fn clear(&mut self) {
        self.data.clear();
    }
}

// Mocking the Item struct for the example
#[derive(Debug, PartialEq)]
struct Item(i32);

impl Item {
    fn new(val: i32) -> Self {
        Item(val)
    }
}

// Helper to create a failing item for testing
fn failing_item_for_push() -> Item {
    Item(0) // Value that signals failure for our simulation
}

// Test scenario for failing push
let mut sv = SafeVector::new();
sv.push(Item::new(5)).unwrap(); // Initial successful push
assert_eq!(sv.len(), 1);

// Attempt to push an item that signals failure
let push_result = sv.push(failing_item_for_push());

assert!(push_result.is_err()); // Operation failed
assert_eq!(sv.len(), 1); // Vector should be unchanged
assert_eq!(sv.get(0).unwrap().0, 5); // The original item is still there

Example 3: Drop behavior and potential for inconsistency

Consider a scenario where the elements T have destructors that might panic. If SafeVector is dropped, and one of its elements' drop implementation panics, the standard behavior is for the panic to unwind and terminate the program. The SafeVector's drop should ensure that even if a panic occurs during element dropping, the vector's own internal state (like its allocated memory) is cleaned up.

struct PanickingDrop(i32);

impl Drop for PanickingDrop {
    fn drop(&mut self) {
        if self.0 == 1 {
            panic!("Panicking during drop!");
        }
        println!("Dropping PanickingDrop({})", self.0);
    }
}

// This test is tricky to write assertively because panics terminate execution.
// The goal is to ensure that if a panic happens in a `T`'s drop, the `SafeVector`
// itself still manages its resources correctly.

// Let's illustrate the *intent* rather than a direct assert.
// If we had a SafeVector<PanickingDrop> and dropped it, and the first element
// triggered a panic in its drop:
// let mut sv = SafeVector::new();
// sv.push(PanickingDrop(1)).unwrap(); // Item that will panic on drop
// sv.push(PanickingDrop(2)).unwrap();
//
// // When `sv` goes out of scope here, its `drop` method will be called.
// // It will attempt to drop PanickingDrop(2), then PanickingDrop(1).
// // Dropping PanickingDrop(1) will panic.
// // The `SafeVector`'s drop implementation should ensure that any memory
// // allocated for `sv` itself is released, even though the element's drop panicked.
// // The panic will propagate out of the `drop` method.

Constraints

• The SafeVector<T> must implement Default.
• The SafeVector<T> must implement Clone if T implements Clone.
• The SafeVector<T> must implement Debug if T implements Debug.
• The operations push and pop should aim for O(1) amortized time complexity.
• The clear operation should aim for O(N) time complexity, where N is the number of elements.
• Crucially, you must not use external crates that provide similar functionality. You need to implement the core logic yourself, focusing on Rust's standard library features (Vec, Result, Drop).

Notes

This challenge is about understanding how to build robust, state-preserving data structures in Rust, particularly when dealing with operations that can fail. The Vec<T> in Rust already provides strong guarantees for its internal memory management. Your task is to build on top of that, ensuring that your SafeVector's own logic for managing its state and integrating with T's lifecycle (creation, dropping) upholds the strong guarantee.

Consider the following patterns:

• Temporary allocations and rollback: If an operation modifies internal state before a potential failure point, you might need to save the original state and restore it upon failure.
• Commit after success: Design operations such that modifications are only finalized (committed) after all internal steps have succeeded.
• Drop implementation for T: Be mindful of how panics in T's Drop can affect the SafeVector's Drop. Rust's Drop implementation for collections typically handles this by unwinding, but ensures the collection's own memory is freed.

The push method's failure simulation in the examples is a placeholder. In a real-world scenario, push could fail due to memory allocation issues (which Vec handles) or if the item being inserted violates some invariant of your SafeVector. Your implementation must account for these possibilities and always leave the SafeVector in a valid state.