Implementing a Simple Garbage Collector in Rust

This challenge focuses on understanding and implementing fundamental memory management techniques. You will create a basic garbage collector (GC) in Rust to automatically reclaim memory that is no longer in use, mimicking a simplified version of what happens in languages with automatic memory management. This is a great way to explore Rust's ownership and borrowing system from a different perspective and understand the trade-offs involved in memory management.

Problem Description

Your task is to build a rudimentary garbage collector for a custom data structure. The GC will operate on a collection of objects, and its primary responsibility will be to identify and deallocate objects that are no longer reachable from a set of "roots."

Key Requirements:

Object Representation: Define a trait, let's call it GCObject, that all objects managed by the GC must implement. This trait should provide a way to access references (pointers) to other GCObjects that the current object might hold.
Garbage Collector Structure: Create a GarbageCollector struct. This struct will:
- Maintain a list of all allocated GCObjects.
- Store a set of "root" references. These are the entry points from which the GC can trace reachable objects.
- Implement a collect method to trigger the garbage collection process.
Tracing Algorithm: The collect method should implement a mark-and-sweep garbage collection algorithm:
- Mark Phase: Start from the roots and traverse all reachable objects. Mark each visited object as "reachable." This traversal should follow the references defined by the GCObject trait.
- Sweep Phase: Iterate through all allocated objects. Any object that was not marked as reachable during the mark phase should be deallocated (its memory reclaimed).
Object Allocation: Provide a mechanism within the GarbageCollector to allocate new GCObjects, adding them to its internal list of managed objects.

Expected Behavior:

When collect is called, only objects that can be reached directly or indirectly from the roots should remain allocated.
Objects that are no longer referenced by any other object or a root should be reclaimed.

Edge Cases:

Circular References: The GC must correctly handle circular references between objects. An object in a cycle is still reachable if any part of the cycle is reachable from a root.
Empty Roots: The GC should handle cases where there are no roots. In this scenario, all objects should be collected.
No Allocated Objects: The GC should not panic if there are no objects to manage.

Examples

Example 1: Simple Linear Reference

// Imagine an object structure like this:
// Root -> ObjectA -> ObjectB
// ObjectB is no longer referenced by Root or ObjectA (after some operation)

// GC Initialization
let mut gc = GarbageCollector::new();

// Allocate objects
let obj_a = gc.allocate(MyObject::new("A")); // obj_a holds a ref to obj_b
let obj_b = gc.allocate(MyObject::new("B"));

// Establish references
// (Assume MyObject has a method to set its internal reference)
obj_a.set_ref(Some(obj_b.clone())); // obj_a now points to obj_b

// Add obj_a to roots
gc.add_root(obj_a.clone());

// ... some operations ...
// Now, let's say we remove obj_a from roots, and obj_a is modified not to point to obj_b
// (for simplicity in this explanation, let's assume obj_a is dropped from the scope and thus its reference is cleared internally)

// If obj_a is no longer a root and its internal reference is cleared, obj_b should be collected.
// For this example, let's simulate removing obj_a as a root AND clearing its reference.
// In a real scenario, this would happen naturally if obj_a goes out of scope or is updated.

// We'll need a way to represent the objects and their references.
// Let's assume 'obj_a' and 'obj_b' are handles or pointers managed by GC.

// Let's refine the example to be more concrete.

// Object structure:
// Root -> A -> B
// Assume A is removed from roots and its pointer to B is cleared. B should be collected.

// Initial State:
// GC has A, B
// Root points to A
// A points to B

// After GC run (assuming A is no longer a root and cleared its reference):
// GC should only have objects reachable from roots.
// If A is no longer a root and cleared its reference, nothing is reachable.
// Therefore, both A and B are collected.

// This example needs concrete types and methods. Let's proceed with the definition of GCObject.

Example 2: Circular Reference

// Object structure:
// Root -> A
// A -> B
// B -> A (circular reference)

// GC Initialization
let mut gc = GarbageCollector::new();

// Allocate objects
let obj_a = gc.allocate(MyObject::new("A"));
let obj_b = gc.allocate(MyObject::new("B"));

// Establish references
obj_a.set_ref(Some(obj_b.clone()));
obj_b.set_ref(Some(obj_a.clone()));

// Add obj_a to roots
gc.add_root(obj_a.clone());

// When gc.collect() is called:
// - Root points to obj_a. obj_a is marked.
// - obj_a points to obj_b. obj_b is marked.
// - obj_b points to obj_a. obj_a is already marked.
// Both obj_a and obj_b are reachable and should NOT be collected.

Example 3: Unreachable Object

// Object structure:
// Root -> A
// B is allocated but not referenced by A or Root.

// GC Initialization
let mut gc = GarbageCollector::new();

// Allocate objects
let obj_a = gc.allocate(MyObject::new("A"));
let obj_b = gc.allocate(MyObject::new("B")); // obj_b is standalone

// Establish reference
obj_a.set_ref(None); // Or no reference set

// Add obj_a to roots
gc.add_root(obj_a.clone());

// When gc.collect() is called:
// - Root points to obj_a. obj_a is marked.
// - obj_a has no outgoing references.
// - obj_b was never reached from the roots.
// obj_b should be collected. obj_a should remain.

Constraints

Your GarbageCollector should manage at least 1000 objects without significant performance degradation.
The references within GCObjects should be managed safely, preventing dangling pointers.
The solution should be written entirely in Rust, leveraging its type system and memory safety features as much as possible.
Avoid using Box or other standard heap allocators for the objects themselves. The GC is responsible for managing the lifetime of these objects, implying a custom allocation scheme. You can use Box for the GarbageCollector struct itself or for internal GC data structures like the list of objects.

Notes

You will need a way to represent "pointers" or "handles" to GCObjects that the GC can manipulate. Rc<RefCell<T>> or a custom pointer type could be useful here. Consider how to represent the "null" or "none" state for references.
The GCObject trait should provide a method like get_references(&self) -> Vec<GcHandle<Self>> (where GcHandle is your pointer/handle type) to allow the GC to traverse the object graph.
Think about how to store the allocated objects. A Vec is a reasonable starting point, but consider the implications of removal during the sweep phase.
The "marking" can be implemented by adding a flag to your object representation or using a separate data structure (e.g., a HashSet of reachable object IDs).
Deallocation means freeing the memory associated with an object. In Rust, this often means dropping the object. You need to ensure that when an object is swept, its Drop implementation is called.
Consider using std::mem::forget or std::mem::drop strategically, but be very careful with memory safety. The goal is to reclaim memory, not leak it.