Implement a Basic Incremental Garbage Collector in Rust

Garbage collection (GC) is a crucial mechanism for automatic memory management in many programming languages. However, traditional stop-the-world GC pauses program execution entirely, leading to noticeable latency. Incremental GC aims to mitigate this by performing GC work in smaller, interleaved chunks, reducing pause times. This challenge asks you to implement a simplified incremental garbage collector for a small, object-based memory model in Rust.

Problem Description

You need to build a rudimentary incremental garbage collector for a hypothetical memory space that manages dynamically allocated objects. The GC will operate on a graph of interconnected objects. The core idea is to implement a mark-and-sweep garbage collector that can be run in an incremental fashion. This means the GC process should be broken down into multiple steps that can be executed over time without excessively long pauses.

Key Requirements:

1. Memory Model: Design a simple memory model to represent allocated objects. Each object should have a unique identifier and a list of references (pointers) to other objects.
2. Garbage Collector Logic: Implement the core mark-and-sweep algorithm.
   • Mark Phase: Identify all reachable objects starting from a set of "roots."
   • Sweep Phase: Deallocate all objects that were not marked as reachable.
3. Incremental Execution: The mark phase needs to be designed to be interruptible and resumable. You should be able to perform a portion of the marking work in one call and then continue from where you left off in subsequent calls.
4. Root Set: The GC needs a defined set of "roots" (e.g., global variables, stack frames) from which reachability is determined. For this challenge, you can simulate a fixed set of root object IDs.
5. Allocation: Implement a simple allocation mechanism to create new objects within the managed memory.

Expected Behavior:

• The system should be able to allocate objects.
• The GC should correctly identify and collect unreachable objects.
• The incremental GC should allow for multiple calls to mark objects without completing the entire marking process at once.
• When the marking phase is complete (all reachable objects are marked), the sweep phase should deallocate the unmarked objects.

Edge Cases:

• Circular references: Objects that reference each other, forming cycles.
• No reachable objects: All objects become unreachable.
• All objects reachable: No objects are collected.
• Allocating and collecting concurrently (simulated): The GC should not crash or exhibit undefined behavior when new objects are allocated between incremental GC steps.

Examples

Example 1: Basic Allocation and Collection

Initial State:
Managed Memory: {} (empty)
Roots: []

1. Allocate object A.
   Managed Memory: {A: {id: A, refs: []}}
   Roots: [A]

2. Allocate object B.
   Managed Memory: {A: {id: A, refs: []}, B: {id: B, refs: []}}
   Roots: [A]

3. Allocate object C.
   Managed Memory: {A: {id: A, refs: []}, B: {id: B, refs: []}, C: {id: C, refs: []}}
   Roots: [A, C]

4. Trigger Incremental GC (partial mark).
   Marking Queue: [A]
   Marked: {A}
   Managed Memory: {A: {id: A, refs: []}, B: {id: B, refs: []}, C: {id: C, refs: []}}
   Roots: [A, C]

5. Trigger Incremental GC (continue mark).
   Marking Queue: [C]
   Marked: {A, C}
   Managed Memory: {A: {id: A, refs: []}, B: {id: B, refs: []}, C: {id: C, refs: []}}
   Roots: [A, C]

6. Incremental GC completes marking. Sweep phase starts.
   Unmarked objects: B
   Managed Memory: {A: {id: A, refs: []}, C: {id: C, refs: []}}
   Roots: [A, C]

Output: Object B is collected.

Example 2: Object Referencing Another

Initial State:
Managed Memory: {}
Roots: []

1. Allocate object X.
   Managed Memory: {X: {id: X, refs: []}}
   Roots: [X]

2. Allocate object Y.
   Managed Memory: {X: {id: X, refs: []}, Y: {id: Y, refs: []}}
   Roots: [X]

3. Make X reference Y.
   Managed Memory: {X: {id: X, refs: [Y]}, Y: {id: Y, refs: []}}
   Roots: [X]

4. Trigger Incremental GC (partial mark).
   Marking Queue: [X]
   Marked: {X}
   Managed Memory: {X: {id: X, refs: [Y]}, Y: {id: Y, refs: []}}
   Roots: [X]

5. Trigger Incremental GC (continue mark).
   Marking Queue: [Y]
   Marked: {X, Y}
   Managed Memory: {X: {id: X, refs: [Y]}, Y: {id: Y, refs: []}}
   Roots: [X]

6. Incremental GC completes marking. Sweep phase starts.
   Unmarked objects: None
   Managed Memory: {X: {id: X, refs: [Y]}, Y: {id: Y, refs: []}}
   Roots: [X]

Output: No objects are collected.

Example 3: Circular Reference and Unreachable Cycle

Initial State:
Managed Memory: {}
Roots: []

1. Allocate object P.
   Managed Memory: {P: {id: P, refs: []}}
   Roots: [P]

2. Allocate object Q.
   Managed Memory: {P: {id: P, refs: []}, Q: {id: Q, refs: []}}
   Roots: [P]

3. Allocate object R.
   Managed Memory: {P: {id: P, refs: []}, Q: {id: Q, refs: []}, R: {id: R, refs: []}}
   Roots: [P]

4. Make P reference Q.
   Managed Memory: {P: {id: P, refs: [Q]}, Q: {id: Q, refs: []}, R: {id: R, refs: []}}
   Roots: [P]

5. Make Q reference R.
   Managed Memory: {P: {id: P, refs: [Q]}, Q: {id: Q, refs: [R]}, R: {id: R, refs: []}}
   Roots: [P]

6. Make R reference Q (creating a cycle Q <-> R).
   Managed Memory: {P: {id: P, refs: [Q]}, Q: {id: Q, refs: [R]}, R: {id: R, refs: [Q]}}
   Roots: [P]

7. Remove P from roots.
   Managed Memory: {P: {id: P, refs: [Q]}, Q: {id: Q, refs: [R]}, R: {id: R, refs: [Q]}}
   Roots: []

8. Trigger Incremental GC (partial mark).
   Marking Queue: [] (Roots are empty)
   Marked: {}
   Managed Memory: {P: {id: P, refs: [Q]}, Q: {id: Q, refs: [R]}, R: {id: R, refs: [Q]}}
   Roots: []

9. Incremental GC completes marking. Sweep phase starts.
   Unmarked objects: P, Q, R
   Managed Memory: {}
   Roots: []

Output: Objects P, Q, and R are collected.

Constraints

• The number of objects in the managed memory at any given time will not exceed 10,000.
• Each object will reference at most 10 other objects.
• The total number of GC cycles should not exceed 100 before memory is exhausted.
• Your GC implementation should strive to make progress on the marking phase with each incremental call, avoiding excessive redundant work.
• The implementation should be purely in Rust, using standard library features where possible. unsafe Rust is discouraged unless absolutely necessary and clearly justified.

Notes

• Consider how to represent object references. usize IDs or raw pointers could be used, but be mindful of Rust's ownership and borrowing rules. You might need to use Rc<RefCell<T>> or a similar construct to manage shared mutable state for objects and their references.
• For the incremental mark phase, a worklist (e.g., a VecDeque) is a common data structure. You can process a fixed number of items from the worklist in each incremental GC step.
• Think about how to signal when the marking phase is truly complete and the sweep phase can begin.
• A simple HashMap could serve as your managed memory, mapping object IDs to object data.
• The "roots" can be represented as a HashSet or Vec of object IDs.
• Feel free to define your Object struct. It should contain at least an identifier and a collection of references to other objects.