Challenge 26: verify Rc/Weak safety in alloc::rc with Kani

[v3risec]

Summary

This PR adds Kani-based verification artifacts for Rc/Weak safety in library/alloc/src/rc.rs for Challenge 26.

The change introduces:

• proof harness modules under #[cfg(kani)] for both required unsafe functions and a broad safe-function subset
• contracts/harnesses that check pointer layout/alignment/allocation consistency and key reference-count invariants
• shared helper-based construction for nondeterministic unsized slice inputs, so Rc<[T]>/Weak<[T]> paths can be exercised in a reusable way

No non-verification runtime behavior is changed in normal builds.

Verification Coverage Report

Unsafe functions (required by Challenge 26)

Coverage: 12 / 12 (100%)

Verified set includes:

• Rc<mem::MaybeUninit<T>,A>::assume_init
• Rc<[mem::MaybeUninit<T>],A>::assume_init
• Rc<T:?Sized>::from_raw
• Rc<T:?Sized>::increment_strong_count
• Rc<T:?Sized>::decrement_strong_count
• Rc<T:?Sized,A:Allocator>::from_raw_in
• Rc<T:?Sized,A:Allocator>::increment_strong_count_in
• Rc<T:?Sized,A:Allocator>::decrement_strong_count_in
• Rc<T:?Sized,A:Allocator>::get_mut_unchecked
• Rc<dyn Any,A:Allocator>::downcast_unchecked
• Weak<T:?Sized>::from_raw
• Weak<T:?Sized,A:Allocator>::from_raw_in

Safe functions (Challenge 26 list)

Coverage: 50 / 54 (92.6%)

This exceeds the challenge threshold (>= 75%).

Covered safe functions (50/54):

• Rc<T:?Sized, A:Allocator>::inner
• Rc<T:?Sized, A:Allocator>::into_inner_with_allocator
• Rc<T>::new
• Rc<T>::new_uninit
• Rc<T>::new_zeroed
• Rc<T>::try_new
• Rc<T>::try_new_uninit
• Rc<T>::try_new_zeroed
• Rc<T>::pin
• Rc<T,A:Allocator>::new_uninit_in
• Rc<T,A:Allocator>::new_zeroed_in
• Rc<T,A:Allocator>::new_cyclic_in
• Rc<T,A:Allocator>::try_new_in
• Rc<T,A:Allocator>::try_new_uninit_in
• Rc<T,A:Allocator>::try_new_zeroed_in
• Rc<T,A:Allocator>::pin_in
• Rc<T,A:Allocator>::try_unwrap
• Rc<[T]>::new_uninit_slice
• Rc<[T]>::new_zeroed_slice
• Rc<[T]>::into_array
• Rc<[T],A:Allocator>::new_uninit_slice_in
• Rc<[T],A:Allocator>::new_zeroed_slice_in
• Rc<T:?Sized,A:Allocator>::as_ptr
• Rc<T:?Sized,A:Allocator>::get_mut
• Rc<T:?Sized+CloneToUninit, A:Allocator+Clone>::make_mut
• Rc<dyn Any,A:Allocator>::downcast
• Rc<T:?Sized,A:Allocator>::from_box_in
• RcFromSlice<T: Copy>::from_slice
• Drop<T: ?Sized, A:Allocator>::drop for Rc
• Clone<T: ?Sized, A:Allocator>::clone for Rc
• Default<T:Default>::default
• Default<str>::default
• From<&str>::from
• From<Vec<T,A:Allocator>>::from
• From<Rc<str>>::from
• ToRcSlice<T, I>::to_rc_slice
• Weak<T:?Sized,A:Allocator>::as_ptr
• Weak<T:?Sized,A:Allocator>::into_raw_with_allocator
• Weak<T:?Sized,A:Allocator>::inner
• Drop<T:?Sized, A:Allocator>::drop for Weak
• RcInnerPtr::inc_strong
• RcInnerPtr::inc_weak
• UniqueRc<T:?Sized,A:Allocator>::into_rc
• UniqueRc<T:?Sized,A:Allocator+Clone>::downgrade
• Deref<T:?Sized,A:Allocator>::deref
• DerefMut<T:?Sized,A:Allocator>::deref_mut
• Drop<T:?Sized, A:Allocator>::drop for UniqueRc
• UniqueRcUninit<T:?Sized, A:Allocator>::new
• UniqueRcUninit<T:?Sized, A:Allocator>::data_ptr
• Drop<T:?Sized, A:Allocator>::drop for UniqueRcUninit

Not yet listed as standalone harness targets (4/54):

• Rc<T:?Sized,A:Allocator>::into_raw_with_allocator
• RcFromSlice<T: Clone>::from_slice
• TryFrom<Rc<[T],A:Allocator>>::try_from
• Weak<T:?Sized,A:Allocator>::upgrade

Three Criteria Met (Challenge 26)

• Required unsafe functions covered: All 12/12 required unsafe functions in Challenge 26 are annotated with contracts and verified.
• Safe-function threshold met: 50/54 safe functions are covered (92.6%), which exceeds the Challenge 26 requirement of at least 75%.
• Challenge scope allowances respected: Generic T is instantiated with allowed representative concrete types, and allocator-focused proofs are limited to standard-library allocator scope (Global).

Approach

The verification strategy combines contracts for unsafe entry points with executable proof harnesses:

1. Contract for unsafe functions

• Attach kani::requires preconditions for pointer validity, alignment soundness, same-allocation checks, and refcount well-formedness.
• Attach postconditions where appropriate (kani::ensures) and mutation footprints (kani::modifies) for refcount-changing operations.

2. Harness-backed behavioral checks

• Add dedicated #[kani::proof] and #[kani::proof_for_contract()] modules for each required unsafe function and safe function coverage.

3. Helper-based unsized generalization

• Introduce shared helper functions for nondeterministic vector/slice setup and reuse them across harnesses that target ?Sized slice-based APIs.
• Use the helpers to exercise unsized slice cases through Rc<[T]>/Weak<[T]> constructions without duplicating per-harness setup logic.

4. Challenge alignment

• Keep all verification code under cfg(kani) so normal std behavior is unchanged.
• Target Challenge 26 success criteria directly: full required unsafe coverage + safe coverage above threshold.

Scope assumptions (per challenge allowance)

• Harnesses instantiate primitive and representative non-primitive types, including i32, u8, u64, bool, (), String, arrays/slices, str, and trait objects (dyn Any).
• Allocator coverage is limited to Global (both explicit Rc<_, Global> / Weak<_, Global> and implicit default Rc/Weak).

Verification

All harnesses pass locally with Kani 0.65.

Resolves #382

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[feliperodri]

You should use macros to reduce code duplication. It'll also make review easier.

[Copilot]

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