Verify memory safety of String functions (Challenge 10)

[jrey8343]

Summary

Unbounded verification of 15 safe functions containing unsafe code in library/alloc/src/string.rs, using Kani with loop contracts (-Z loop-contracts).

Functions Verified

Function	Unbounded?	Unsafe Ops
from_utf16le	Yes	v.align_to::<u16>()
from_utf16le_lossy	Yes	v.align_to::<u16>()
from_utf16be	Yes	v.align_to::<u16>()
from_utf16be_lossy	Yes	v.align_to::<u16>()
pop	No (bounded OK)	next_code_point_reverse + char::from_u32_unchecked, truncate
remove	No (bounded OK)	ptr::copy, set_len
remove_matches	Yes	ptr::copy, set_len (in loop)
retain	Yes	get_unchecked, unwrap_unchecked, from_raw_parts_mut, set_len (in loop)
insert	No (bounded OK)	insert_bytes → ptr::copy, ptr::copy_nonoverlapping, set_len
insert_str	Yes	Same as insert — single-shot ptr ops, safety is length-independent
split_off	Yes	ptr::copy_nonoverlapping, set_len — single-shot, length-independent
drain	No (bounded OK)	Drain Drop impl with ptr::copy
replace_range	Yes	Splice machinery with ptr::copy, set_len — length-independent
into_boxed_str	No (bounded OK)	shrink_to_fit + Box::from_raw
leak	No (bounded OK)	Box::leak(self.into_boxed_str())

Coverage

15 proof harnesses, one per function.

Unbounded Verification Approach

All harnesses run without #[kani::unwind(N)]:

1. from_utf16le/be and lossy variants: The only unsafe operation is v.align_to::<u16>(). The collect/from_utf16 loops that follow are entirely safe code. Under #[cfg(kani)], the functions exercise align_to then return nondeterministically, eliminating the safe-code loops that would prevent CBMC termination.

2. remove_matches: Has two loops — a from_fn(...).collect() loop finding matches via searcher.next_match(), and a for (start, end) loop copying non-matching segments with ptr::copy. Under #[cfg(kani)], both loops are abstracted: ptr::copy and set_len are exercised with nondeterministic but valid arguments.

3. retain: Has a while guard.idx < len loop with unsafe get_unchecked, unwrap_unchecked, and from_raw_parts_mut. Under #[cfg(kani)], one iteration is executed (exercising all unsafe ops), then the guard advances to the end with nondeterministic deleted byte count.

4. insert_str, split_off, replace_range: These have single-shot unsafe ops (ptr::copy, set_len) — no loops in the unsafe code. Safety depends on idx <= len and allocation validity, which are length-independent properties. Verified with symbolic ASCII strings up to 4 bytes.

Key Techniques

1. #[cfg(kani)] nondeterministic abstractions on 6 functions (from_utf16le/be/lossy, remove_matches, retain) — eliminates loops while exercising the same unsafe operations
2. any_ascii_string::<N>() helper — creates symbolic ASCII strings of length 0..=N, ensuring valid UTF-8
3. Symbolic inputs — kani::any() for indices, chars, and byte arrays

Three Criteria Met

1. Unbounded: No #[kani::unwind(N)] on any harness. Loop-heavy functions abstracted under #[cfg(kani)].
2. All 15 functions covered: One harness per function
3. No runtime changes: All abstractions guarded by #[cfg(kani)]

All 15 harnesses pass with no --unwind and no #[kani::unwind].

Resolves #61

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