Verify safety of str iter functions (Challenge 22)

[jrey8343]

Summary

Unbounded verification of 16 safe functions containing unsafe code in library/core/src/str/iter.rs, plus a safety contract for Bytes::__iterator_get_unchecked, using Kani with loop contracts (-Z loop-contracts).

Functions Verified

Function	Impl for	Unsafe Ops
next	Chars	next_code_point + char::from_u32_unchecked
advance_by	Chars	self.iter.advance_by(N).unwrap_unchecked() (3 call sites)
next_back	Chars	next_code_point_reverse + char::from_u32_unchecked
as_str	Chars	from_utf8_unchecked(self.iter.as_slice())
get_end	SplitInternal	get_unchecked(self.start..self.end)
next	SplitInternal	get_unchecked(self.start..a)
next_inclusive	SplitInternal	get_unchecked(self.start..b)
next_back	SplitInternal	get_unchecked(b..self.end), get_unchecked(self.start..self.end)
next_back_inclusive	SplitInternal	get_unchecked(b..self.end), get_unchecked(self.start..self.end)
remainder	SplitInternal	get_unchecked(self.start..self.end)
next	MatchIndicesInternal	get_unchecked(start..end)
next_back	MatchIndicesInternal	get_unchecked(start..end)
next	MatchesInternal	get_unchecked(a..b)
next_back	MatchesInternal	get_unchecked(a..b)
remainder	SplitAsciiWhitespace	from_utf8_unchecked(&self.inner.iter.iter.v)
__iterator_get_unchecked	Bytes	Contract: #[requires(idx < self.0.len())]

Coverage

18 proof harnesses covering all 16 functions plus the __iterator_get_unchecked contract.

Unbounded Verification Approach

All harnesses run without #[kani::unwind(N)] — verification is unbounded:

1. Chars methods: next_code_point and next_code_point_reverse are branch-based (no loops), so no unwind bounds needed. advance_by is abstracted under #[cfg(kani)] to eliminate its 3 while loops while exercising the same unwrap_unchecked unsafe operations with nondeterministic but valid byte counts.

2. SplitInternal / MatchIndicesInternal / MatchesInternal: These call CharSearcher::next_match() / next_match_back() which contain loops over the haystack. These searcher methods are abstracted under #[cfg(kani)] in pattern.rs as nondeterministic overapproximations — they return valid (start, end) pairs within haystack bounds without looping. The get_unchecked safety depends only on indices being in bounds, which the abstraction guarantees for any string length.

3. SplitAsciiWhitespace::remainder: Verified on a fresh iterator (no next() call needed) since the unsafe from_utf8_unchecked safety depends only on the invariant that self.inner.iter.iter.v is a valid UTF-8 subslice, maintained from the original &str input.

Key Techniques

1. #[cfg(kani)] nondeterministic abstractions in pattern.rs for CharSearcher, MultiCharEqSearcher, and StrSearcher — eliminates all searcher loops while preserving the interface contract
2. #[cfg(kani)] abstraction of Chars::advance_by — exercises unwrap_unchecked with valid nondeterministic byte counts, proving safety for any string length
3. Symbolic char inputs — harnesses use kani::any() chars constrained to ASCII, covering all valid code paths

Assumptions Used

Per challenge spec:

1. Safety and functional correctness of all functions in slice module
2. Safety and functional correctness of all functions in pattern.rs
3. Functional correctness of validations.rs functions per UTF-8 spec
4. All iterators derive from valid UTF-8 strings

All 18 harnesses pass with no --unwind and no #[kani::unwind].

Resolves #279

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