Fix integer overflow in BitBufferMut::truncate and append_n

[ksj1230]

Summary

Several methods on BitBufferMut use unchecked arithmetic on offset/length values, which can overflow in release mode and lead to undefined behavior reachable entirely from safe code (#![forbid(unsafe_code)]).

Affected methods:

• truncate: self.offset + len overflows, causing the buffer to be truncated to an incorrect size.
• append_n: self.offset + self.len + n overflows, causing incorrect capacity calculation.

Fix: add overflow assertions before the unchecked addition.

Reproduction (all use #![forbid(unsafe_code)]):

// 1. truncate
let buffer = ByteBufferMut::zeroed(10);
let mut bb = BitBufferMut::from_buffer(buffer, usize::MAX - 2, 80);
bb.truncate(3);
bb.set(2);

// 2. append_n
let buffer = ByteBufferMut::zeroed(2);
let mut bb = BitBufferMut::from_buffer(buffer, 1000, 0);
bb.append_n(true, usize::MAX);
bb.set(10000);

Related: #7979 (the BufferMut::zeroed_aligned overflow I reported to maintainers in April 2025; this PR addresses additional overflow sites found during the same audit)

Testing

Existing cargo test -p vortex-buffer passes (789 tests + 6 doc-tests).
Both PoCs now panic with overflow message instead of UB.

Happy to adjust the approach if maintainers prefer a different fix strategy.

[miniex]

sign off the commit to pass DCO: git commit --amend -s --no-edit && git push -f

[AdamGS]

stylistically - I think we use error messagess that read more like "Operation X on Y overflowed"

[codspeed-hq]

Merging this PR will improve performance by 13.04%

⚡ 5 improved benchmarks
✅ 1632 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	encode_varbin[(10000, 2)]	977.1 µs	836.1 µs	+16.87%
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	244.4 ns	215.3 ns	+13.55%
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	307.4 µs	271.5 µs	+13.21%
⚡	Simulation	compare_int_constant	298.7 µs	268.8 µs	+11.1%
⚡	Simulation	bitwise_not_vortex_buffer_mut[1024]	304.7 ns	275.6 ns	+10.58%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ksj1230:fix/bitbuffermut-checked-arithmetic (0d12875) with develop (d2b2378)}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

This PR has been marked as stale because it has been open for 14 days with no activity. Please comment or remove the stale label if you wish to keep it active, otherwise it will be closed in 7 days

[github-actions]

This PR was closed because it has been inactive for 7 days since being marked as stale.

[robert3005]

@AdamGS anything missing here?

[AdamGS]

No, just lost track of it

[AdamGS]

@ksj1230 thank you for you contribution! I apologize for taking so long.

[ksj1230]

Thanks for merging! Happy to help.