Assert the nbytes range in sign_extend

[thatssoheil]

This adds the assertion #832 asks for, plus the two tests that pin the range it asserts.

sign_extend is private, and all four callers (get_int, get_int_le, try_get_int, try_get_int_le) reach it through get_uint / get_uint_le / try_get_uint / try_get_uint_le, which reject more than 8 bytes via panic_does_not_fit before sign_extend runs. So nbytes > 8 there means a caller broke that contract rather than that a read came up short, and the subtraction is best not done blind:

debug_assert!(nbytes <= 8, "nbytes must be in [0, 8]");

One note on the assertion suggested in the issue itself, debug_assert!(nbytes >= 1 && ...): it cannot be used as written. A zero-length read is a real input here, handled by the existing if nbytes == 0 branch that returns 0 instead of shifting by 64, and asserting nbytes >= 1 panics on it and fails 27 existing tests. This asserts the upper bound only and leaves that branch as it was.

No release-mode behaviour changes: debug_assert is compiled out, and the assert cannot fire on any input the public API previously accepted.

Tests

cargo test passes in debug and release, and cargo fmt --check is clean.

Two tests in a #[cfg(test)] mod tests at the end of the file:

• sign_extend_asserts_more_than_eight_bytes: #[should_panic(expected = "nbytes must be in [0, 8]")] for sign_extend(0, 9), gated with #[cfg(debug_assertions)] because the assertion is compiled out of a release build.
• sign_extend_accepts_a_zero_length_read: pins the zero-length read at 0.

Deleting the assertion makes the first fail with "panic did not contain expected string", so both exercise the change.

Disclosure

AI-assisted. I ran the checks above and can explain every line during review.

Fixes #832

[Copilot]

🟢 Approval recommended

The assertion and tests cover the intended bounds without changing valid behavior.

Pull request overview

Adds a debug-only upper-bound assertion to sign_extend while preserving zero-length reads.

Changes:

• Assert nbytes <= 8.
• Add tests for oversized and zero-length inputs.

File summaries

File	Description
src/buf/buf_impl.rs	Adds the assertion and focused unit tests.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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