Add FromByteSlice: fallible variable-length integer parse (0.1.2)

[kaidokert]

Opens the 0.1.2-alpha line and lands capability (b) from the byte-trait ecosystem plan: a fallible variable-length integer parse.

Why

FromBytes mirrors core's from_le_bytes — fixed width, infallible — so it can't parse a &[u8] whose length the caller doesn't match to the target width, and has no channel to reject over-length input. FromByteSlice fills that gap:

pub trait FromByteSlice: Sized {
    fn from_be_slice(bytes: &[u8]) -> Result<Self, ByteSliceError>;
    fn from_le_slice(bytes: &[u8]) -> Result<Self, ByteSliceError>;
}

Shorter-than-width input zero-extends; equal is exact; wider → ByteSliceError { Overflow } (never truncates); empty → ByteSliceError { Empty }.

Properties

• Additive — no existing item changes signature. Patch-slot alpha.
• Unsigned-only (u8–u128, usize): zero-extension has no sign-extension analogue.
• Crate-owned error (ByteSliceError + ByteSliceErrorKind), mirroring AsciiParseError — constructible on stable, no std type.
• Const-callable on nightly (canary in const_nightly.rs).
• Root + prelude re-exports.

Design feedback (from the earlier proposal doc)

• Empty → error, not Ok(0) — adopted. A truncated/missing buffer can't masquerade as a valid zero; matches FromAscii.
• from_ne_slice — declined. Native endianness is for fixed-width reinterpretation (from_ne_bytes on [u8; N]); a variable-length parse in "native" order is semantically dubious and invites misuse. Additive if ever wanted.
• "Fits" semantics — length-based (matches the live rsa/new_reduce check): len > width → Overflow regardless of whether surplus high bytes are zero.

Not included

The backend-side changes (fixed-bigint FromBytes::Bytes = [u8] flip, &Backend ToBytes impl, const-eval inherent) and the ed25519/rsa rebindings land in their own repos. This PR is only the additive const-num-traits primitive.

[coderabbitai]

📝 Walkthrough

Walkthrough

The crate version was bumped, and a new byte_slice parsing API was added, re-exported from the crate root, and exercised in nightly const tests.

Changes

Byte slice parsing API

Layer / File(s)	Summary
Version bump Cargo.toml	Package version updated from 0.1.1 to 0.1.2-alpha.0.
Module exports src/ops/mod.rs, src/lib.rs	The new byte_slice module is included in the module tree, and its error types and trait are re-exported through the crate root and prelude.
Error and trait contract src/ops/byte_slice.rs	Defines ByteSliceErrorKind, ByteSliceError, and the FromByteSlice const trait for big-endian and little-endian parsing from &[u8].
Implementations and tests src/ops/byte_slice.rs, tests/const_nightly.rs	Implements FromByteSlice for unsigned integer types with length-based padding and errors, and adds a const test using from_be_slice.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding the fallible variable-length FromByteSlice parser.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch proposal/byte-trait-ecosystem

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[gemini-code-assist]

Code Review

This pull request bumps the crate version to 0.1.2-alpha.0 and adds a comprehensive proposal document detailing a byte-trait ecosystem for backend-agnostic numeric consumers. The review feedback suggests correcting inconsistent method names in the capability table, adding a native-endian parsing method (from_ne_slice) for completeness, and treating empty slice inputs as errors rather than defaulting to zero to prevent potential bugs.

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[sourcery-ai]

Hey - I've left some high level feedback:

• The design note uses c0nst::c0nst! and similar pseudo-identifiers; consider adding a short note up front that this is illustrative syntax and mapping it explicitly to the real macro/paths to avoid confusion for readers trying to correlate with the actual code.
• The Open decisions section is very helpful; you might want to turn these into explicit, numbered action items (with tentative resolutions or acceptance criteria) so it’s easier to track what must be settled before landing the FromByteSlice/ByteSliceError implementation.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The design note uses `c0nst::c0nst!` and similar pseudo-identifiers; consider adding a short note up front that this is illustrative syntax and mapping it explicitly to the real macro/paths to avoid confusion for readers trying to correlate with the actual code.
- The `Open decisions` section is very helpful; you might want to turn these into explicit, numbered action items (with tentative resolutions or acceptance criteria) so it’s easier to track what must be settled before landing the `FromByteSlice`/`ByteSliceError` implementation.

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