Migrate to Copy-on-Write (Cow) for zero-copy decoding support

[moreal]

Add lifetime parameters to BencodexValue<'a> and BencodexKey<'a>, replacing internal Vec<u8>/String with Cow<'a, [u8]>/Cow<'a, str>. This enables zero-copy decoding via the new decode_borrowed() API that borrows directly from the input slice. Also changes Encode trait from consuming self to &self, eliminating unnecessary clones during encoding.

Summary by CodeRabbit

• New Features

  • Added zero-copy decoding via new decode_borrowed function for improved performance with borrowed data.
  • Extended benchmarking suite with zero-copy benchmark paths to measure performance improvements.

• Bug Fixes

  • Eliminated unnecessary clones in encode and benchmark paths by adopting borrowing semantics.

• Refactor

  • Changed Encode trait to borrow values instead of consuming them.
  • Enhanced internal data handling to support both borrowed and owned data throughout the codec pipeline.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

This pull request introduces zero-copy borrowing semantics throughout the bencodex codec by adding lifetime parameters to BencodexValue and BencodexKey using Cow-backed storage, refactoring the Encode trait to accept borrowed references, and providing dual decode paths for both owned and borrowed data across all codec implementations.

Changes

Cohort / File(s)	Summary
Core Type System src/codec/types.rs	Introduces lifetime-parametric BencodexValue<'a> and BencodexKey<'a> with Cow-backed Binary/Text variants; updates BencodexList/BencodexDictionary to use lifetimes; adds into_owned() conversion methods; updates all From trait impls to leverage Cow for borrowed/owned data.
Decode Implementation src/codec/decode.rs, src/codec/simd/mod.rs, src/codec/simd/stage2.rs	Adds const generic BORROW flag for dual-path decoding; introduces decode_borrowed(input: &[u8]) → Result<BencodexValue<'_>, DecodeError> API; refactors all internal decode functions to be generic over BORROW with helper functions cow_binary/cow_text; updates SIMD parser to propagate lifetimes through parse operations and return borrowed Cow variants; updates all decode trait implementations to return BencodexValue<'static>.
Encode Implementation src/codec/encode.rs	Changes Encode trait method signature from encode(self, ...) to encode(&self, ...) across all implementations; adds encode_binary and encode_text helper functions; updates Vec<BencodexValue<'>> and BTreeMap<BencodexKey<'>, BencodexValue<'_>> to use lifetime parameters; refactors key/value encoding to use new helpers.
JSON Support src/json/decode.rs, src/json/encode.rs	Updates from_json functions to return BencodexValue<'static> with Cow::Owned variants; adds lifetime annotations to BencodexJsonEncoder and related functions; updates to_json/to_json_with_options to accept BencodexValue<'_> references.
Library API src/lib.rs, src/prelude.rs	Adds decode_borrowed to public codec::decode re-exports; replaces ToOwned with Cow in prelude.
Benchmarks benches/decode.rs, benches/encode.rs	Introduces decode_borrowed benchmark path with macro support; updates decode_simd benchmark to use zero-copy path; converts encode benchmarks to use borrowed references instead of clones; updates benchmark groups to register new paths.
Examples & Test Data examples/generate_test_data.rs	Updates random_key, random_primitive, random_value, generate_large_value signatures to return BencodexKey<'static>/BencodexValue<'static>; wraps generated data with Cow::Owned; adds Cow import.
Tests tests/codec/encode.rs, tests/codec/utils.rs, tests/fuzz/mod.rs, tests/fuzz/simd_decode.rs	Updates Spec and TestsuiteYamlLoader to use BencodexValue<'static'> and BencodexKey<'static'>; modifies YAML loader to construct Cow::Owned variants; updates fuzz generators to produce 'static lifetimes with Cow-wrapped data; adds .into_owned() calls on SIMD decode results; removes unnecessary clones in encoding tests.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• Support SIMD #33: SIMD decode API signature changes and decode_simd zero-copy implementation directly parallel the decode_borrowed functionality introduced here.
• Replace BigInt with usize for string length prefix parsing #36: Modifications to string and byte-string parsing paths in decode.rs overlap with the BORROW-generic refactoring of decode functions.
• fix: correct json representation #28: JSON encoder lifetime and Cow-based type changes align with the broader lifetime-parametric type system established in this PR.

Poem

🐰 Hops through borrowed lands so free,
Where Cow owns what data be,
No clones today, just lifetimes bright,
Zero-copy hops—what a delight!
Static and borrowed in harmony.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch copy-on-write

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.82759% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.23%. Comparing base (5bcb841) to head (2e02f33).

Files with missing lines	Patch %	Lines
src/codec/decode.rs	80.64%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   74.78%   75.23%   +0.44%     
==========================================
  Files          14       14              
  Lines         714      751      +37     
==========================================
+ Hits          534      565      +31     
- Misses        180      186       +6     

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by ×3.5

_{Comparing copy-on-write (2e02f33) with main (5bcb841)}

Summary

⚡ 4 improved benchmarks
✅ 6 untouched benchmarks
🆕 3 new benchmarks
⏩ 4 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	large_random_0 (36.5MB)	N/A	412.8 ms	N/A
⚡	bigint (9223372036854775807)	3.7 µs	3.3 µs	+14.42%
🆕	ncinventory_1 (45.6KB)	N/A	1.6 ms	N/A
⚡	ncinventory_1 (45.6KB)	2,656.7 µs	933.7 µs	×2.8
⚡	ncavatar_1 (20.6KB)	1,155.6 µs	327.5 µs	×3.5
⚡	large_random_0 (36.5MB)	535 ms	344.5 ms	+55.28%
🆕	ncavatar_1 (20.6KB)	N/A	696.4 µs	N/A

Footnotes

1. 4 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. ↩