feat(multitude): add arena-aware Serde and harden allocation safety

[geeknoid]

Summary

• Add allocator-backed deserialization traits, derive support, recursive containers, borrow-or-arena strings, dynamic values, capacity reuse, resource limits, and optional JSON helpers.
• Harden chunk ownership and cross-thread teardown by sharing allocator state safely, tighten Send/Sync bounds, and fix ZST drain and Rc overflow handling.
• Align allocation benchmarks and performance reporting with bumpalo, add representative Serde benchmarks and examples, and refresh architecture and performance documentation.

Validation

• Comprehensive unit, integration, property, malformed-input, Miri, and Loom coverage.
• 100% LLVM line coverage for the new serialization runtime and derive implementation.

[Copilot]

Pull request overview

This PR hardens multitude’s chunk layout and concurrency guarantees by moving the backing allocator into a shared Arc<A> in chunk metadata, preventing allocator state from inflating the chunk header past the first 64 KiB tile (which smart-pointer header recovery relies on). It also fixes a ZST-specific Drain drop edge case, expands safety documentation for assume_init, and rewrites docs/DESIGN.md into a higher-level architecture and invariants overview.

Changes:

• Store the backing allocator as Arc<A> in Chunk/ChunkProvider and tighten Send/Sync bounds to require A: Send + Sync where cross-thread chunk teardown is possible.
• Fix Drain’s drop path for zero-sized types by avoiding offset_from on dangling ZST pointers; add regression test coverage.
• Add tests reproducing the “large allocator state displaces header” issue and expand assume_init documentation; rewrite DESIGN documentation.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/multitude/tests/send_sync.rs	Adds a regression test ensuring Arena<A> is not Send when A is !Sync.
crates/multitude/tests/audit_repro.rs	Adds a large-allocator-state regression test ensuring smart pointers remain valid after arena drop.
crates/multitude/tests/arena_vec.rs	Adds a test ensuring ZST drain(..) drop restores tail and runs drops correctly.
crates/multitude/src/vec/drain.rs	Fixes ZST Drain drop behavior by avoiding pointer offset computation for ZSTs.
crates/multitude/src/rc.rs	Documents assume_init clone/leak footgun for Rc<MaybeUninit<...>>.
crates/multitude/src/internal/chunk.rs	Stores allocator as Arc<A>, updates alignment/header computations and related tests.
crates/multitude/src/internal/chunk_provider.rs	Stores allocator as Arc<A> and tightens Send/Sync impl bounds to A: Send + Sync.
crates/multitude/src/internal/chunk_mutator.rs	Tightens Send impl bound to A: Send + Sync to match allocator sharing semantics.
crates/multitude/src/arena/retired_local.rs	Tightens Send impl bound to A: Send + Sync for moved arenas with pinned chunks.
crates/multitude/src/arena/mod.rs	Updates Arena: Send rationale to reflect A: Send + Sync requirement.
crates/multitude/src/arc.rs	Documents assume_init clone/leak footgun for Arc<MaybeUninit<...>>.
crates/multitude/docs/DESIGN.md	Rewrites internal design doc into an architecture overview with invariants, failure modes, and concurrency model.

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[github-actions]

✅ Version increments look sufficient

cargo semver-checks compared the 3 crate(s) this PR publishes against their previous version-bump commit in git history. Every version increment is sufficient for the detected API changes.

Crate	Baseline	Baseline commit	This PR	Minimum required	Status
multitude	0.6.1	ddbfcd1	0.6.2	0.6.2	✅ ok
multitude_macros	new crate	—	0.1.0	0.1.0	✅ ok
multitude_macros_impl	new crate	—	0.1.0	0.1.0	✅ ok

This check is informational and does not block the merge.

View the check run

[Copilot]

Pull request overview

Copilot reviewed 15 out of 16 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (ba34d62) to head (29837b6).

Additional details and impacted files

@@            Coverage Diff            @@
##             main     #587     +/-   ##
=========================================
  Coverage   100.0%   100.0%             
=========================================
  Files         359      375     +16     
  Lines       27885    30664   +2779     
=========================================
+ Hits        27885    30664   +2779     

Flag	Coverage Δ
linux	90.4% <100.0%> (-9.6%)	⬇️
linux-arm	90.4% <100.0%> (-9.6%)	⬇️
scheduled	?
windows	90.4% <100.0%> (-9.6%)	⬇️

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[Copilot]

Pull request overview

Copilot reviewed 15 out of 16 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 17 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 17 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 27 out of 28 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 90 out of 92 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

crates/multitude_macros/README.md:2

• README references ./logo.png, but no such file exists in this crate, so the header image will be broken on GitHub/crates.io/docs.rs renders. Either add the image (non-LFS, included in the published crate) or point this to an existing asset (e.g. the repo-level logo.svg).
  crates/multitude_macros_impl/README.md:2
• README references ./logo.png, but no such file exists in this crate, so the header image will be broken on GitHub/crates.io/docs.rs renders. Either add the image (non-LFS, included in the published crate) or point this to an existing asset (e.g. the repo-level logo.svg).

[Copilot]

Pull request overview

Copilot reviewed 90 out of 92 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

crates/multitude_macros/README.md:2

• README references ./logo.png, but that file is not present in this crate directory, so the image will render as a broken link. Either add the missing asset or point to an existing logo file in the repo.
  crates/multitude_macros_impl/README.md:2
• README references ./logo.png, but that file is not present in this crate directory, so the image will render as a broken link. Either add the missing asset or point to an existing logo file in the repo.

[Copilot]

Pull request overview

Copilot reviewed 90 out of 92 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

crates/multitude_macros/README.md:3

• This README references local assets (./logo.png and ../../logo.svg). The workspace packaging allowlist (workspace.package include) does not include images or repo-root assets like logo.svg, so these links will be broken on crates.io/docs.rs. Prefer absolute URLs for badges/logos (also aligns with the packaging guideline to reference binary assets by URL).
  crates/multitude_macros_impl/README.md:3
• This README references local assets (./logo.png and ../../logo.svg). The workspace packaging allowlist (workspace.package include) does not include images or repo-root assets like logo.svg, so these links will be broken on crates.io/docs.rs. Prefer absolute URLs for badges/logos (also aligns with the packaging guideline to reference binary assets by URL).

[Copilot]

Pull request overview

Copilot reviewed 90 out of 92 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

crates/multitude_macros/README.md:2

• The README references ./logo.png, but crates/multitude_macros/logo.png is not present in the repo, so the image will be broken anywhere the README is rendered. Add the expected logo.png (matching other crates’ README template) or adjust the README/template to point at an existing asset.
  crates/multitude_macros_impl/README.md:2
• The README references ./logo.png, but crates/multitude_macros_impl/logo.png is not present in the repo, so the image will be broken anywhere the README is rendered. Add the expected logo.png (matching other crates’ README template) or adjust the README/template to point at an existing asset.

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