feat(multitude): add arena-aware Serde and Cow by geeknoid · Pull Request #587 · microsoft/oxidizer

geeknoid · 2026-07-17T15:27:05Z

Summary

Add a generic arena-aware Cow with borrowed and arena-owned forms, fallible and infallible copy-on-write mutation, and explicit allocator-aware cloning.
Add allocator-aware deserialization under multitude::de, including derive support, recursive containers, arena-backed dynamic values, resource limits, JSON helpers, and ordinary-Serde interoperability.
Add local and escape-capable root ownership plus inherent reusable deserialization for Multitude strings and vectors.
Harden chunk ownership and cross-thread teardown, tighten Send/Sync bounds, and fix ZST drain and Rc overflow handling.
Add matched Criterion and Callgrind benchmarks for Serde and allocation teardown, plus updated examples, architecture documentation, and generated performance reporting.

Validation

Full unit, integration, property, doctest, Miri, and Loom coverage.
Regression coverage for panic-safe cleanup, deserialization-limit enforcement, numeric replay, and derive hygiene.
100% LLVM line coverage for the new deserialization runtime, generic Cow, 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 · 2026-07-17T15:33:42Z

✅ 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

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Pull request overview

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

codecov · 2026-07-17T15:40:47Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (1feffd7) to head (20bbb5e).

Additional details and impacted files

@@            Coverage Diff            @@
##             main     #587     +/-   ##
=========================================
  Coverage   100.0%   100.0%             
=========================================
  Files         393      408     +15     
  Lines       33520    36702   +3182     
=========================================
+ Hits        33520    36702   +3182

Flag	Coverage Δ
linux	`84.3% <100.0%> (?)`
linux-arm	`84.3% <100.0%> (?)`
windows	`84.3% <100.0%> (?)`

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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