perf(multitude): optimize hot allocation paths and harden chunk lifecycle

[geeknoid]

Follow-up to the multitude rewrite focused on allocation throughput, memory-safety hardening, and richer runtime accounting.

[github-actions]

⚠️ Breaking Changes Detected

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/inherent_method_missing.ron

Failed in:
  Utf16String::into_arena_box_utf16_str, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/1f0b5970b95b02ee08d19ac3d38733fabb2e06f6/crates/multitude/src/strings/utf16_string.rs:320
  String::into_arena_box_str, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/1f0b5970b95b02ee08d19ac3d38733fabb2e06f6/crates/multitude/src/strings/string.rs:320
  Vec::into_arena_arc, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/1f0b5970b95b02ee08d19ac3d38733fabb2e06f6/crates/multitude/src/vec/freeze.rs:23
  Vec::try_into_arena_arc, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/1f0b5970b95b02ee08d19ac3d38733fabb2e06f6/crates/multitude/src/vec/freeze.rs:45
  Vec::into_arena_box, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/1f0b5970b95b02ee08d19ac3d38733fabb2e06f6/crates/multitude/src/vec/freeze.rs:74

If the breaking changes are intentional then everything is fine - this message is merely informative.

Remember to apply a version number bump with the correct severity when publishing a version with breaking changes (1.x.x -> 2.x.x or 0.1.x -> 0.2.x).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (fc13464) to head (97db72b).

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #498    +/-   ##
=======================================
  Coverage   99.9%   100.0%            
=======================================
  Files        336      337     +1     
  Lines      24829    25213   +384     
=======================================
+ Hits       24818    25213   +395     
+ Misses        11        0    -11     

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[Copilot]

Pull request overview

This PR is a follow-up set of changes in multitude aimed at improving allocation throughput and strengthening chunk lifecycle correctness, while expanding runtime accounting and updating tests/benches/docs to reflect the new behaviors.

Changes:

• Reworked shared-chunk refcounting (pre-credited surplus + local counter reconciliation) and hardened shared/local chunk lifecycle + freelist linkage.
• Refactored hot allocation paths to reduce per-iteration overhead (precomputed sizes, cold-path extraction, len==0 fast paths, fewer overflow checks in tight loops).
• Updated Vec freezing APIs (into_arc/into_box renames, plus new into_slice/into_slice_mut) and adjusted tests/benchmarks/perf reporting accordingly.

Reviewed changes

Copilot reviewed 50 out of 50 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
crates/multitude/tests/utf16.rs	Renames a test to match updated Vec/freeze naming.
crates/multitude/tests/mutants_extras.rs	Updates mutant-kill tests and comments for renamed/changed freeze APIs.
crates/multitude/tests/mutant_kills_post_fix.rs	Removes stats byte-count mutant-kill tests aligned with new stats semantics.
crates/multitude/tests/mutant_kills_more.rs	Narrows mutant-kill scope now that stats byte-count recording is removed.
crates/multitude/tests/dst.rs	Updates into_* calls and related commentary to into_box.
crates/multitude/tests/coverage_gaps.rs	Renames try_into_* coverage module/tests to try_into_arc.
crates/multitude/tests/coverage_extras.rs	Updates coverage tests for into_arc/into_box naming.
crates/multitude/tests/bolero.rs	Updates lifecycle fuzz tests to use into_arc/into_box; adjusts stats assertions.
crates/multitude/tests/audit_repro.rs	Adds regression test for ZST shared handouts advancing cursor (audit finding).
crates/multitude/tests/arena.rs	Updates a stats/mutant test to use into_box.
crates/multitude/tests/arena_vec.rs	Adds stats regression test for releasing retired oversized chunks on growth.
crates/multitude/tests/alloc_ref.rs	Replaces/expands stats tests to validate live wasted_tail_bytes behavior.
crates/multitude/src/vec/mutate.rs	Minor doc tweak related to shrink_to_fit arithmetic helper.
crates/multitude/src/vec/mod.rs	Updates module docs for new into_arc/into_box names; grow path releases retired oversized chunks.
crates/multitude/src/vec/freeze.rs	Renames freeze APIs and adds into_slice/into_slice_mut for O(1) arena-tied freezing.
crates/multitude/src/vec/basic.rs	Optimizes extend_from_slice capacity check to avoid hot-path overflow guards.
crates/multitude/src/strings/utf16_string.rs	Simplifies imports/paths and uses Vec/FromIteratorIn re-exports consistently.
crates/multitude/src/strings/string.rs	Same as above for UTF-8 String builder.
crates/multitude/src/lib.rs	Updates top-level docs for renamed freeze APIs and updated usage wording.
crates/multitude/src/internal/uninit.rs	Minor formatting cleanup.
crates/multitude/src/internal/shared_chunk.rs	Moves cache freelist link into header, adds wasted-tail retire tracking, and adds ref surplus helpers.
crates/multitude/src/internal/local_chunk.rs	Adds unified intrusive next link, wasted-tail retire tracking, and payload-contains helper.
crates/multitude/src/internal/drop_entry.rs	Fixes drop-entry placement for unaligned payload starts; adds regression test.
crates/multitude/src/internal/chunk_provider.rs	Adds live counters (bytes outstanding, wasted tail), alignment slack for oversized payloads, and updates cache linkage.
crates/multitude/src/internal/chunk_ops.rs	Adds stats retire accounting hook for chunk mutator drop/forget paths.
crates/multitude/src/internal/chunk_mutator.rs	Refactors payload range math, adds wasted-tail computation, and de-duplicates prefixed-slice allocation internals.
crates/multitude/src/internal/chunk_alloc.rs	Pads DST allocation layout to alignment to avoid UB when forming fat references.
crates/multitude/src/internal/arena_buf.rs	Removes empty-slice early return; relies on well-defined zero-length memcpy.
crates/multitude/src/arena/retired_local.rs	Introduces intrusive retired-local-chunk list replacing Vec<ChunkMutator<...>>.
crates/multitude/src/arena/reserve.rs	Adds “with precomputed size” reserve helpers for hot slice-copy paths.
crates/multitude/src/arena/mod.rs	Adds shared ref surplus tracking, retired-local list integration, and updated stats aggregation.
crates/multitude/src/arena/alloc_value.rs	Extracts cold oversized paths; adds ZST shared-handout cursor-advance guard; integrates new shared-ref acquisition.
crates/multitude/src/arena/alloc_utf16.rs	Adjusts utf16 alloc APIs to use new shared-ref acquisition and import paths.
crates/multitude/src/arena/alloc_unsized.rs	Uses new shared-ref acquisition for DST allocations; removes old stats byte accounting.
crates/multitude/src/arena/alloc_str.rs	Removes old stats byte accounting on string alloc paths.
crates/multitude/src/arena/alloc_slice_ref.rs	Adds len==0 fast paths + precomputed-size helpers; extracts cold oversized/callsite-spill avoidance.
crates/multitude/src/arena/alloc_slice_box.rs	Hoists payload byte computations; uses new shared-slice reserve-with-size helper.
crates/multitude/src/arena/alloc_slice_arc.rs	Same as above for Arc<[T]> slice allocations.
crates/multitude/src/arena/alloc_prefixed.rs	Uses new shared-ref acquisition; minor cleanup of DropEntry size/align usage.
crates/multitude/src/arena_stats.rs	Updates high-level docs to reflect which fields are live gauges vs lifetime counters.
crates/multitude/src/arc.rs	Updates From<Vec> conversion to use into_arc.
crates/multitude/src/allocator_impl.rs	Uses new shared-ref acquisition for allocator-API2 bridge allocations.
crates/multitude/scripts/perf_report.rs	Renames benchmark identifiers/groups for consistency with updated benches.
crates/multitude/README.md	Regenerates crate README to reflect renamed APIs and updated perf naming.
crates/multitude/examples/multitude_basic.rs	Updates example to use into_arc.
crates/multitude/docs/TODO.md	Removes now-obsolete TODO list after landing the described items.
crates/multitude/docs/PERF.md	Updates published perf table entries/names for the new benchmark identifiers.
crates/multitude/benches/gungraun_drop/linux.rs	Renames drop benchmarks to match perf-report naming.
crates/multitude/benches/gungraun_alloc/linux.rs	Updates bench names and reduces unnecessary black_box(&x) patterns; uses into_slice_mut for vec benches.
crates/multitude/benches/criterion_alloc.rs	Splits warm_arena into local/shared variants to avoid dead-weight chunk priming; updates vec/string freeze benches.

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[Copilot]

Pull request overview

Copilot reviewed 58 out of 58 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

Copilot reviewed 58 out of 58 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 59 out of 59 changed files in this pull request and generated 9 comments.

Comments suppressed due to low confidence (1)

crates/multitude/src/lib.rs:270

• The crate-level docs still claim the UTF-16 builder freeze is O(1) and “reuses the buffer in place”, but Utf16String::into_box now delegates to Arena::alloc_utf16_str_box(...), which copies into a fresh shared allocation (O(n)). This is user-facing and will mislead performance expectations.