feat: expose mimalloc memory stats

[Wodann]

Expose mimalloc memory statistics behind a memory-stats feature flag

Purpose

EDR uses mimalloc as the global allocator of the N-API addon, which makes its native memory invisible to standard profiling tools: a heaptrack run against Hardhat 3's end-to-end benchmarks saw only 1.87 GB of malloc-attributed memory while the process RSS was 15.25 GB. Consumers currently have to approximate EDR's memory as rss − v8HeapTotal − external − arrayBuffers.

This PR exposes mimalloc's own statistics API through N-API, behind a new opt-in cargo feature memory-stats:

• memoryStats(): MiMemoryStats — wraps mi_process_info; returns elapsed/user/system time (ms), current/peak RSS, current/peak commit, and hard page faults as BigInts. The doc comments distinguish OS process-level values (RSS) from mimalloc-internal ones (commit), so consumers don't conflate them.
• memoryReport(): string — wraps mi_stats_print_out; returns mimalloc's human-readable stats report.

A resetMemoryStats() wrapper was considered and dropped: mi_stats_reset is deprecated and effectively a no-op in the vendored mimalloc v3 (it merges thread-local stats instead of resetting, so peaks survive), which would silently break per-phase measurements. Per-phase deltas for the current values can be computed by diffing two memoryStats() snapshots; peaks genuinely cannot be reset in v3.

The bindings come from a direct optional dependency on libmimalloc-sys (with its extended feature), pinned to the version already locked via the mimalloc crate — cargo unifies the two, so this adds bindings only, not a second allocator build. The existing mimalloc dependency and #[global_allocator] are unchanged.

Packaging

Published npm packages deliberately do not ship this feature: build:publish and build:typingFile remain --features op, so the released binaries and the committed index.d.ts are unchanged. The APIs are only available in local builds via the new pnpm build:memory-stats script (mirroring build:scenarios/build:tracing).

Stats detail

The depth of memoryReport() is fixed at compile time by mimalloc's MI_STAT level. EDR's builds get MI_STAT=0 ("only essential") in every cargo profile — libmimalloc-sys only raises it via its debug feature (which also enables mimalloc's internal assertions), and we don't enable it; a dev-profile report is byte-identical to a release one.

MI_STAT	What the report exposes	Cost
0 (what EDR builds, all profiles)	Arena stats (reserved, committed current+peak, purge/mmap/commit counters), page stats (touched, pages, abandoned, reclaim/retire counters), process info (threads, times, peak RSS/commit, page faults)	none
1	Adds per-heap block allocation totals (binned/huge/total bytes of live blocks), tracked on every alloc/free	small
2 (requires libmimalloc-sys debug feature)	Adds per-size-class bin table, allocation counts, and malloc req (exact requested bytes)	measurable, plus internal assertions

Note the practical consequence: the report attributes memory at the arena/commit level — which is the number that explains the RSS gap motivating this PR — but the per-size-class malloc breakdown readers might expect from mimalloc stats is compiled out in EDR's configuration. Everything memoryStats() returns is MI_STAT=0 data, so it is fully accurate in our builds.

Testing

• New Rust unit tests constrain the out-param→field mapping of the mi_process_info wrapper: peak_commit >= current_commit must hold, a touched 256 MiB allocation must raise current_commit by ≥200 MiB and drive peak_rss ≥200 MiB, and memoryReport() must be non-empty. They pass under --features memory-stats and --all-features, so CI's llvm-cov --all-features job runs them automatically. (Note: peak_rss >= current_commit is deliberately not asserted — zeroed allocations commit pages without touching them, so commit can legitimately exceed RSS.)
  • This is the first Rust test in edr_napi, linking via the existing dyn-symbols dev-dependency. Before the introduction of dyn-symbols having Rust tests was not possible.
• cargo check --no-default-features --all-targets and cargo clippy --all-targets --all-features -- -D warnings are clean; cargo tree confirms a single libmimalloc-sys in the graph.
• Manual smoke test: built the addon with pnpm build:memory-stats and loaded it in Node — memoryStats() returns BigInt values and currentCommit moved from 4.65 MB to 9.04 MB when allocating through an EDR API; memoryReport() returns a real mimalloc stats report. Notably, Buffer.alloc does not move the commit figures (Node's heap isn't mimalloc), which is exactly the RSS-vs-commit distinction the API documents.
• Committed index.d.ts/index.js are untouched (generated with --features op as before), keeping the typings-check CI job green.

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Latest commit: 1c8f1e9

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[Copilot]

Pull request overview

Adds an opt-in memory-stats Cargo feature to expose mimalloc process/statistics data through the edr_napi N-API surface, enabling consumers to measure native allocator usage that’s otherwise hidden from typical JS/V8 profiling.

Changes:

• Introduces N-API exports for memoryStats(), memoryReport(), and resetMemoryStats() (feature-gated).
• Adds libmimalloc-sys as an optional dependency behind the new memory-stats feature and wires up a local build script (pnpm build:memory-stats).
• Adds Rust unit tests for the new memory stats/report/reset functionality.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/edr_napi/src/memory_stats.rs	New feature-gated N-API bindings to mimalloc stats APIs plus unit tests.
crates/edr_napi/src/lib.rs	Exposes the new memory_stats module from the crate root.
crates/edr_napi/src/cast.rs	Adds usize -> BigInt conversion used by mimalloc stats out-params.
crates/edr_napi/package.json	Adds build:memory-stats script for local builds.
crates/edr_napi/Cargo.toml	Adds optional libmimalloc-sys dependency and memory-stats feature.
Cargo.lock	Lockfile updates for libmimalloc-sys and transitive deps.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.93%. Comparing base (a69221b) to head (1c8f1e9).

Files with missing lines	Patch %	Lines
crates/edr_napi/src/cast.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1602       +/-   ##
===========================================
- Coverage   79.89%   25.93%   -53.96%     
===========================================
  Files         452      423       -29     
  Lines       78956    67715    -11241     
  Branches    78956    67715    -11241     
===========================================
- Hits        63078    17562    -45516     
- Misses      13697    49231    +35534     
+ Partials     2181      922     -1259     

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