perf(next-custom-transforms): Replace non-Sync Rc<DashMap<_, _>> usage with Rc<RefCell<FxHashMap<_, _>>>

[bgw]

This is a follow-up to #75007

DashMap is needed for situations where you have many threads (or tokio tasks) modifying the same data at the same time. It introduces a lot of memory and CPU overhead to create individually lockable RwLock shards. This type of data structure implements Sync, indicating that it can be shared between threads.

Rc is not Sync (Arc is Sync), so wrapping DashMap in Rc doesn't make a lot of sense: You're paying a pretty high overhead for the ability to share data between threads, and wrapping it in something that does not support sharing data between threads.

This visitor code is entirely single-threaded, so just use Rc<RefCell<FxHashMap>> instead. RefCell is a very cheap single-threaded version of a Mutex. FxHashMap is just a HashMap with the faster FxHash algorithm used.

Closes PACK-3871

[bgw]

• perf(turbopack): Migrate as many data structures to FxHash as possible #75546
• refactor(turbopack): Define and use an FxDashMap alias everywhere #75538
• perf(next-custom-transforms): Replace non-Sync Rc<DashMap<_, _>> usage with Rc<RefCell<FxHashMap<_, _>>> #75534 👈 (View in Graphite)
• perf(turbopack): Update rustc-hash from 1.1.0 to 2.1.0 #75531
• chore(turbopack): Remove unmaintained fxhash crate, use rustc-hash instead #75530
• canary

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[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js bgw/use-cache-fxhashmap	Change
buildDuration	19.2s	17.7s	N/A
buildDurationCached	16.9s	14.3s	N/A
nodeModulesSize	391 MB	391 MB	✓
nextStartRea..uration (ms)	467ms	452ms	N/A

Client Bundles (main, webpack)

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build cache Overall increase ⚠️

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index.pack gzip	74 kB	74.6 kB	⚠️ +541 B
Overall change	2.18 MB	2.18 MB	⚠️ +1.03 kB

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Diff for main-HASH.js

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Commit: 967c0cd

[bgw]

Merge activity

• Feb 3, 10:23 PM EST: A user merged this pull request with Graphite.