Shrink hello-world bundle from 251 KB to 173 KB (-31%) via bundler hints

[NullVoxPopuli-ai-agent]

Two purely-additive bundler hints — no source-file restructuring — that let consumer bundlers (vite/rolldown, webpack, esbuild) tree-shake aggressively through ember-source's internal barrel re-exports.

	raw	gzip
before	251.05 KB	79.75 KB
after	172.99 KB	55.31 KB

Measured against smoke-tests/v2-app-hello-world-template. Classic v2-app-template and v1 app-template smoke tests still build and pass.

Changes

1. "sideEffects": false on ember-source/package.json

Declares that no module in this package has top-level side effects that need to be preserved if the module's exports are unused. Bundlers can then DCE re-exports through index.ts barrels that currently anchor the rest of the graph in place.

This is safe in practice because rollup's chunking groups symbols with their side effects:

• The chunk containing the classic Component class also contains the setInternalComponentManager(CURLY_COMPONENT_MANAGER, Component) call.
• The chunk containing helper.ts exports also contains its top-level setHelperManager(...) registrations.
• The chunk containing setupApplicationRegistry also contains environment.ts's setGlobalContext({...}) call.
• Opcode handlers, runloop init, validator init — same pattern.

Importing a symbol from a chunk pulls the chunk's side effects along; apps that don't reach those symbols don't need their side effects either.

It's a contract: anyone refactoring ember-source's internals later must ensure side-effect-only modules remain reachable through an exported symbol, or explicitly switch to a sideEffects array listing the relevant chunk patterns.

2. treeshake.moduleSideEffects callback in rollup.config.mjs

The package-level sideEffects: false declarations on @glimmer/debug, @glimmer/debug-util, and @glimmer/local-debug-flags get lost when rolldown emits shared chunks (debug code from these packages can leak into chunks that the renderer-only path then pulls in). The callback re-asserts module purity at the chunk level so leaked debug code drops out of the renderer-only path.

Why this works without source changes

Bundlers default to assuming any imported file might have load-bearing top-level side effects, so they preserve every imported file conservatively. sideEffects: false is a contract telling the bundler "you can trust me — none of my files have side effects worth preserving on their own."

That unlocks aggressive tree-shaking through barrel index.ts files: a barrel that does export { foo } from './foo'; export { bar } from './bar' no longer has to load ./bar when only foo is used.

Relationship to other PRs

• Shrink hello-world bundle from 243 KB to 109 KB (-55%) #21359 (the structural refactor on top of Remove barrel file imports from internal code for better tree-shaking #21350) gets to ~109 KB / ~34 KB gzip (-55%).
• Shrink hello-world bundle from 251 KB to 174 KB (-30%) #21360 ports Shrink hello-world bundle from 243 KB to 109 KB (-55%) #21359 to apply against main directly, landing at ~174 KB / ~56 KB gzip (-30%).
• This PR is the config-only subset of Shrink hello-world bundle from 251 KB to 174 KB (-30%) #21360 — virtually all of Shrink hello-world bundle from 251 KB to 174 KB (-30%) #21360's gain (172.99 vs 174.28 KB) with zero source-file changes, just two declarative additions in two files.

These three PRs stack cleanly: bundler hints (this PR) → structural refactor (#21360 / #21359) → barrel-import cleanup (#21350).

Test plan

• smoke-tests/v2-app-hello-world-template builds (251 → 173 KB / 80 → 55 KB gzip)
• smoke-tests/v2-app-template (classic v2 app) builds + 1/1 test passes
• smoke-tests/app-template (v1 app) builds + 1/1 test passes
• pnpm test:node 20/20
• pnpm vite build --mode development --minify false (full dev test suite app) builds clean
• Browser tests pass in CI

🤖 Generated with Claude Code

[NullVoxPopuli]

📊 Size report

Tarball size — 1.2 MB → 1.2 MB

dist/dev   -1.52%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./packages/@glimmer/destroyable/index.js	6.4 kB / 1.5 kB	-94.6%↓346 B / -89.1%↓160 B
./packages/@glimmer/opcode-compiler/index.js	1.4 kB / 443 B	-16.8%↓1.1 kB / -17.8%↓364 B
./packages/@glimmer/vm/index.js	565 B / 288 B	130%↑1.3 kB / 89%↑545 B
./packages/shared-chunks/assert-{hash}.js	619 B / 299 B	-85.3%↓91 B / -72.2%↓83 B
./packages/shared-chunks/capabilities-{hash}.js	1.8 kB / 519 B	-11%↓1.6 kB / -13.1%↓451 B
./packages/shared-chunks/compiler-{hash}.js	178.8 kB / 34.2 kB	-0.07%↓178.7 kB / -0.11%↓34.2 kB
./packages/shared-chunks/curried-D5PiYPSB.js	—	163 B / 100 B
./packages/shared-chunks/debug-to-string-CFb7h0lY.js	1.7 kB / 585 B	—
./packages/shared-chunks/fragment-Cc5k9Oy4.js	30.4 kB / 7.4 kB	—
./packages/shared-chunks/immediate-untnQeUA.js	—	1 kB / 350 B
./packages/shared-chunks/registers-ylirb0dq.js	973 B / 365 B	—
Total (Includes all files)	2 MB / 476.3 kB	-1.52%↓2 MB / -1.66%↓468.4 kB

---

dist/prod   -1.45%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./packages/@glimmer/opcode-compiler/index.js	1.3 kB / 416 B	-10.6%↓1.1 kB / -10.1%↓374 B
./packages/@glimmer/vm/index.js	565 B / 288 B	130%↑1.3 kB / 89%↑545 B
./packages/shared-chunks/capabilities-{hash}.js	1.5 kB / 392 B	-7.44%↓1.3 kB / -13.5%↓339 B
./packages/shared-chunks/curried-D5PiYPSB.js	—	163 B / 100 B
./packages/shared-chunks/fragment-D7nBU9ae.js	30.9 kB / 7.5 kB	—
./packages/shared-chunks/immediate-untnQeUA.js	—	1 kB / 350 B
./packages/shared-chunks/registers-ylirb0dq.js	973 B / 365 B	—
Total (Includes all files)	1.9 MB / 436 kB	-1.45%↓1.8 MB / -1.52%↓429.4 kB

---

smoke-tests/v2-app-hello-world-template/dist   -31.1%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./assets/main-{hash}.js	251.1 kB / 68.6 kB	-31.1%↓173 kB / -30.7%↓47.6 kB
Total (Includes all files)	251.4 kB / 68.8 kB	-31.1%↓173.3 kB / -30.6%↓47.7 kB

_{🤖 This report was automatically generated by wyvox/pkg-size}