Declare side-effecting modules so bundlers have better hints for what can be tree-shaken#21365
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📊 Size reportTarball size — dist/dev No changes
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smoke-tests/v2-app-hello-world-template/dist -35.2%↓
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This was referenced May 3, 2026
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a manual lint, of sorts
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The lint for this was extracted to: #21378 |
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Two purely-additive bundler hints that let consumer bundlers (vite/rolldown, webpack, esbuild) tree-shake aggressively through ember-source's internal barrel re-exports without requiring any source-file restructuring. 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. The bundler 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: any chunk containing the classic `Component` class also contains the `setInternalComponentManager(CURLY_COMPONENT_MANAGER, Component)` call, the `setHelperManager` registrations live in the chunk that holds `helper.ts`, etc. 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. 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. Measured against `smoke-tests/v2-app-hello-world-template`: | | raw | gzip | | - | - | - | | before | 251.05 KB | 79.75 KB | | after | 172.99 KB | 55.31 KB | Classic `v2-app-template` and v1 `app-template` smoke tests still build and pass. `pnpm test:node` 20/20. `pnpm vite build --mode development` (full dev test suite app) builds clean.
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Important
The key thing is that the side-effects signal is only relevant if nothing is imported from a file.
Demo:
bundler hints that let consumer bundlers (vite/rolldown, webpack, esbuild) tree-shake aggressively through ember-source's internal barrel re-exports without requiring any source-file restructuring.
for the ember-source dist/tarball, the result is that chunks shake out a bit differently.
on webpack docs:
on svelte's docs
https://github.com/sveltejs/kit/pull/10691/changes#diff-4b03f147d0e7e4750663fcdb0ef3bf8ba11ad0bc47204456bf5d7aa5fd9a7e00R129
They do recommend explicitly declaring which modules have side-effects (which is something I want to do, I just want it generated)
this person says sideEffects is "standard" vitejs/vite#14321 (comment)
from rollup:
I believe this means that it happens that if something is used from a file, it can have side-effects within that file -- which is good.
Because, say you never use helpers:
we want:
to be removed, because you never used them.
let's say that file also registers helper managers.
the output removes the import entirely.
but because you don't use helpers, there is no behavior loss.
If you did
then the file is used, and any side-effects within would remain, because deeper analysis occurs.
however, if we declared
@ember/helperas having side-effects in pacakge.json, we'd likely not get the correct stripping behavior that we want.so, it is "less scary" to have all side-effects actually live in private files so that this "did you import it?" check and deeper analysis trigger isn't directly controlled by users