feat(plugin-legacy): make terser optional if renderLegacyChunk false

[bluwy]

Description

terser is only needed if we're rendering legacy chunks. If renderLegacyChunks: false, we don't need terser as the existing code also actually doesn't need it, per

vite/packages/plugin-legacy/src/index.ts

Lines 349 to 360 in 5f50693

	if (!genLegacy) {
	return null
	}
	// @ts-ignore avoid esbuild transform on legacy chunks since it produces
	// legacy-unsafe code - e.g. rewriting object properties into shorthands
	opts.__vite_skip_esbuild__ = true
	// @ts-ignore force terser for legacy chunks. This only takes effect if
	// minification isn't disabled, because that leaves out the terser plugin
	// entirely.
	opts.__vite_force_terser__ = true

where const genLegacy = options.renderLegacyChunks !== false.

Ref #9435 (comment)

Additional context

So far tested manually with the legacy playground only.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Note: This function is only called in two places. One for building the modern polyfilll chunk, two for legacy polyfill chunk. They pass es and iife respectively. So this changes that modern polyfills uses esbuild instead, which should be fine.

[sapphi-red]

I'm not sure about this. Most users would be using plugin-legacy for legacy chunks and those users will need to install terser by themselves. Also we don't recommend users to include polyfill automatically for modern chunks.

[bluwy]

I think plugin-legacy is also viable for generating modern-polyfills only as there's no other official/simple way to do so.

Also we don't recommend users to include polyfill automatically for modern chunks.

I think that only applies for modernPolyfills: true which I agree, it's better to specify as an array instead so we get control of what is being polyfilled, which is what I've done in the past. (Also great for build perf too)

[sapphi-red]

If someone is only using renderLegacyChunk: false + modernPolyfills: ['es.promise.finally'], I think this could be done without this plugin like this: stackblitz
This injects polyfills during dev too. But with transformIndexHtml, it could be removed for dev if needed.

[bluwy]

Interesting, I guess doing it that way is an option too, but I think it's more laborious for those who want to quickly set it up or not familiar with the tools, and harder to achieve this trick too. A plugin would simplify all the steps. 🤔

[sapphi-red]

If we want to provide this feature, I think providing it with a different plugin is better.
plugin-legacy depends on @babel/standalone which is 7.16MB (packagephobia) and this is far more bigger than terser which is 3.83MB (packagephobia).
For this feature (injecting selected polyfill), babel is not needed and if we want to cut terser, we probably should cut @babel/standalone first. (We can cut regenerator-runtime and systemjs)

	@babel/standalone	terser
generate modern chunk	❌	❌
generate legacy chunk	⭕	⭕
detect required polyfill	⭕	❌

[bluwy]

Closing as terser is also now required for polyfills chunk minifying since #9635