[Breaking Change] Put plugin-legacy logic inside Vite core, for simplicity & flexibility and avoiding future breaking changes

[Tal500]

Description

If I understand correctly, since Vite must use terser for minification of legacy chunks (and not the esbuild one that do some issues), and Vite didn't want to introduce the terser dependency by default, we have that plugin-legacy is not embedded in Vite core.
But this is problematic, since the plugin-legacy isn't fully orthogonal from Vite core plugins (meaning that some plugins depends on plugin-legacy behavior), we may benefit from embedding this plugin as a core plugin, so we can modify its internal behaviour without the worry of breaking changes, without worrying to not be compatible with some plugin-legacy versions.

Suggested solution

To embed the plugin-legacy logic in Vite core, without introducing the terser dependency, I suggest to put all the logic of plugin-legacy inside an internal Vite core plugin, say:

// plugin-legacy-internal.ts
export interface LegacyConfig {
};
function pluginLegacyInternal(config: LegacyConfig) {
// ...
}

And then, the plugin-legacy will be a "dummy" package, containing the dependency to the terser package, together with the following typescript file:

// plugin-legacy-public.ts
import { pluginLegacyInternal, type LegacyConfig } from '/path/to/plugin-legacy-internal.ts';

export { type LegacyConfig, pluginLegacyInternal as pluginLegacy };
// ...
}

This way, we introduce the ability to have flexible changes in the logic of plugin-legacy, while keeping the user experience similar to what happens today.

I suggest this breaking change now, since Vite is working on the breaking version 4.0.

Alternative

No response

Additional context

As a proof of concept, this will make implementing #10383 very easy, while currently it feels you need to juggle with magic token strings and be extra careful to implement it right and safely!

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[patak-dev]

It isn't only about Terser, but also about the need for Babel. I think plugin-legacy living in its own package has served us well, even considering the issues you raise here.

Lets close this issue for the moment and try to rework the current coupling by exposing some of Vite's internal APIs or other config options so we also allow other implementations of plugin legacy in the future, for example using SWC.