Does Rspack configuration only affect the modern build (web.browser)?

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1

I’m using Meteor with the Rspack integration and I’m trying to understand how the build pipeline is split between the modern and legacy web architectures.

What I tested:

  • I set Rspack jsc.target to esNext.
  • Then I ran meteor build and inspected the output bundle under bundle/programs/.

What I observed:

  • In programs/web.browser (modern), the generated JS still contains lots of modern syntax such as ?? (and other ES2020+ features), which matches the esNext expectation.
  • In programs/web.browser.legacy (legacy), those modern syntaxes (e.g. ??) are completely absent. It looks like everything is transpiled down to ES5.

Questions:

  • Is it expected that Rspack config (like jsc.target / env) only affects the modern client build (web.browser), while the legacy build (web.browser.legacy) is still handled by Meteor’s legacy toolchain?
  • If yes, could this be documented more explicitly? I suspect many users assume Rspack config applies to both modern and legacy outputs.
  • Looking forward: is there a plan to let Rspack also take over web.browser.legacy? For example, introducing something like Meteor.modern() (or a similar API) so developers can switch Rspack targets/jsc.target/env between modern and legacy builds explicitly.

Thanks in advance for any clarification!