fix: error at compile time on unsupported TypeScript language features

[Rich-Harris]

part of #11502 - to close it completely, we also need to look at using and possibly implement heuristics within bundler plugins to give more details

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[Rich-Harris]

It seems that acorn-typescript doesn't parse enums using (we should probably open an issue), while acorn does parse decorators but esrap doesn't print them (we should probably open an issue).

What other language features do we need to consider?

[dummdidumm]

• constructor(private/public/protected x)
• namespace X { export function y() { ... } } (though it seems we don't strip namespaces that only consist of types currently, and neither do we strip declare module, nor declare const/function/etc, which we should)
• class X { accessor y = .. }
• using ... (because it's not stage 4 yet / not in acorn)

[Rich-Harris]

oh wait i mean using instead of enums above

[Rich-Harris]

Also I misspoke when I said Acorn parses decorators — it's actually acorn-typescript doing that.

There is an effort underway to add TS support to esrap, which would include decorators and everything else. If that landed, would it in fact make sense to just support the features in question?

[dummdidumm]

I mean theoretically yes, but it would certainly be a non-trivial effort. The transformations that TS does under the hood are somewhat involved (you can look at the JS output tab in their playground).

[Rich-Harris]

I'm not suggesting we transform TypeScript syntax, just that we understand it enough that it doesn't mess up the analysis, and then we emit TS rather than JS

[dummdidumm]

Oh so you're saying that whatever comes after Svelte has to then know "ok this is a typescript file now so I'm gonna transform it using esbuild/tsc/whatever"? @dominikg how would that impact the toolchain?

[dominikg]

allowing the svelte compiler to emit ts that then has to be postprocessed is going to create a lot of effort. even if it works in vite, what about all the other bundlers or even just our repl?

[Rich-Harris]

Unfortunately this is the kind of thing the bundler designers of the world didn't anticipate — everything is more or less based on file extensions, so you'd need to configure the downstream transformers to keep an eye out for .svelte files as well.

Have thought for a while that the ecosystem would be more robust if instead of just returning { code, map } we could return various well-known attributes like language/version/whatever, so that a TS plugin could say 'oh, this .potato file generated some TypeScript — guess I'm up'. Hindsight is 20/20.

[dummdidumm]

I added validation for all features we can validate (i.e. everything that came to my mind except using which ts-acorn doesn't parse yet). This should be a good stop gap solution until we figure out the next steps (or decide that this is the end state, which I would also be good with, given that changing toolchains is a big effort)