Generic AST for EcmaScript

[kdy1]

I can make EcmaScript AST generic, but it has pros and cons.

It will make, e.g. migrating built tools for Vue or Svelte to Rust much easier, but it requires a decent amount of work, and it will make compilation slower.
So I want to know if there's a team that wants to do that.

Idea

API will look similar to Stylesheet of lightningcss, but with much more complexity.

pub trait AstExt {
    type Stmt;
    type Expr;
    type Pat;
}

pub type Never = Infallible;

pub struct Standard;

impl AstExt for Standard {
    type Stmt = Never;
    type Expr = Never;
    type Pat = Never;
}


pub struct Program<E: AstExt = Standard> {
    Module(Module<E>),
    Script(Script<E>),
}

pub struct Expr<E: AstExt = Standard> {
     // Many generics variants...
    Ext(E::Expr),
}

Pros

Reduced binary size & Improved performance of core transforms

We can extract TypeScript nodes as extensions, and it will make all other visitors faster and smaller.

Cons

Compile time

[kdy1]

Note that this will make compilation much slower.

This is cc to hear opinions about the compile time.

@dsherret
@devongovett
@Boshen

[devongovett]

Maybe it's possible to use dyn? You lose some performance to dynamic dispatch but would improve compile time. If you expect a lot of different extensions, maybe that's the way to go. For lightningcss we basically only expect 1 per program, so the compile time isn't really affected. It was pretty complicated to pass all the generics everywhere though. 😉

[kdy1]

Hmm.. I didn't think deeply about dyn because of Wasm plugins, but I found that we don't need them. I'll think about it more. Thank you! I think it will work.

[kdy1]

It does not work because of various traits. e.g. PartialEq

[kdy1]

Oh I found a way