Consider migrating away from `tsd` to support TS Native

[chriskrycho]

Prerequisites

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Issue

Today, Mongoose’s type tests use tsd, which uses a fork of the TS compiler to validate the types. This makes it impossible to execute the test suite against the TypeScript native rewrite, and it is unclear what the future of tsd will be. Switching to use a type assertion library like expect-type (or any other that purely works with types, rather than forking the compiler) would make it possible to begin regularly testing Mongoose’s types against tsgo.

That in turn will allow early adopters (like Vanta, where I am actively trying to get us to be able to use tsgo!) to flush out issues and help get them fixes in the appropriate place.

(Really sorry about my off-topic comments. Please ignore. I decided to suspend my account.)

[vkarpov15]

This should be doable with some caveats. The only helpers we use from tsd are expectType, expectAssignable, expectNotAssignable, and expectError. The first 3 are one-liners (at least with how Mongoose uses them). expectError is trickier to replace, but we could just use // @ts-expect-error for cases when we want to test that a certain bit of code is invalid.

We'll try this out in the next couple of weeks and see how it goes.

[chriskrycho]

I’m pushing pretty hard on this for us, so I’m happy to see if I can push one up in the next couple days if you’d like! I can either use expect-type (which has the most robust type assertions I know of, and is my go-to for these kinds of things) or I can use the one-liners inline. Have a preference?