Improve IDE integration for inferred object properties to support Go To Definition, Rename Symbol, etc.

[bentefay]

This PR fixes the issue I raised here: #1115

To summarise, the properties of types declared using z.infer on z.object({ ... }) do not support IDE language server features such as Find All References, Rename Symbol or Go To Definition.

To take a concrete example using the latest version of Zod in VS Code, the language server features Find All References, Rename Symbol and Go To Definition do not work when you select the property prop in the code below:

const Example = z.object({ prop: z.string() });

type Example = z.infer<typeof Example>;

const instance: Example = { prop: "value" };

As you no doubt know, these features are incredibly useful for working on TypeScript projects, particularly as they grow in size.

These features are broken because z.object({ ... }) uses a mapped type, addQuestionMarks, that transforms its keys using requiredKeys and optionalKeys. Unfortunately, due to a limitation in the TypeScript compiler (microsoft/TypeScript#47813), transforming keys in a mapped type prevents the compiler from linking the mapped type properties to their definition.

Fortunately, it is possible to preserve the behaviour of addQuestionMarks without mapping the keys on one side of the intersection (by taking advantage of the fact that (T | undefined) & T equals T):

export type addQuestionMarks<T extends object> = {
    [k in keyof T]?: T[k];
  } & { [k in requiredKeys<T>]: T[k] };

It appears that the built-in type helpers Omit and Pick are also special cased to preserve the linkage between mapped keys, so I was able to use those for object({}).extend(), object({}).pick() and object({}).omit():

export type extendShape<A, B> = Omit<A, keyof B> & B;

These changes preserve the existing behaviour while enabling the IDE features for z.object({ ... }) as well as when using z.object({ ... }).merge(z.object({ ... })), z.object({ ... }).partial(), z.union(), z.object().pick and z.object().omit.

I have added a number of tests that use the TypeScript language server (via the library ts-morph) to verify that Go To Definition (and by extension the other features) work correctly. I have also added additional tests around the types inferred from z.object to ensure I have not broken existing behaviour.

What do you think? Happy to make any changes. Thanks!