Support destructuring/pattern matching of record rest elements

[tsnobip]

Today, we support pattern matching of variant subtypes:

let greetAnimal = (animal: animals) => {
  switch animal {
  | ...pets as pet => greetPet(pet)
  | ...fish as fish => greetFish(fish)
  }
}

The goal of this issue is to design a way to pattern match on the rest elements of a record:

module PackageManagerConfig = {
  type t = {
    packageManagerName?: string,
    runtimeVersion?: string,
  }
}

module StyleConfig = {
  type t = {
    styleName?: string,
    theme?: string,
  }
}

module Config = {
  type t = {
    ...PackageManagerConfig.t,
    ...StyleConfig.t,
  }
}

let config = {Config.packageManagerName: "yarn", theme: "dark"}
let { ?Config.packageManagerName, ?runtimeVersion, ...StyleConfig.t as rest } = config

This would create a value rest of type StyleConfig.t.

Ideally, the compiler should check that no fileds that are not in StyleConfig.t are left in the rest value, this way the shape of the rest record is indeed correct and doesn't have extra fields.

This would allow to write in rescript a common pattern in JS/TS in react:

function MyComponent({ className, children, ...props }) {
  return (
    <div className={`w-full ${className}`} ...props >
     {children}
    </div>);
}

Today, this requires props drilling which has two drawbacks:

• it can be cumbersome when you have many props
• it changes unprovided props into undefined, which changes the behavior of props spreading when it's not the first argument of the component (undefined props erased overridden props while absent props do nothing).