Add validation for keys in PickProperties

[dmnorc]

Keys Validation for PickProperties

Description

This PR introduces key validation for the pickProperties decorator in the TypeSpec compiler. The changes ensure that only valid keys are used when picking properties from a model. If an invalid key is provided, a diagnostic error will be emitted.

Changes

• Updated the decorators.ts file to include the validation checks.
• Added corresponding tests in decorators.test.ts to verify the new validation logic.

Related Issues:

#4369

[dmnorc]

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[witemple-msft]

This seems like a good idea.

Could you run pnpm -w change add to create a change record for this (and fill in a small message describing the change)? We use this to create release notes etc.

I have some comments on the implementation and some things that should be accounted for.

[witemple-msft]

I think we need to account for and test for correct handling of inheritance here. The property might not be a property of the template argument itself, but one of its ancestors.

model X {
  a: string;
}

model Y extends X {}

model Test is PickProperties<Y, "a">

[witemple-msft]

I don't think this is the right error message to give. We use this diagnostic internally when we are checking the assignability of objects, like this:

const a: {} = #{ b: "abc" };

Object value may only specify known properties, and 'a' does not exist in type '{}'.

I think we need to add a better/shorter message for this case. Something like

Property 'notMee' does not exist in type 'OriginalModel'.

[witemple-msft]

We need to be careful here, because this will put the error on the invocation of the decorator, which is on the template. We would want to put it at least on the second template argument of the template, and best if we can put it specifically on the string that isn't a property.