[Typekit] - Add builtin.is(type: Type): boolean

[joheredi]

Introduce builtin.is(type: Type): boolean, which returns true for any type defined in the global TypeSpec namespace (i.e. built-in/standard library types).

[Copilot]

Pull Request Overview

This PR adds a new builtin.is(type: Type): boolean method to detect built-in types (including those defined in the global TypeSpec namespace and pure intrinsics) and supplements compiler logic with comprehensive tests.

• Implemented builtin.is() in the builtin kit with a helper to locate the proper namespace ancestor.
• Added extensive Vitest scenarios covering models, properties, unions, enums, scalars, aliases, and intrinsic types.
• Introduced getImmediateChildOfGlobal to support namespace ancestry resolution.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/compiler/src/typekit/kits/builtin.ts	Added is() method and getImmediateChildOfGlobal to identify built-in types based on namespace ancestry.
packages/compiler/test/typekit/builtin.test.ts	Added a suite of tests for builtin.is() covering various type scenarios and edge cases.

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

Show changes

@typespec/compiler - feature ✏️

Introduce builtin.is(type: Type): boolean, which returns true for any type defined in the global TypeSpec namespace (i.e. built-in/standard library types).

[timotheeguerin]

not sure this is worth it?

[joheredi]

Updated the tests to show when this is helpful

namespace TypeSpec {
        model InTypeSpec {
          foo: string;
        }
      }

      model Foo {
        bar: TypeSpec.InTypeSpec.foo;
      } 

Without this $.builtin.is(Foo.bar.type) would return true

[timotheeguerin]

is that always working when using mutators?

[bterlson]

I seem to recall at some point that we had special-cased TypeSpec so as to avoid mutating built-in types. Is that not the case? If it's not, then I think we might have more bugs in other places, e.g. the intrinsic is checks use reference equality.

[azure-sdk]

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