[WIP] compiler: extern fn

[witemple-msft]

This is a proof-of-concept for extern functions, (related: #7296)

• Re-enables fn declarations, with a required extern keyword, which bind a type FunctionType with kind: "Function".
• Adds FunctionType to the Type union.
• Like decorators, fn declarations must have an associated JS implementation (for now -- we may eventually allow implementations to be defined in TypeSpec itself).
• Changed parsing of fn declarations to allow mixed constraints in return type position.
• Implemented tspd extern signature generation for functions alongside decorators.
• Partially implements checking for CallExpression where target: FunctionType

This strategy replaces the extern alias proof of concept, allowing aliases to simply invoke functions:

extern fn transformVisibility(t: Reflection.Type, filter: valueof VisibilityFilter, nameTemplate: valueof string): Reflection.Type;

alias Read<
  T,
  NameTemplate extends valueof string = "Read{name}"
> = transformVisibility(t, #{ all: #[Lifecycle.Read] }, NameTemplate);

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/microsoft/typespec/@typespec/compiler@8119

commit: ed664a3

[github-actions]

❌ There is undocummented changes. Run chronus add to add a changeset or click here.

The following packages have changes but are not documented.

• ❌@typespec/compiler
• ❌@typespec/events
• ❌@typespec/html-program-viewer
• ❌@typespec/http
• ❌@typespec/json-schema
• ❌@typespec/openapi
• ❌@typespec/openapi3
• ❌@typespec/protobuf
• ❌@typespec/rest
• ❌@typespec/spector
• ❌@typespec/sse
• ❌@typespec/streams
• ❌@typespec/tspd
• ❌@typespec/versioning
• ❌@typespec/xml

Show changes

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[witemple-msft]

Some basic examples: https://cadlplayground.z22.web.core.windows.net/prs/8119/?e=%40typespec%2Fopenapi3&options=%7B%22options%22%3A%7B%22%40typespec%2Fopenapi3%22%3A%7B%22openapi-versions%22%3A%5B%223.1.0%22%5D%7D%7D%7D&c=bW9kZWwgRm9vIHsKICBiYXI6IEV4YW1wbGU8c3RyaW5nPjvFGHrKGGludDMyxRdxdXjKFyJhc2RmIscYzBnJSMVJc2U6IE15QWxpYXPKGn0KCmHEEskaVD4gPSBUW107Cg%3D%3D&vs=%7B%7D

You can see that the two properties where the type is Example<string> have the same first field (you need to use OpenAPI3 to render tuples, since the implementation in the Example template returns a tuple) because the instantiation is cached.

[markcowl]

Will review in detail in bit. Thinking about rolling this out:

• We should validate that there are no compile breaks in typespec-azure or typespec-azure-pr
• We should talk to TCGC folks about whether they want to support this for some scenarios
• We should think about preventing usage in Azure specs until TCGC is ready to support (linting rules)
• We should think about some http-specs scenarios for this
• We should add some website conceptual documentation