feat(compiler): allow inline decorators on declaration expressions

[timotheeguerin]

Note

Stacked on top of #11019 (declarations-as-expressions). This PR targets main, so its diff currently includes the decl-expr commits as well — it should be reviewed/merged after #11019, or rebased once that lands.

Summary

Builds on the declaration-expressions feature to allow decorators to be applied inline to model, enum, union, and scalar declarations used in expression position.

model Foo {
  status: @doc("the current status") enum { active, inactive };
  inner: @doc("nested model") model Inner { x: string };
}

Changes (incremental over #11019)

• Parser (parser.ts): the @ case in parsePrimaryExpression now parses a decorator list and, when followed by a model/enum/union/scalar keyword, dispatches to the corresponding parse*Statement (with pos captured at @ so the node span includes the decorators). Decorators before any other expression still report invalid-decorator-location ("Cannot decorate expression."). No checker changes were required — parse*Statement already plumbs decorators onto the AST node and the checker already applies them.
• Formatter (printer.ts): the four print*Statement printers now pass tryInline: isInExpressionPosition(path), keeping decorators on the same line in expression position while statement-position decorators still break to their own line. Output is idempotent.
• Grammar spec (spec.emu.html): added the optional DecoratorList? prefix to the four *DeclarationExpression productions.

Tests

• Checker tests verifying the decorator is applied (via getDoc) for anonymous enum, named model, keyword union, and scalar declaration expressions, plus a negative test that non-declaration expressions still reject decorators.
• Formatter tests covering inline formatting for each kind.

Validation

• @typespec/compiler builds clean
• declaration-expressions.test.ts and formatter.test.ts pass (254 tests)
• Prettier + oxlint clean on changed files

[pkg-pr-new]

Open in StackBlitz

@typespec/compiler

npm i https://pkg.pr.new/@typespec/compiler@11035

@typespec/html-program-viewer

npm i https://pkg.pr.new/@typespec/html-program-viewer@11035

@typespec/json-schema

npm i https://pkg.pr.new/@typespec/json-schema@11035

@typespec/openapi

npm i https://pkg.pr.new/@typespec/openapi@11035

@typespec/openapi3

npm i https://pkg.pr.new/@typespec/openapi3@11035

@typespec/versioning

npm i https://pkg.pr.new/@typespec/versioning@11035

commit: 3de49f4

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler
• ✅ @typespec/html-program-viewer
• ✅ @typespec/json-schema
• ✅ @typespec/openapi
• ✅ @typespec/openapi3
• ✅ @typespec/versioning

Show changes

@typespec/compiler - feature ✏️

Allow augment decorators (@@) to target model, enum, union, and scalar declarations used in expression position (reached via a navigation reference such as ::type).,> ,> tsp,> model Foo {,> status: enum { active, inactive };,> },> ,> @@doc(Foo.status::type, "the current status");,>

@typespec/compiler - feature ✏️

Allow decorators to be applied inline to model, enum, union, and scalar declarations used in expression position.,> ,> tsp,> model Foo {,> status: @doc("the current status") enum { active, inactive };,> inner: @doc("nested model") model Inner { x: string };,> },>

@typespec/json-schema - feature ✏️

Support model, enum, union, and scalar declarations used in expression position. Anonymous declaration expressions are inlined, while named ones are hoisted into their own schema.,> ,> tsp,> model Foo {,> status: enum { active, inactive }; // inlined,> unit: scalar extends string; // inlined,> inner: model Inner { x: string }; // hoisted as `Inner.json`,> },>

@typespec/openapi - feature ✏️

Support model, enum, union, and scalar declarations used in expression position. Anonymous declaration expressions are inlined, while named ones are hoisted into a referenced component.,> ,> tsp,> model Foo {,> status: enum { active, inactive }; // inlined,> unit: scalar extends string; // inlined,> inner: model Inner { x: string }; // hoisted as component `Inner`,> },>

@typespec/openapi3 - feature ✏️

Support model, enum, union, and scalar declarations used in expression position. Anonymous declaration expressions are inlined, while named ones are hoisted into a referenced component.,> ,> tsp,> model Foo {,> status: enum { active, inactive }; // inlined,> unit: scalar extends string; // inlined,> inner: model Inner { x: string }; // hoisted as component `Inner`,> },>

@typespec/compiler - feature ✏️

$.enum.create now produces an enum expression (expression: true) when given an empty name, mirroring $.model.create.

@typespec/versioning - feature ✏️

Validate the variants of a keyword-form union expression (union { ... }) used in expression position like the variants of a named union, so versioning incompatibilities on decorated variants are reported.

@typespec/compiler - feature ✏️

Allow model, enum, union, and scalar declarations to be used as expressions. A declaration used in expression position has its corresponding type marked with expression: true and is not registered in the enclosing namespace. It may be named or anonymous (in which case its name is "").,> ,> tsp,> alias Foo = enum {,> a,,> b,,> };,> ,> model Bar {,> status: enum { active, inactive };,> unit: scalar extends string;,> inner: model Inner { x: string };,> },>

@typespec/html-program-viewer - feature ✏️

Display the new expression property on Model, Enum, and Scalar types in the program viewer.

[azure-sdk-automation]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension