Replace legacy emitter with foundation skeleton and diagnostics

Removes the procedural GraphQL emitter and stands up the data-pipeline
skeleton that the upcoming Alloy component-based emitter will build on.

Deleted:
- graphql-emitter.ts (old procedural emitter)
- schema-emitter.ts (old schema-specific emitter)
- registry.ts (old type registry)
- type-maps.ts (old type mapping logic)
- test/emitter.test.ts tests for the legacy emitter

Rewrote src/emitter.ts as a four-phase data pipeline:
- Phase 1: type usage tracking (reachability / input-output marking)
- Phase 2: mutation (GraphQL naming via the mutation engine)
- Phase 3: classification (interfaces, output models, input models, ops)
- Phase 4: model variant lookups

Phase 5 (SDL rendering) is intentionally left as a TODO. Component-based
rendering, file emission, and the \`.tsx\` conversion land in follow-up
PRs in the chain.

Also introduces two new diagnostics that the pipeline reports today:
- empty-schema: fires when a schema has no query root (GraphQL requires one)
- void-operation-return: fires when an operation returns void (no GraphQL
  equivalent; the operation is filtered out of the schema)

Test coverage added for both diagnostics in test/emitter.test.ts.

Author: FionaBronwen

packages/graphql/src/emitter.ts

@@ -1,37 +1,371 @@
-import type { EmitContext, NewLine } from "@typespec/compiler";
-import { resolvePath } from "@typespec/compiler";
-import { createGraphQLEmitter } from "./graphql-emitter.js";
-import type { GraphQLEmitterOptions } from "./lib.js";
-
-const defaultOptions = {
-  "new-line": "lf",
-  "omit-unreachable-types": false,
-  strict: false,
-} as const;
+import {
+  getEncode,
+  isArrayModelType,
+  isUnknownType,
+  isVoidType,
+  navigateTypesInNamespace,
+  type EmitContext,
+  type Enum,
+  type Model,
+  type ModelProperty,
+  type Namespace,
+  type Operation,
+  type Program,
+  type Scalar,
+  type Type,
+  type Union,
+} from "@typespec/compiler";
+import { isInterface } from "./lib/interface.js";
+import { getOperationKind } from "./lib/operation-kind.js";
+import { type GraphQLEmitterOptions, reportDiagnostic } from "./lib.js";
+import { resolveTypeUsage, GraphQLTypeUsage, type TypeUsageResolver } from "./type-usage.js";
+import { listSchemas } from "./lib/schema.js";
+import { createGraphQLMutationEngine, GraphQLTypeContext } from "./mutation-engine/index.js";
+import type { ClassifiedTypes, ModelVariants, ScalarVariant } from "./context/index.js";
+import { unwrapNullableUnion } from "./lib/type-utils.js";
+import { getGraphQLBuiltinName, getScalarMapping } from "./lib/scalar-mappings.js";
+import { getSpecifiedBy } from "./lib/specified-by.js";
 
+/**
+ * Main emitter entry point for GraphQL SDL generation.
+ *
+ * Runs the full data pipeline (type usage → mutation → classification) but
+ * does not yet render output. Component-based SDL rendering will be added
+ * in follow-up PRs.
+ */
 export async function $onEmit(context: EmitContext<GraphQLEmitterOptions>) {
-  const options = resolveOptions(context);
-  const emitter = createGraphQLEmitter(context, options);
-  await emitter.emitGraphQL();
+  const schemas = listSchemas(context.program);
+  if (schemas.length === 0) {
+    schemas.push({ type: context.program.getGlobalNamespaceType() });
+  }
+
+  for (const schema of schemas) {
+    await emitSchema(context, schema);
+  }
 }
 
-export interface ResolvedGraphQLEmitterOptions {
-  outputFile: string;
-  newLine: NewLine;
-  omitUnreachableTypes: boolean;
-  strict: boolean;
+/**
+ * Process a single GraphQL schema through the data pipeline.
+ */
+async function emitSchema(
+  context: EmitContext<GraphQLEmitterOptions>,
+  schema: { type: Namespace; name?: string },
+) {
+  // Phase 1: Type usage tracking — determine which types are reachable from operations.
+  // Must run before mutation so we can filter on original (pre-clone) type objects.
+  const omitUnreachable = context.options["omit-unreachable-types"] ?? false;
+  const typeUsage = resolveTypeUsage(schema.type, omitUnreachable);
+
+  // Phase 2: Mutation — transform TypeSpec types with GraphQL naming conventions.
+  // Unreachable enums/unions are skipped during mutation (avoids identity mismatch with cloned objects).
+  const { mutatedTypes, scalarSpecifications, originalToMutated } = mutateTypes(
+    context,
+    schema,
+    typeUsage,
+  );
+
+  // Phase 3: Classification — separate types by category.
+  const classifiedTypes = classifyTypes(
+    context.program,
+    mutatedTypes,
+    originalToMutated,
+    typeUsage,
+  );
+
+  // Phase 4: Build model variant lookups.
+  const _modelVariants = buildModelVariants(classifiedTypes);
+
+  // GraphQL requires at least a Query root type. If there are no query operations,
+  // the schema cannot be built. Emit a diagnostic and skip rendering.
+  if (classifiedTypes.queries.length === 0) {
+    reportDiagnostic(context.program, {
+      code: "empty-schema",
+      target: schema.type,
+    });
+    return;
+  }
+
+  // Phase 5: Component-based SDL rendering.
+  // TODO: Render using Alloy JSX components (added in follow-up PRs).
+  void _modelVariants;
+  void scalarSpecifications;
 }
 
-export function resolveOptions(
+// ---------------------------------------------------------------------------
+// Phase 2: Mutation
+// ---------------------------------------------------------------------------
+
+/**
+ * Mutate all types in a schema namespace using the mutation engine.
+ * Unreachable enums/unions are skipped based on the type usage resolver.
+ */
+function mutateTypes(
   context: EmitContext<GraphQLEmitterOptions>,
-): ResolvedGraphQLEmitterOptions {
-  const resolvedOptions = { ...defaultOptions, ...context.options };
-  const outputFile = resolvedOptions["output-file"] ?? "{schema-name}.graphql";
+  schema: { type: Namespace },
+  typeUsage: TypeUsageResolver,
+) {
+  const engine = createGraphQLMutationEngine(context.program);
+  const mutatedModels: Model[] = [];
+  const mutatedEnums: Enum[] = [];
+  const mutatedScalars: Scalar[] = [];
+  const mutatedUnions: Union[] = [];
+  const mutatedOperations: Operation[] = [];
+  const wrapperModels: Model[] = [];
+  const scalarSpecifications = new Map<string, string>();
+  const originalToMutated = new Map<Model, Model>();
+  const processedScalars = new Set<string>();
+  const scalarVariantsMap = new Map<string, ScalarVariant>();
+
+  const processScalar = (node: Scalar): void => {
+    const isDirectGraphQLBuiltin =
+      context.program.checker.isStdType(node, "string") ||
+      context.program.checker.isStdType(node, "int32") ||
+      context.program.checker.isStdType(node, "float32") ||
+      context.program.checker.isStdType(node, "float64") ||
+      context.program.checker.isStdType(node, "boolean");
+
+    if (isDirectGraphQLBuiltin) return;
+
+    const mutation = engine.mutateScalar(node);
+    const graphqlName = mutation.mutatedType.name;
+
+    if (!processedScalars.has(graphqlName)) {
+      processedScalars.add(graphqlName);
+      mutatedScalars.push(mutation.mutatedType);
+
+      const specUrl = getSpecifiedBy(context.program, mutation.mutatedType);
+      if (specUrl) {
+        scalarSpecifications.set(graphqlName, specUrl);
+      }
+    }
+  };
+
+  const processScalarVariant = (target: ModelProperty): void => {
+    if (isUnknownType(target.type)) {
+      if (!scalarVariantsMap.has("Unknown")) {
+        scalarVariantsMap.set("Unknown", {
+          sourceScalar: target.type,
+          encoding: "default",
+          graphqlName: "Unknown",
+          specificationUrl: undefined,
+        });
+      }
+      return;
+    }
+    if (
+      target.type.kind === "Scalar" &&
+      context.program.checker.isStdType(target.type) &&
+      !getGraphQLBuiltinName(context.program, target.type)
+    ) {
+      const encodeData = getEncode(context.program, target);
+      const encoding = encodeData?.encoding;
+      const mapping = getScalarMapping(context.program, target.type, encoding);
+      if (mapping && !scalarVariantsMap.has(mapping.graphqlName)) {
+        scalarVariantsMap.set(mapping.graphqlName, {
+          sourceScalar: target.type,
+          encoding: encoding || "default",
+          graphqlName: mapping.graphqlName,
+          specificationUrl: mapping.specificationUrl,
+        });
+      }
+    }
+  };
+
+  navigateTypesInNamespace(schema.type, {
+    model: (node: Model) => {
+      if (isArrayModelType(context.program, node)) return;
+      const mutation = engine.mutateModel(node, GraphQLTypeContext.Output);
+      mutatedModels.push(mutation.mutatedType);
+      originalToMutated.set(node, mutation.mutatedType);
+    },
+    enum: (node: Enum) => {
+      if (typeUsage.isUnreachable(node)) return;
+      const mutation = engine.mutateEnum(node);
+      mutatedEnums.push(mutation.mutatedType);
+    },
+    scalar: (node: Scalar) => {
+      processScalar(node);
+    },
+    union: (node: Union) => {
+      // Skip nullable unions (e.g., string | null) — they're not union declarations.
+      // Nullability for these is detected at render time in GraphQLTypeExpression.
+      if (unwrapNullableUnion(node) !== undefined) {
+        return;
+      }
+      if (typeUsage.isUnreachable(node)) return;
+      const mutation = engine.mutateUnion(node, GraphQLTypeContext.Output);
+      mutatedUnions.push(mutation.mutatedType as Union);
+      wrapperModels.push(...mutation.wrapperModels);
+    },
+    operation: (node: Operation) => {
+      // Operations are passed through unmutated. This is load-bearing:
+      // typeUsage walked operation params/returns on original types to mark input/output.
+      // classifyTypes reverse-maps mutated models via originalToMutated.
+      mutatedOperations.push(node);
+    },
+  });
+
+  // Collect referenced scalars from model properties and operations.
+  // Standard library scalars like int64, utcDateTime are not declared in the schema,
+  // but are referenced in model properties — we need to collect and mutate them.
+  const visitedTypes = new Set<Type>();
+
+  const collectReferencedScalars = (type: Type): void => {
+    if (visitedTypes.has(type)) return;
+    visitedTypes.add(type);
+
+    if (type.kind === "Scalar") {
+      processScalar(type);
+    } else if (type.kind === "Model" && isArrayModelType(context.program, type)) {
+      if (type.indexer?.value) {
+        collectReferencedScalars(type.indexer.value);
+      }
+    } else if (type.kind === "Model") {
+      for (const prop of type.properties.values()) {
+        collectReferencedScalars(prop.type);
+      }
+    } else if (type.kind === "Union") {
+      for (const variant of type.variants.values()) {
+        collectReferencedScalars(variant.type);
+      }
+    }
+  };
+
+  // Uses original (pre-mutation) models because mutated type refs won't match processedScalars.
+  const originalModels = Array.from(originalToMutated.keys());
+  for (const model of originalModels) {
+    for (const prop of model.properties.values()) {
+      collectReferencedScalars(prop.type);
+      processScalarVariant(prop);
+    }
+  }
+
+  for (const op of mutatedOperations) {
+    for (const param of op.parameters.properties.values()) {
+      collectReferencedScalars(param.type);
+      processScalarVariant(param);
+    }
+    collectReferencedScalars(op.returnType);
+  }
 
   return {
-    outputFile: resolvePath(context.emitterOutputDir, outputFile),
-    newLine: resolvedOptions["new-line"],
-    omitUnreachableTypes: resolvedOptions["omit-unreachable-types"],
-    strict: resolvedOptions["strict"],
+    mutatedTypes: {
+      models: mutatedModels,
+      enums: mutatedEnums,
+      scalars: mutatedScalars,
+      unions: mutatedUnions,
+      operations: mutatedOperations,
+      wrapperModels,
+      scalarVariants: Array.from(scalarVariantsMap.values()),
+    },
+    scalarSpecifications,
+    originalToMutated,
   };
 }
+
+// ---------------------------------------------------------------------------
+// Phase 3: Classification
+// ---------------------------------------------------------------------------
+
+/**
+ * Classify types into categories (interfaces, output types, input types, operations).
+ */
+function classifyTypes(
+  program: Program,
+  mutatedTypes: ReturnType<typeof mutateTypes>["mutatedTypes"],
+  originalToMutated: Map<Model, Model>,
+  typeUsage: TypeUsageResolver,
+): ClassifiedTypes {
+  const interfaces: Model[] = [];
+  const outputModels: Model[] = [];
+  const inputModels: Model[] = [];
+  const queries: Operation[] = [];
+  const mutations: Operation[] = [];
+  const subscriptions: Operation[] = [];
+
+  // Create reverse mapping
+  const mutatedToOriginal = new Map<Model, Model>();
+  for (const [orig, mut] of originalToMutated) {
+    mutatedToOriginal.set(mut, orig);
+  }
+
+  for (const model of mutatedTypes.models) {
+    const originalModel = mutatedToOriginal.get(model) || model;
+    if (typeUsage.isUnreachable(originalModel)) {
+      continue;
+    }
+
+    // Check @Interface on the original (pre-clone) model, since decorator state
+    // is stored against original type identity, not mutated clones.
+    if (isInterface(program, originalModel)) {
+      interfaces.push(model);
+    } else {
+      const usage = typeUsage.getUsage(originalModel);
+      const usedAsInput = usage?.has(GraphQLTypeUsage.Input) ?? false;
+      const usedAsOutput = usage?.has(GraphQLTypeUsage.Output) ?? false;
+
+      if (!usedAsInput && !usedAsOutput) {
+        // Reachable but not referenced by any operation — default to Output
+        outputModels.push(model);
+      } else {
+        if (usedAsOutput) outputModels.push(model);
+        if (usedAsInput) inputModels.push(model);
+      }
+    }
+  }
+
+  // Add wrapper models created by union mutations (always used as output)
+  outputModels.push(...mutatedTypes.wrapperModels);
+
+  // Classify operations by kind, filtering out void-returning operations
+  for (const op of mutatedTypes.operations) {
+    if (isVoidType(op.returnType)) {
+      reportDiagnostic(program, {
+        code: "void-operation-return",
+        format: { name: op.name },
+        target: op,
+      });
+      continue;
+    }
+    const kind = getOperationKind(program, op);
+    if (kind === "Query") queries.push(op);
+    else if (kind === "Mutation") mutations.push(op);
+    else if (kind === "Subscription") subscriptions.push(op);
+  }
+
+  return {
+    interfaces,
+    outputModels,
+    inputModels,
+    enums: mutatedTypes.enums,
+    scalars: mutatedTypes.scalars,
+    scalarVariants: mutatedTypes.scalarVariants,
+    unions: mutatedTypes.unions,
+    queries,
+    mutations,
+    subscriptions,
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Phase 4: Model variant lookups
+// ---------------------------------------------------------------------------
+
+/**
+ * Build model variant lookups for checking which variants exist.
+ */
+function buildModelVariants(classifiedTypes: ClassifiedTypes): ModelVariants {
+  const modelVariants: ModelVariants = {
+    outputModels: new Map(),
+    inputModels: new Map(),
+  };
+
+  classifiedTypes.outputModels.forEach((m) =>
+    modelVariants.outputModels.set(m.name, m),
+  );
+  classifiedTypes.inputModels.forEach((m) =>
+    modelVariants.inputModels.set(m.name, m),
+  );
+
+  return modelVariants;
+}