Fix handling of DU members that are also DUs (#901)

Avoid spurious import statement for DU types when in unions.rs.
Added Spector coverage for the impacted scenario.

Author: jhendrixMSFT

packages/typespec-rust/.scripts/tspcompile.js

@@ -88,6 +88,7 @@ const azureHttpSpecsGroup = {
   'spector_clientloc_move4': {input: 'azure/client-generator-core/client-location/move-to-root-client'},
   'spector_emptystringasnone': {input: 'azure/client-generator-core/deserialize-empty-string-as-null'},
   'spector_flattenproperty': {input: 'azure/client-generator-core/flatten-property'},
+  'spector_hierarchy_building': {input: 'azure/client-generator-core/hierarchy-building'},
   'spector_corenextlinkverb': {input: 'azure/client-generator-core/next-link-verb'},
   'spector_coreoverride': {input: 'azure/client-generator-core/override/client.tsp'},
   'spector_coreusage': {input: 'azure/client-generator-core/usage'},

packages/typespec-rust/CHANGELOG.md

@@ -1,5 +1,11 @@
 # Release History
 
+## 0.37.1 (unreleased)
+
+### Bugs Fixed
+
+* Fixed an issue with discriminated unions containing other discriminated unions.
+
 ## 0.37.0 (2026-03-11)
 
 ### Breaking Changes

packages/typespec-rust/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@azure-tools/typespec-rust",
-  "version": "0.37.0",
+  "version": "0.37.1",
   "description": "TypeSpec emitter for Rust SDKs",
   "type": "module",
   "packageManager": "pnpm@10.30.3",

packages/typespec-rust/src/codegen/unions.ts

@@ -66,7 +66,11 @@ export function emitUnions(module: rust.ModuleContainer, context: Context): Unio
       body += `${helpers.emitVisibility(rustUnion.visibility)}enum ${rustUnion.name} {\n`;
 
       for (const member of rustUnion.members) {
-        use.addForType(member.type);
+        // Use isn't granular enough to realize we're in unions.rs
+        // so skip adding when the member is also a DU
+        if (member.type.kind !== 'discriminatedUnion') {
+          use.addForType(member.type);
+        }
 
         const docs = helpers.formatDocComment(member.docs, true);
         if (docs.length > 0) {

packages/typespec-rust/src/codemodel/types.ts

@@ -172,7 +172,7 @@ export interface DiscriminatedUnionMember {
   docs: Docs;
 
   /** the type of the discriminated union member */
-  type: Model;
+  type: DiscriminatedUnion | Model;
 
   /** discriminator property value */
   discriminantValue: string;
@@ -893,7 +893,7 @@ export class DiscriminatedUnionEnvelope implements DiscriminatedUnionEnvelope {
 }
 
 export class DiscriminatedUnionMember implements DiscriminatedUnionMember {
-  constructor(type: Model, discriminantValue: string) {
+  constructor(type: DiscriminatedUnion | Model, discriminantValue: string) {
     this.kind = 'discriminatedUnionMember';
     this.type = type;
     this.discriminantValue = discriminantValue;

packages/typespec-rust/src/tcgcadapter/adapter.ts

@@ -626,7 +626,7 @@ export class Adapter {
           if (!subType.discriminatorValue) {
             throw new AdapterError('InternalError', `model ${subType.name} has no discriminator value`, subType.__raw?.node);
           }
-          const unionMemberType = this.getModel(subType);
+          const unionMemberType = subType.discriminatedSubtypes ? this.getDiscriminatedUnion(subType) : this.getModel(subType);
           const rustUnionMember = new rust.DiscriminatedUnionMember(unionMemberType, subType.discriminatorValue);
           rustUnion.members.push(rustUnionMember);
         }

packages/typespec-rust/test/Cargo.lock

@@ -29,6 +29,7 @@ members = [
     "spector/azure/client-generator-core/client-location/move-to-root-client",
     "spector/azure/client-generator-core/deserialize-empty-string-as-null",
     "spector/azure/client-generator-core/flatten-property",
+    "spector/azure/client-generator-core/hierarchy-building",
     "spector/azure/client-generator-core/next-link-verb",
     "spector/azure/client-generator-core/override",
     "spector/azure/client-generator-core/usage",

packages/typespec-rust/test/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "spector_hierarchy_building"
+version = "0.1.0"
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+
+[features]
+default = ["azure_core/default"]
+
+[dependencies]
+azure_core = { workspace = true }
+serde = { workspace = true }
+
+[dev-dependencies]
+tokio = { workspace = true }