Add no case mismatch rule (#3475)

fix [#3394](#3394)

---------

Co-authored-by: Mark Cowlishaw <[email protected]>

Author: timotheeguerin

.chronus/changes/no-case-mismatch-rule-2025-10-4-9-7-18.md

@@ -0,0 +1,8 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-azure-core"
+  - "@azure-tools/typespec-azure-rulesets"
+---
+
+Add new `no-case-mismatch` rule checking for types with names only differing by case

packages/typespec-azure-core/README.md

@@ -37,6 +37,7 @@ Available ruleSets:
 | `@azure-tools/typespec-azure-core/composition-over-inheritance`                                                                                                          | Check that if a model is used in an operation and has derived models that it has a discriminator or recommend to use composition via spread or `is`. |
 | `@azure-tools/typespec-azure-core/known-encoding`                                                                                                                        | Check for supported encodings.                                                                                                                       |
 | `@azure-tools/typespec-azure-core/long-running-polling-operation-required`                                                                                               | Long-running operations should have a linked polling operation.                                                                                      |
+| [`@azure-tools/typespec-azure-core/no-case-mismatch`](https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-case-mismatch)                           | Validate that no two types have the same name with different casing.                                                                                 |
 | [`@azure-tools/typespec-azure-core/no-closed-literal-union`](https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-closed-literal-union)             | Unions of literals should include the base scalar type to mark them as open enum.                                                                    |
 | [`@azure-tools/typespec-azure-core/no-enum`](https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-enum)                                             | Azure services should not use enums.                                                                                                                 |
 | `@azure-tools/typespec-azure-core/no-error-status-codes`                                                                                                                 | Recommend using the error response defined by Azure REST API guidelines.                                                                             |

packages/typespec-azure-core/src/linter.ts

@@ -7,6 +7,7 @@ import { compositionOverInheritanceRule } from "./rules/composition-over-inherit
 import { friendlyNameRule } from "./rules/friendly-name.js";
 import { knownEncodingRule } from "./rules/known-encoding.js";
 import { longRunningOperationsRequirePollingOperation } from "./rules/lro-polling-operation.js";
+import { noCaseMismatchRule } from "./rules/no-case-mismatch.js";
 import { noClosedLiteralUnionRule } from "./rules/no-closed-literal-union.js";
 import { noEnumRule } from "./rules/no-enum.js";
 import { noErrorStatusCodesRule } from "./rules/no-error-status-codes.js";
@@ -47,6 +48,7 @@ const rules = [
   compositionOverInheritanceRule,
   knownEncodingRule,
   longRunningOperationsRequirePollingOperation,
+  noCaseMismatchRule,
   noClosedLiteralUnionRule,
   noEnumRule,
   noErrorStatusCodesRule,

packages/typespec-azure-core/src/rules/no-case-mismatch.ts

@@ -0,0 +1,60 @@
+import {
+  type Enum,
+  type Model,
+  type Namespace,
+  type Union,
+  createRule,
+  isTemplateInstance,
+  paramMessage,
+} from "@typespec/compiler";
+import { DuplicateTracker } from "@typespec/compiler/utils";
+
+type DataType = Model | Union | Enum;
+
+export const noCaseMismatchRule = createRule({
+  name: "no-case-mismatch",
+  description: "Validate that no two types have the same name with different casing.",
+  severity: "warning",
+  url: "https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-case-mismatch",
+  messages: {
+    default: paramMessage`Type '${"typeName"}' has a name that differs only by casing from another type: ${"otherNames"}`,
+  },
+  create(context) {
+    const duplicateTrackers = new Map<Namespace, DuplicateTracker<string, DataType>>();
+
+    const track = (type: DataType) => {
+      if (!(type.namespace && type.name) || isTemplateInstance(type)) {
+        return;
+      }
+      let tracker = duplicateTrackers.get(type.namespace);
+      if (tracker === undefined) {
+        tracker = new DuplicateTracker<string, DataType>();
+        duplicateTrackers.set(type.namespace, tracker);
+      }
+      tracker.track(type.name.toLowerCase(), type);
+    };
+    return {
+      model: (en: Model) => track(en),
+      union: (en: Union) => track(en),
+      enum: (en: Enum) => track(en),
+      exit: () => {
+        for (const [_, tracker] of duplicateTrackers) {
+          for (const [_k, duplicates] of tracker.entries()) {
+            for (const duplicate of duplicates) {
+              context.reportDiagnostic({
+                format: {
+                  typeName: duplicate.name!,
+                  otherNames: duplicates
+                    .map((d) => d.name)
+                    .filter((name) => name !== duplicate.name)
+                    .join(", "),
+                },
+                target: duplicate,
+              });
+            }
+          }
+        }
+      },
+    };
+  },
+});

packages/typespec-azure-core/test/rules/no-case-mismatch.test.ts

@@ -0,0 +1,91 @@
+import { Tester } from "#test/test-host.js";
+import { LinterRuleTester, createLinterRuleTester } from "@typespec/compiler/testing";
+import { beforeEach, describe, it } from "vitest";
+import { noCaseMismatchRule } from "../../src/rules/no-case-mismatch.js";
+
+let tester: LinterRuleTester;
+
+beforeEach(async () => {
+  const runner = await Tester.createInstance();
+  tester = createLinterRuleTester(runner, noCaseMismatchRule, "@azure-tools/typespec-azure-core");
+});
+
+describe("flags models with names that just differ by casing", () => {
+  it.each(["model", "enum", "union"])("%s", async (type) => {
+    await tester
+      .expect(
+        `
+        ${type} FailOverProperties {}
+        ${type} FailoverProperties {}
+      `,
+      )
+      .toEmitDiagnostics([
+        {
+          code: "@azure-tools/typespec-azure-core/no-case-mismatch",
+          message:
+            "Type 'FailOverProperties' has a name that differs only by casing from another type: FailoverProperties",
+        },
+        {
+          code: "@azure-tools/typespec-azure-core/no-case-mismatch",
+          message:
+            "Type 'FailoverProperties' has a name that differs only by casing from another type: FailOverProperties",
+        },
+      ]);
+  });
+});
+
+it("flags 3 or more types", async () => {
+  await tester
+    .expect(
+      `
+        model FailOverProperties {}
+        model FailoverProperties {}
+        model Failoverproperties {}
+      `,
+    )
+    .toEmitDiagnostics([
+      {
+        code: "@azure-tools/typespec-azure-core/no-case-mismatch",
+        message:
+          "Type 'FailOverProperties' has a name that differs only by casing from another type: FailoverProperties, Failoverproperties",
+      },
+      {
+        code: "@azure-tools/typespec-azure-core/no-case-mismatch",
+        message:
+          "Type 'FailoverProperties' has a name that differs only by casing from another type: FailOverProperties, Failoverproperties",
+      },
+      {
+        code: "@azure-tools/typespec-azure-core/no-case-mismatch",
+        message:
+          "Type 'Failoverproperties' has a name that differs only by casing from another type: FailOverProperties, FailoverProperties",
+      },
+    ]);
+});
+
+it("doesn't flag if names are differ by more than casing", async () => {
+  await tester
+    .expect(
+      `
+        model FailedOver {}
+        model FailOver {}
+      `,
+    )
+    .toBeValid();
+});
+
+it("doesn't flag template instances", async () => {
+  await tester
+    .expect(
+      `
+        model Template<T> {
+          t: T;
+        }
+
+        model Test {
+          a: Template<string>;
+          b: Template<int32>;
+        }
+      `,
+    )
+    .toBeValid();
+});

packages/typespec-azure-rulesets/src/rulesets/data-plane.ts

@@ -11,6 +11,7 @@ export default {
     "@azure-tools/typespec-azure-core/use-extensible-enum": true,
     "@azure-tools/typespec-azure-core/known-encoding": true,
     "@azure-tools/typespec-azure-core/long-running-polling-operation-required": true,
+    "@azure-tools/typespec-azure-core/no-case-mismatch": true,
     "@azure-tools/typespec-azure-core/no-closed-literal-union": true,
     "@azure-tools/typespec-azure-core/no-enum": true,
     "@azure-tools/typespec-azure-core/no-error-status-codes": true,

packages/typespec-azure-rulesets/src/rulesets/resource-manager.ts

@@ -12,6 +12,7 @@ export default {
     "@azure-tools/typespec-azure-core/use-extensible-enum": true,
     "@azure-tools/typespec-azure-core/known-encoding": true,
     "@azure-tools/typespec-azure-core/long-running-polling-operation-required": true,
+    "@azure-tools/typespec-azure-core/no-case-mismatch": true,
     "@azure-tools/typespec-azure-core/no-closed-literal-union": true,
     "@azure-tools/typespec-azure-core/no-enum": true,
     "@azure-tools/typespec-azure-core/no-error-status-codes": true,

website/src/content/docs/docs/libraries/azure-core/reference/linter.md

@@ -31,6 +31,7 @@ Available ruleSets:
 | `@azure-tools/typespec-azure-core/composition-over-inheritance`                                                                  | Check that if a model is used in an operation and has derived models that it has a discriminator or recommend to use composition via spread or `is`. |
 | `@azure-tools/typespec-azure-core/known-encoding`                                                                                | Check for supported encodings.                                                                                                                       |
 | `@azure-tools/typespec-azure-core/long-running-polling-operation-required`                                                       | Long-running operations should have a linked polling operation.                                                                                      |
+| [`@azure-tools/typespec-azure-core/no-case-mismatch`](/libraries/azure-core/rules/no-case-mismatch.md)                           | Validate that no two types have the same name with different casing.                                                                                 |
 | [`@azure-tools/typespec-azure-core/no-closed-literal-union`](/libraries/azure-core/rules/no-closed-literal-union.md)             | Unions of literals should include the base scalar type to mark them as open enum.                                                                    |
 | [`@azure-tools/typespec-azure-core/no-enum`](/libraries/azure-core/rules/no-enum.md)                                             | Azure services should not use enums.                                                                                                                 |
 | `@azure-tools/typespec-azure-core/no-error-status-codes`                                                                         | Recommend using the error response defined by Azure REST API guidelines.                                                                             |