Add linter rule to block service namespace redefinition in client.tsp

[Copilot]

client.tsp customizations were increasingly reopening the service namespace and injecting new models or operations into the service surface. This change adds a client-generation linter rule to keep client customizations API-neutral: use augment decorators for service types, and put new helper types outside the service namespace.

• New TCGC rule

  • Adds @azure-tools/typespec-client-generator-core/no-service-namespace-redefinition
  • Reports when client.tsp declares:
    • the service namespace itself
    • any child namespace under the service namespace
  • Allows valid customization patterns such as:
    • @@... augment decorators
    • separate namespaces like Customizations

• Ruleset enablement

  • Enables the new rule in Azure data-plane and resource-manager rulesets so it applies to client-customized specs by default

• Docs and examples

  • Adds rule documentation and updates generated linter reference tables
  • Documents the intended split between service definitions and client-only customization code

• Coverage

  • Adds focused tests for:
    • valid augment-only client.tsp
    • valid helper types in Customizations
    • invalid redefinition of the service namespace
    • invalid declaration of child namespaces under the service namespace

Example:

// main.tsp
@service
namespace Contoso.Widget;

// client.tsp
import "./main.tsp";
import "@azure-tools/typespec-client-generator-core";

using Azure.ClientGenerator.Core;
using Contoso.Widget;

namespace Customizations;

model WidgetClientOptions {
  mode: string;
}

@@clientName(Widget.name, "widgetName");

[timotheeguerin]

@copilot suggest some other names, this doesn't feel specific enough, thinking it should be more related to client(like use-client-namespace-isolation

[Copilot]

Addressed in 1fb8c05 — I renamed the rule to use-client-namespace-isolation across the implementation, tests, rulesets, and docs.

[timotheeguerin]

@copilot I asked for suggestion not to rename yet

[timotheeguerin]

@copilot please reply with an answer

[Copilot]

Some options I think fit better are:

• use-client-namespace-isolation
• keep-client-customizations-outside-service-namespace
• prevent-service-namespace-reopening-in-client
• no-client-service-namespace-redefinition

My preference is use-client-namespace-isolation if you want it phrased as guidance rather than as a prohibition.

[timotheeguerin]

@copilot what about no-service-ns-in-client, lets use that

[Copilot]

Done in efbcfad.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@azure-tools/typespec-azure-rulesets@4869

npm i https://pkg.pr.new/@azure-tools/typespec-client-generator-core@4869

commit: efbcfad

[timotheeguerin]

@copilot format

[Copilot]

@copilot format

Fixed in e22d38d.