Fix: Prevent @description decorator on metadata declarations

[Copilot]

Summary

This PR prevents the @description decorator from being used on metadata declarations and removes it from language server completion suggestions for metadata. The @description decorator has no effect on the generated ARM template when used on metadata, making it misleading and incorrect.

Issues Fixed

1. Validation Error: @description on metadata now produces BCP269 error
2. Language Server: No longer suggests @description when typing decorators for metadata

Before

@description('description')
metadata foo = 'foo'

✅ Compiles without error (but @description has no effect on generated template)
💡 Language server suggests @description for metadata

After

@description('description')
metadata foo = 'foo'

❌ BCP269 Error: Function "description" cannot be used as a metadata decorator
💡 Language server does not suggest @description for metadata

Implementation

Core Changes

1. Added AnyDecoratorExceptMetadata flag in FunctionFlags.cs

   • New flag that includes all decorator types except MetadataDecorator
   • Provides precise control over decorator usage

2. Updated @description decorator in SystemNamespaceType.cs

   • Changed from FunctionFlags.AnyDecorator to FunctionFlags.AnyDecoratorExceptMetadata
   • Prevents usage on metadata while preserving functionality elsewhere

How It Works

• Validation: When @description is used on metadata, the validation system checks for the MetadataDecorator flag. Since @description now uses AnyDecoratorExceptMetadata, it fails validation and produces BCP269 error.
• Language Server: The completion provider filters decorators by required flags. For metadata symbols, it only shows decorators with MetadataDecorator flag, automatically excluding @description.

Testing

• Updated test baselines in InvalidMetadata_CRLF to expect BCP269 error
• Added comprehensive integration tests:
  • DescriptionDecorator_OnMetadata_ShouldProduceError(): Verifies error occurs
  • DescriptionDecorator_OnValidTargets_ShouldWork(): Ensures @description still works on parameters/outputs

Verification

The fix leverages existing validation infrastructure and is minimal/surgical:

• ✅ @description on metadata → BCP269 error
• ✅ @description on parameters/outputs → works correctly
• ✅ Language server no longer suggests @description for metadata
• ✅ All existing functionality preserved

Fixes #10714.

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[github-actions]

Test this change out locally with the following install scripts (Action run 16058989084)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 16058989084

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 16058989084"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 16058989084

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 16058989084"

[anthony-c-martin]

Remove the MetadataDecorator flag entirely, and clean up references to it. It was only added pre-emptively, and isn't actually used anywhere.

[anthony-c-martin]

See comment