Enforce banned-API checks for reference assembly

Add banned API analyzer and rule list to prevent leaking implementation-only types into the published reference assembly.

- Add Microsoft.CodeAnalysis.BannedApiAnalyzers package to Directory.Packages.props
- Add src/WinRT.Runtime2/BannedSymbols.txt listing the WindowsRuntimeImplementationOnlyMember attribute as banned for reference builds
- Update WinRT.Runtime.csproj to enable ProduceReferenceAssembly=false for implementation builds and to define WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY / WINDOWS_RUNTIME_REFERENCE_ASSEMBLY appropriately
- Treat the banned-API diagnostic (RS0030) as an error for both implementation and reference assembly build configurations
- Wire the banned symbols file into AdditionalFiles and add the analyzer package as a PrivateAssets dependency when building the reference assembly

This ensures implementation-only types cannot be accidentally exposed in the public API surface of WinRT.Runtime.

Author: Sergio0694

src/Directory.Packages.props

@@ -13,6 +13,7 @@
     <PackageVersion Include="coverlet.collector" Version="1.3.0" />
     <PackageVersion Include="MarkdownLog.NS20" Version="0.10.1" />
     <PackageVersion Include="Microsoft.CodeAnalysis.Analyzers" Version="3.11.0" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.BannedApiAnalyzers" Version="4.14.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.Common" Version="5.0.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="5.0.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp.Analyzer.Testing" Version="1.1.2" />

src/WinRT.Runtime2/BannedSymbols.txt

@@ -0,0 +1,7 @@
+# We use the '[WindowsRuntimeImplementationOnlyMember]' attribute to mark
+# all implementation-only types in 'WinRT.Runtime.dll'. These types are
+# only included when building the implementation assembly, while entirely
+# skipped when producing the reference assembly. This list of banned
+# symbols is only included when building the reference assembly, which
+# ensures that we never accidentally leak any of them in the public API.
+T:WindowsRuntime.WindowsRuntimeImplementationOnlyMemberAttribute

src/WinRT.Runtime2/WinRT.Runtime.csproj

@@ -79,28 +79,33 @@
     <AssemblyOriginatorKeyFile>key.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
 
-  <!--
-    Disable the default 'ProduceReferenceAssembly' behavior. WinRT.Runtime provides its own custom
-    reference assembly (built with 'CsWinRTBuildReferenceAssembly=true') that strips private implementation
-    detail types via '#if WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY'. The SDK-generated ref assembly would
-    include those types (since 'WINDOWS_RUNTIME_REFERENCE_ASSEMBLY' is not defined in normal builds), which
-    would leak abstract members to downstream ProjectReference consumers and break reference projections.
-  -->
+  <!-- Settings for the implementation assembly -->
   <PropertyGroup Condition="'$(CsWinRTBuildReferenceAssembly)' != 'true'">
+
+    <!--
+      Disable the default 'ProduceReferenceAssembly' behavior. WinRT.Runtime provides its own custom
+      reference assembly (built with 'CsWinRTBuildReferenceAssembly=true') that strips private implementation
+      detail types via '#if WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY'. The SDK-generated ref assembly would
+      include those types (since 'WINDOWS_RUNTIME_REFERENCE_ASSEMBLY' is not defined in normal builds), which
+      would leak abstract members to downstream ProjectReference consumers and break reference projections.
+    -->
     <ProduceReferenceAssembly>false</ProduceReferenceAssembly>
 
     <!-- Build constant to simplify '#ifdef'-s, so they don't need negation expressions -->
     <DefineConstants>$(DefineConstants);WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY</DefineConstants>
+      <WarningsAsErrors>$(WarningsAsErrors);RS0030</WarningsAsErrors>
   </PropertyGroup>
 
-  <!--
-    When producing a reference assembly for packaging, define 'WINDOWS_RUNTIME_REFERENCE_ASSEMBLY'. This is used
-    to completely strip out the private implementation detail types from the reference assembly, so they're
-    further hidden. This is conditioned on 'CsWinRTBuildReferenceAssembly' (and NOT 'ProduceReferenceAssembly'),
-    because 'ProduceReferenceAssembly' is set to 'true' by default by the .NET SDK for library projects. The
-    stripping should only happen during the explicit reference assembly build step for NuGet packaging.
-  -->
+  <!-- Settings for the reference assembly -->
   <PropertyGroup Condition="'$(CsWinRTBuildReferenceAssembly)' == 'true'">
+
+    <!--
+      When producing a reference assembly for packaging, define 'WINDOWS_RUNTIME_REFERENCE_ASSEMBLY'. This is used
+      to completely strip out the private implementation detail types from the reference assembly, so they're
+      further hidden. This is conditioned on 'CsWinRTBuildReferenceAssembly' (and NOT 'ProduceReferenceAssembly'),
+      because 'ProduceReferenceAssembly' is set to 'true' by default by the .NET SDK for library projects. The
+      stripping should only happen during the explicit reference assembly build step for NuGet packaging.
+    -->
     <DefineConstants>$(DefineConstants);WINDOWS_RUNTIME_REFERENCE_ASSEMBLY</DefineConstants>
 
     <!--
@@ -111,7 +116,19 @@
         - IDE0380: 'unsafe' modifier is unnecessary (some types have 'unsafe' for excluded members).
     -->
     <NoWarn>$(NoWarn);CS8597;IDE0005;IDE0380</NoWarn>
+
+    <!-- Always make the banned API analyzer warnings errors, since these should always be blocking -->
+    <WarningsAsErrors>$(WarningsAsErrors);RS0030</WarningsAsErrors>
   </PropertyGroup>
+  <ItemGroup Condition="'$(CsWinRTBuildReferenceAssembly)' == 'true'">
+
+    <!-- Include the banned API list, for the implementation-only marker attribute -->
+    <None Remove="BannedSymbols.txt" />
+    <AdditionalFiles Include="BannedSymbols.txt" />
+
+    <!-- Include the banned API analyzer, which we use to block implementation-only types in the reference assembly -->
+    <PackageReference Include="Microsoft.CodeAnalysis.BannedApiAnalyzers" PrivateAssets="all" />
+  </ItemGroup>
 
   <!--
     Strip all files whose top-level type is annotated with '[WindowsRuntimeImplementationOnlyMember]'