Make CollectDeclaredReferences target incremental

src/Package/build/ReferenceTrimmer.targets

@@ -9,7 +9,11 @@
     <CompilerVisibleProperty Include="EnableReferenceTrimmerDiagnostics"/>
   </ItemGroup>
 
-  <Target Name="CollectDeclaredReferences" DependsOnTargets="ResolveAssemblyReferences;PrepareProjectReferences" Condition="'$(EnableReferenceTrimmer)' != 'false'">
+  <Target Name="CollectDeclaredReferences"
+          DependsOnTargets="ResolveAssemblyReferences;PrepareProjectReferences"
+          Condition="'$(EnableReferenceTrimmer)' != 'false'"
+          Inputs="$(MSBuildProjectFullPath);$(ProjectAssetsFile)"
+          Outputs="$(MSBuildProjectDirectory)\$(IntermediateOutputPath)_ReferenceTrimmer_DeclaredReferences.tsv">
     <PropertyGroup>
       <_ReferenceTrimmerDeclaredReferencesFile>$([System.IO.Path]::GetFullPath('$(IntermediateOutputPath)_ReferenceTrimmer_DeclaredReferences.tsv'))</_ReferenceTrimmerDeclaredReferencesFile>
       <_ReferenceTrimmerUsedReferencesFile>$([System.IO.Path]::GetFullPath('$(IntermediateOutputPath)_ReferenceTrimmer_UsedReferences.log'))</_ReferenceTrimmerUsedReferencesFile>

src/Tasks/CollectDeclaredReferencesTask.cs

@@ -506,6 +506,9 @@ private static void SaveDeclaredReferences(IReadOnlyList<DeclaredReference> decl
             string existing = File.ReadAllText(filePath);
             if (string.Equals(existing, newContent, StringComparison.OrdinalIgnoreCase))
             {
+                // Touch the file so that MSBuild's Inputs/Outputs incrementality check
+                // sees a fresh timestamp and can skip the target on subsequent builds.
+                File.SetLastWriteTimeUtc(filePath, DateTime.UtcNow);
                 return;
             }
         }