Add RazorGenerate items to UpToDateCheckInput

Fixes #2048

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Sdk.Razor.CurrentVersion.targets

@@ -158,6 +158,8 @@ Copyright (c) .NET Foundation. All rights reserved.
     <RazorIntermediateAssembly Condition="'$(RazorIntermediateAssembly)'==''" Include="$(IntermediateOutputPath)$(RazorTargetName).dll" />
     <!-- Used in Compilation.targets -->   
     <_RazorDebugSymbolsIntermediatePath Condition="'$(_RazorDebugSymbolsProduced)'=='true'" Include="$(IntermediateOutputPath)$(RazorTargetName).pdb" />
+    <!-- Add all cshtml files to UpToDateCheckInput - a collection used by the IDE's project system to determine if a project needs to be rebuilt -->
+    <UpToDateCheckInput Condition="'$(RazorCompileOnBuild)'=='true'" Include="@(Content->WithMetadataValue('Extension', '.cshtml'))" />
   </ItemGroup>
 
   <!--

test/Microsoft.AspNetCore.Razor.Design.Test/IntegrationTests/BuildIntrospectionTest.cs

@@ -0,0 +1,23 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.IO;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
+{
+    public class BuildIntrospectionTest : MSBuildIntegrationTestBase
+    {
+        [Fact]
+        [InitializeTestProject("SimpleMvc")]
+        public async Task RazorSdk_AddsCshtmlFilesToUpToDateCheckInput()
+        {
+            var result = await DotnetMSBuild("_IntrospectUpToDateCheckInput", "/p:RazorCompileOnBuild=true");
+
+            Assert.BuildPassed(result);
+            Assert.BuildOutputContainsLine(result, $"UpToDateCheckInput: {Path.Combine("Views", "Home", "Index.cshtml")}");
+            Assert.BuildOutputContainsLine(result, $"UpToDateCheckInput: {Path.Combine("Views", "_ViewStart.cshtml")}");
+        }
+    }
+}

test/testapps/RazorTest.Introspection.targets

@@ -10,4 +10,8 @@
   <Target Name="_IntrospectPreserveCompilationContext">
     <Message Text="PreserveCompilationContext: $(PreserveCompilationContext)" Importance="High" />
   </Target>
+
+  <Target Name="_IntrospectUpToDateCheckInput">
+    <Message Text="UpToDateCheckInput: %(UpToDateCheckInput.Identity)" Importance="High" />
+  </Target>
 </Project>