Add tests for disabling JsonSerializer.IsReflectionEnabledByDefault on PublishTrimmed=true (#33757)

Author: eiriktsarpalis

src/Tests/Microsoft.NET.Publish.Tests/GivenThatWeWantToRunILLink.cs

@@ -970,6 +970,12 @@ public void TrimmingOptions_are_defaulted_correctly_on_trimmed_apps(string targe
                     .Should().BeFalse();
                 configProperties["System.Threading.Thread.EnableAutoreleasePool"].Value<bool>()
                     .Should().BeFalse();
+
+                if (parsedVersion.Major >= 8)
+                {
+                    configProperties["System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault"].Value<bool>()
+                        .Should().BeFalse();
+                }
             }
             else
             {
@@ -1698,8 +1704,12 @@ public void Build_respects_PublishTrimmed_property(string targetFramework)
 
             // runtimeconfig has trim settings
             JObject runtimeConfig = JObject.Parse(File.ReadAllText(runtimeConfigPath));
-            JToken startupHookSupport = runtimeConfig["runtimeOptions"]["configProperties"]["System.StartupHookProvider.IsSupported"];
-            startupHookSupport.Value<bool>().Should().BeFalse();
+            JToken configProperties = runtimeConfig["runtimeOptions"]["configProperties"];
+
+            configProperties["System.StartupHookProvider.IsSupported"].Value<bool>().Should().BeFalse();
+
+            // Build with PublishTrimmed enabled should disable System.Text.Json reflection
+            configProperties["System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault"].Value<bool>().Should().BeFalse();
 
             // just setting PublishTrimmed doesn't inject the IsTrimmable attribute
             AssemblyInfo.Get(assemblyPath).ContainsKey("AssemblyMetadataAttribute").Should().BeFalse();

src/WebSdk/ProjectSystem/Targets/Microsoft.NET.Sdk.Web.ProjectSystem.targets

@@ -46,11 +46,6 @@ Copyright (c) .NET Foundation. All rights reserved.
     <EnableUnsafeBinaryFormatterSerialization Condition="'$(EnableUnsafeBinaryFormatterSerialization)' == ''">false</EnableUnsafeBinaryFormatterSerialization>
   </PropertyGroup>
 
-  <!-- Default settings for trimmed and aot'd apps. -->
-  <PropertyGroup Condition="'$(PublishTrimmed)' == 'true' Or '$(PublishAot)' == 'true'">
-    <JsonSerializerIsReflectionEnabledByDefault Condition="'$(JsonSerializerIsReflectionEnabledByDefault)' == ''">false</JsonSerializerIsReflectionEnabledByDefault>
-  </PropertyGroup>
-
   <!-- Default settings for aot'd apps. -->
   <PropertyGroup Condition="'$(PublishAot)' == 'true'">
     <!-- Enable the built-in source generators. -->