Suppress warnings about [Obsolete] member usage in JSON src-gen'd code (#63496)

Author: layomia

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

@@ -148,7 +148,10 @@ private void AddSource(string fileName, string source, bool isRootContextDef = f
                 bool isInGlobalNamespace = @namespace == JsonConstants.GlobalNamespaceValue;
 
                 StringBuilder sb = new(@"// <auto-generated/>
-#nullable enable");
+#nullable enable
+
+// Suppress warnings about [Obsolete] member usage in generated code.
+#pragma warning disable CS0618");
 
                 if (!isInGlobalNamespace)
                 {

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

@@ -716,5 +716,36 @@ public class MyType
             Assert.Equal(1, types.Count);
             Assert.Equal("HelloWorld.MyType", types.Keys.First());
         }
+
+        [Fact]
+        public static void NoWarningsDueToObsoleteMembers()
+        {
+                string source = @"using System;
+using System.Text.Json.Serialization;
+
+namespace Test
+{
+    [JsonSerializable(typeof(ClassWithObsolete))]
+    public partial class JsonContext : JsonSerializerContext { }
+
+    public class ClassWithObsolete
+    {
+        [Obsolete(""This is a test"")]
+        public bool Test { get; set; }
+    }
+}
+";
+
+            Compilation compilation = CompilationHelper.CreateCompilation(source);
+            JsonSourceGenerator generator = new JsonSourceGenerator();
+
+            Compilation newCompilation = CompilationHelper.RunGenerators(compilation, out _, generator);
+            ImmutableArray<Diagnostic> generatorDiags = newCompilation.GetDiagnostics();
+
+            // No diagnostics expected.
+            CompilationHelper.CheckDiagnosticMessages(DiagnosticSeverity.Info, generatorDiags, Array.Empty<(Location, string)>());
+            CompilationHelper.CheckDiagnosticMessages(DiagnosticSeverity.Warning, generatorDiags, Array.Empty<(Location, string)>());
+            CompilationHelper.CheckDiagnosticMessages(DiagnosticSeverity.Error, generatorDiags, Array.Empty<(Location, string)>());
+        }
     }
 }