Fix AOT trimming

Author: xoofx

src/Tomlyn/Serialization/Internal/TomlTypeInfoResolverPipeline.cs

@@ -20,6 +20,7 @@ internal static class TomlTypeInfoResolverPipeline
     private const string ReflectionBasedSerializationMessage =
         "Reflection-based TOML serialization is not compatible with trimming/NativeAOT. " +
         "Use a source-generated TomlSerializerContext or pass a TomlTypeInfo instance.";
+    private const string ReflectionSwitchName = TomlSerializerFeatureSwitches.ReflectionSwitchName;
 
     private static readonly ConditionalWeakTable<TomlSerializerOptions, ConcurrentDictionary<Type, TomlTypeInfo>> CacheByOptions = new();
 
@@ -81,7 +82,7 @@ private static TomlTypeInfo ResolveUncached(TomlSerializerOptions options, Type
                 $"Dictionaries must have string keys to be representable as TOML tables. Type '{type.FullName}' is not supported without a custom converter.");
         }
 
-        if (TomlSerializerFeatureSwitches.IsReflectionEnabledByDefaultCalculated)
+        if (TomlSerializer.IsReflectionEnabledByDefault)
         {
             var typeInfo = ResolveFromReflection(options, type);
             return TomlPolymorphicTypeInfo.TryWrap(typeInfo);
@@ -94,8 +95,9 @@ private static TomlTypeInfo ResolveUncached(TomlSerializerOptions options, Type
         }
 
         throw new TomlException(
-            $"No TOML metadata is available for type '{type.FullName}'. " +
-            $"Provide {nameof(TomlSerializerOptions)}.{nameof(TomlSerializerOptions.TypeInfoResolver)} (source generation) or a custom resolver.");
+            $"Reflection serialization is disabled and no TOML metadata was found for type '{type.FullName}'. " +
+            $"Provide {nameof(TomlSerializerOptions)}.{nameof(TomlSerializerOptions.TypeInfoResolver)} (source generation) or a custom resolver, " +
+            $"or enable the '{ReflectionSwitchName}' AppContext switch.");
     }
 
     [RequiresUnreferencedCode(ReflectionBasedSerializationMessage)]

src/Tomlyn/Serialization/TomlSerializerFeatureSwitches.cs

@@ -16,7 +16,7 @@ internal static class TomlSerializerFeatureSwitches
     [FeatureSwitchDefinition(ReflectionSwitchName)]
 #endif
     public static bool IsReflectionEnabledByDefault
-        => AppContext.TryGetSwitch(ReflectionSwitchName, out var enabled) ? enabled : true;
+        => !AppContext.TryGetSwitch(ReflectionSwitchName, out var enabled) || enabled;
 
     public static readonly bool IsReflectionEnabledByDefaultCalculated = IsReflectionEnabledByDefault;
 }

src/Tomlyn/TomlSerializer.cs

@@ -27,13 +27,12 @@ public static class TomlSerializer
         "Reflection-based TOML serialization is not compatible with trimming/NativeAOT. " +
         "Use a source-generated TomlSerializerContext or pass a TomlTypeInfo instance.";
 
-    private static readonly bool ReflectionEnabledByDefault = TomlSerializerFeatureSwitches.IsReflectionEnabledByDefaultCalculated;
     private static readonly Encoding DefaultStreamEncoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
 
     /// <summary>
     /// Gets a value indicating whether reflection-based serialization is enabled by default.
     /// </summary>
-    public static bool IsReflectionEnabledByDefault => ReflectionEnabledByDefault;
+    public static bool IsReflectionEnabledByDefault => TomlSerializerFeatureSwitches.IsReflectionEnabledByDefaultCalculated;
 
     /// <summary>
     /// Serializes a value into TOML text.