Refactor code so it doesn't hit mono aot limitation

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/IReadBufferState.cs

@@ -26,6 +26,9 @@ public abstract ValueTask<TReadBufferState> ReadAsync(
 
         public abstract void Advance(long bytesConsumed);
 
-        public abstract Utf8JsonReader GetReader(JsonReaderState jsonReaderState);
+        // This would normally be implemented as returning a Utf8JsonReader, but this pattern hits a limitation
+        // in mono aot that is not trivial to fix. For now use the alternative pattern of returning via an out
+        // argument. Tracking issue: https://github.com/dotnet/runtime/issues/118697
+        public abstract void GetReader(JsonReaderState jsonReaderState, out Utf8JsonReader reader);
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfoOfT.ReadHelper.cs

@@ -136,7 +136,7 @@ internal bool ContinueDeserialize<TReadBufferState, TStream>(
             out T? value)
             where TReadBufferState : struct, IReadBufferState<TReadBufferState, TStream>
         {
-            Utf8JsonReader reader = bufferState.GetReader(jsonReaderState);
+            bufferState.GetReader(jsonReaderState, out Utf8JsonReader reader);
 
             try
             {

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/PipeReadBufferState.cs

@@ -75,11 +75,11 @@ public async ValueTask<PipeReadBufferState> ReadAsync(PipeReader utf8Json, Cance
 
         public void Read(PipeReader utf8Json) => throw new NotImplementedException();
 
-        public Utf8JsonReader GetReader(JsonReaderState jsonReaderState)
+        public void GetReader(JsonReaderState jsonReaderState, out Utf8JsonReader reader)
         {
             if (_sequence.IsSingleSegment)
             {
-                return new Utf8JsonReader(
+                reader = new Utf8JsonReader(
 #if NET
                     _sequence.FirstSpan,
 #else
@@ -89,7 +89,7 @@ public Utf8JsonReader GetReader(JsonReaderState jsonReaderState)
             }
             else
             {
-                return new Utf8JsonReader(_sequence, IsFinalBlock, jsonReaderState);
+                reader = new Utf8JsonReader(_sequence, IsFinalBlock, jsonReaderState);
             }
         }
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/StreamReadBufferState.cs

@@ -157,9 +157,9 @@ public void Advance(long bytesConsumed)
             _offset = 0;
         }
 
-        public Utf8JsonReader GetReader(JsonReaderState jsonReaderState)
+        public void GetReader(JsonReaderState jsonReaderState, out Utf8JsonReader reader)
         {
-            return new Utf8JsonReader(
+            reader = new Utf8JsonReader(
                 _buffer.AsSpan(_offset, _count),
                 IsFinalBlock,
                 jsonReaderState);