Optimize ReusableStringWriter.Write/WriteLine(ReadOnlySpan<char>) methods

Original StringWriter.Write/WriteLine(ReadOnlySpan<char>) methods are optimized only for the StringWriter type itself, not derived classes. For derived classes, these methods copy the provided Span to an array from ArrayPool and fall back to Write(char[], int, int) method for backward compatibility with code created before the Span type existed.

Explicit overrides in the ReusableStringWriter class that call StringBuilder.Append directly help to avoid the overhead of ArrayPool.Rent/Return and copying.

Author: epeshk

src/Serilog/Rendering/ReusableStringWriter.cs

@@ -3,7 +3,7 @@ namespace Serilog.Rendering;
 /// <summary>
 /// Class that provides reusable StringWriters to reduce memory allocations
 /// </summary>
-class ReusableStringWriter : StringWriter
+sealed class ReusableStringWriter : StringWriter
 {
     [ThreadStatic]
     static ReusableStringWriter? _pooledWriter;
@@ -52,8 +52,20 @@ protected override void Dispose(bool disposing)
             return;
         }
         // We don't call base.Dispose because all it does is mark the writer as closed so it can't be
-        // written to and we want to keep it open as reusable writer.
+        // written to, and we want to keep it open as reusable writer.
         sb.Clear();
         _pooledWriter = this;
     }
+
+#if FEATURE_SPAN
+    public override void Write(ReadOnlySpan<char> buffer)
+    {
+        GetStringBuilder().Append(buffer);
+    }
+
+    public override void WriteLine(ReadOnlySpan<char> buffer)
+    {
+        GetStringBuilder().Append(buffer).AppendLine();
+    }
+#endif
 }