Fix IBufferWriterExtensions Write for unmanaged types

Author: pziezio

src/CommunityToolkit.HighPerformance/Extensions/IBufferWriterExtensions.cs

@@ -49,7 +49,7 @@ public static unsafe void Write<T>(this IBufferWriter<byte> writer, T value)
         where T : unmanaged
     {
         int length = sizeof(T);
-        Span<byte> span = writer.GetSpan(1);
+        Span<byte> span = writer.GetSpan(length);
 
         if (span.Length < length)
         {

tests/CommunityToolkit.HighPerformance.UnitTests/Extensions/Test_IBufferWriterExtensions.cs

@@ -118,4 +118,20 @@ public void Test_IBufferWriterExtensions_WriteReadOverItems_ReadOnlySpan()
 
         Assert.IsTrue(span.SequenceEqual(buffer.AsSpan()));
     }
+
+    [TestMethod]
+    public unsafe void Test_IBufferWriterExtensions_WriteExceedingFreeCapacity()
+    {
+        ArrayPoolBufferWriter<byte> writer = new();
+
+        // Leave only one byte of free capacity
+        int count = writer.Capacity - 1;
+        for (int i = 0; i < count; i++)
+        {
+            writer.Write<byte>(0);
+        }
+
+        // Write 4 bytes
+        writer.Write(1);
+    }
 }