Copy only formatted data into right-aligned buffer output

src/ZString.Unity/Assets/Scripts/ZString/FormatHelper.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Text;
 using System.Buffers;
 using System.Runtime.CompilerServices;
@@ -82,7 +82,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }
@@ -163,7 +163,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }

src/ZString.Unity/Assets/Scripts/ZString/Utf8ValueStringBuilder.cs

@@ -460,7 +460,7 @@ private void AppendFormatInternal<T>(T arg, int width, StandardFormat format, st
                         Append(' ', padding);  // TODO Fill Method is too slow.
                     }
 
-                    s.CopyTo(GetSpan(charsWritten));
+                    s.Slice(0, charsWritten).CopyTo(GetSpan(charsWritten));
                     Advance(charsWritten);
                 }
             }

src/ZString/FormatHelper.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Text;
 using System.Buffers;
 using System.Runtime.CompilerServices;
@@ -82,7 +82,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }
@@ -163,7 +163,7 @@ private static void FormatToRightJustify<TBufferWriter, T>(ref TBufferWriter sb,
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }

src/ZString/FormatHelper.tt

@@ -94,7 +94,7 @@ namespace Cysharp.Text
                 }
 
                 var span = sb.GetSpan(charsWritten);
-                s.CopyTo(span);
+                s.Slice(0, charsWritten).CopyTo(span);
                 sb.Advance(charsWritten);
             }
         }

src/ZString/Utf8ValueStringBuilder.cs

@@ -460,7 +460,7 @@ private void AppendFormatInternal<T>(T arg, int width, StandardFormat format, st
                         Append(' ', padding);  // TODO Fill Method is too slow.
                     }
 
-                    s.CopyTo(GetSpan(charsWritten));
+                    s.Slice(0, charsWritten).CopyTo(GetSpan(charsWritten));
                     Advance(charsWritten);
                 }
             }

tests/ZString.Tests/RightAlignedBufferTest.cs

@@ -0,0 +1,53 @@
+using Cysharp.Text;
+using FluentAssertions;
+using System;
+using System.Buffers;
+using System.Text;
+using Xunit;
+
+namespace ZStringTests
+{
+    public class RightAlignedBufferTest
+    {
+        [Theory]
+        [InlineData(1)]
+        [InlineData(100)]
+        public void PreparedUtf16UsesOnlyFormattedCharacters(int value)
+        {
+            var writer = new ArrayBufferWriter<char>(1);
+            ZString.PrepareUtf16<int>("{0,2}").FormatTo(ref writer, value);
+            new string(writer.WrittenSpan).Should().Be(string.Format("{0,2}", value));
+        }
+
+        [Theory]
+        [InlineData(false, 1)]
+        [InlineData(false, 100)]
+        [InlineData(true, 1)]
+        [InlineData(true, 100)]
+        public void Utf8UsesOnlyFormattedBytes(bool prepared, int value)
+        {
+            var writer = new ArrayBufferWriter<byte>(1);
+            if (prepared)
+            {
+                ZString.PrepareUtf8<int>("{0,2}").FormatTo(ref writer, value);
+            }
+            else
+            {
+                ZString.Utf8Format(writer, "{0,2}", value);
+            }
+            Encoding.UTF8.GetString(writer.WrittenSpan).Should().Be(string.Format("{0,2}", value));
+        }
+
+        [Fact]
+        public void Utf8BuilderUsesOnlyFormattedBytesNearCapacity()
+        {
+            using (var builder = ZString.CreateUtf8StringBuilder())
+            {
+                var prefix = new string('x', builder.GetSpan(0).Length - 2);
+                builder.Append(prefix);
+                builder.AppendFormat("{0,1}", 1);
+                builder.ToString().Should().Be(prefix + "1");
+            }
+        }
+    }
+}