Improve DateTime{Offset} formatting further in a variety of cases

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -128,8 +128,6 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\FormattingHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Boolean.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Date.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Date.G.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Date.L.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Decimal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Float.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Guid.cs" />

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Utf8Constants.cs

@@ -14,12 +14,6 @@ internal static partial class Utf8Constants
         public const byte Space = (byte)' ';
         public const byte Hyphen = (byte)'-';
 
-        // Invariant formatting uses groups of 3 for each number group separated by commas.
-        //   ex. 1,234,567,890
-        public const int GroupSize = 3;
-
-        public static readonly TimeSpan NullUtcOffset = TimeSpan.MinValue;  // Utc offsets must range from -14:00 to 14:00 so this is never a valid offset.
-
         public const int DateTimeMaxUtcOffsetHours = 14; // The UTC offset portion of a TimeSpan or DateTime can be no more than 14 hours and no less than -14 hours.
 
         public const int DateTimeNumFractionDigits = 7;  // TimeSpan and DateTime formats allow exactly up to many digits for specifying the fraction after the seconds.

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs

@@ -1,6 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+using System.Text;
+
 namespace System.Buffers.Text
 {
     public static partial class Utf8Formatter
@@ -29,18 +32,15 @@ public static partial class Utf8Formatter
         /// </exceptions>
         public static bool TryFormat(DateTimeOffset value, Span<byte> destination, out int bytesWritten, StandardFormat format = default)
         {
-            TimeSpan offset = Utf8Constants.NullUtcOffset;
-            char symbol = format.Symbol;
             if (format.IsDefault)
             {
-                symbol = 'G';
-                offset = value.Offset;
+                return DateTimeFormat.TryFormatInvariantG(value.DateTime, value.Offset, destination, out bytesWritten);
             }
 
-            switch (symbol)
+            switch (format.Symbol)
             {
                 case 'R':
-                    return DateTimeFormat.TryFormatR(value.UtcDateTime, new TimeSpan(DateTimeFormat.NullOffset), destination, out bytesWritten);
+                    return DateTimeFormat.TryFormatR(value.UtcDateTime, NullOffset, destination, out bytesWritten);
 
                 case 'O':
                     return DateTimeFormat.TryFormatO(value.DateTime, value.Offset, destination, out bytesWritten);
@@ -49,7 +49,7 @@ public static bool TryFormat(DateTimeOffset value, Span<byte> destination, out i
                     return TryFormatDateTimeL(value.UtcDateTime, destination, out bytesWritten);
 
                 case 'G':
-                    return TryFormatDateTimeG(value.DateTime, offset, destination, out bytesWritten);
+                    return DateTimeFormat.TryFormatInvariantG(value.DateTime, NullOffset, destination, out bytesWritten);
 
                 default:
                     ThrowHelper.ThrowFormatException_BadFormatSpecifier();
@@ -83,21 +83,36 @@ public static bool TryFormat(DateTime value, Span<byte> destination, out int byt
             switch (FormattingHelpers.GetSymbolOrDefault(format, 'G'))
             {
                 case 'R':
-                    return DateTimeFormat.TryFormatR(value, new TimeSpan(DateTimeFormat.NullOffset), destination, out bytesWritten);
+                    return DateTimeFormat.TryFormatR(value, NullOffset, destination, out bytesWritten);
 
                 case 'O':
-                    return DateTimeFormat.TryFormatO(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
+                    return DateTimeFormat.TryFormatO(value, NullOffset, destination, out bytesWritten);
 
                 case 'l':
                     return TryFormatDateTimeL(value, destination, out bytesWritten);
 
                 case 'G':
-                    return TryFormatDateTimeG(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
+                    return DateTimeFormat.TryFormatInvariantG(value, NullOffset, destination, out bytesWritten);
 
                 default:
                     ThrowHelper.ThrowFormatException_BadFormatSpecifier();
                     goto case 'R'; // unreachable
             }
         }
+
+        // Rfc1123 lowercased
+        private static bool TryFormatDateTimeL(DateTime value, Span<byte> destination, out int bytesWritten)
+        {
+            if (DateTimeFormat.TryFormatR(value, NullOffset, destination, out bytesWritten))
+            {
+                Debug.Assert(bytesWritten == DateTimeFormat.FormatRLength);
+                Ascii.ToLowerInPlace(destination.Slice(0, bytesWritten), out bytesWritten);
+                return true;
+            }
+
+            return false;
+        }
+
+        private static TimeSpan NullOffset => new TimeSpan(DateTimeFormat.NullOffset);
     }
 }

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ValueListBuilder.cs

@@ -166,6 +166,11 @@ public void Dispose()
             }
         }
 
+        // Note that consuming implementations depend on the list only growing if it's absolutely
+        // required.  If the list is already large enough to hold the additional items be added,
+        // it must not grow. The list is used in a number of places where the reference is checked
+        // and it's expected to match the initial reference provided to the constructor if that
+        // span was sufficiently large.
         private void Grow(int additionalCapacityRequired = 1)
         {
             const int ArrayMaxLength = 0x7FFFFFC7; // same as Array.MaxLength

src/libraries/System.Private.CoreLib/src/System/DateOnly.cs

@@ -772,7 +772,7 @@ public string ToString([StringSyntax(StringSyntaxAttribute.DateOnlyFormat)] stri
                 }
             }
 
-            DateTimeFormat.IsValidCustomDateFormat(format.AsSpan(), throwOnError: true);
+            DateTimeFormat.IsValidCustomDateOnlyFormat(format.AsSpan(), throwOnError: true);
             return DateTimeFormat.Format(GetEquivalentDateTime(), format, provider);
         }
 
@@ -820,7 +820,7 @@ private bool TryFormatCore<TChar>(Span<TChar> destination, out int charsWritten,
                 }
             }
 
-            if (!DateTimeFormat.IsValidCustomDateFormat(format, throwOnError: false))
+            if (!DateTimeFormat.IsValidCustomDateOnlyFormat(format, throwOnError: false))
             {
                 throw new FormatException(SR.Format(SR.Format_DateTimeOnlyContainsNoneDateParts, format.ToString(), nameof(DateOnly)));
             }