Protoype for nonallocating string formatting

corefxlab.sln

@@ -98,6 +98,10 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.Experimental.Coll
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.Experimental.Collections.Tests", "tests\Microsoft.Experimental.Collections.Tests\Microsoft.Experimental.Collections.Tests.csproj", "{71D6F42F-95F7-486D-A659-22503A6D3D46}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.ValueBuilder", "src\System.Text.ValueBuilder\System.Text.ValueBuilder.csproj", "{A679FE55-5C11-4CE2-A634-B78AC8C76C29}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.ValueBuilder.Tests", "tests\System.Text.ValueBuilder.Tests\System.Text.ValueBuilder.Tests.csproj", "{20F82CD0-49EE-4A2A-945C-22BC34B136D3}"
+EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Security.Cryptography.Asn1.Experimental", "src\System.Security.Cryptography.Asn1.Experimental\System.Security.Cryptography.Asn1.Experimental.csproj", "{D117AC96-F53F-47A8-81DF-F4A7CCEDB58E}"
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Security.Cryptography.Asn1.Experimental.Tests", "tests\System.Security.Cryptography.Asn1.Experimental.Tests\System.Security.Cryptography.Asn1.Experimental.Tests.csproj", "{C58BC591-9C3F-44E8-8408-7C57B1806B21}"
@@ -616,6 +620,30 @@ Global
 		{71D6F42F-95F7-486D-A659-22503A6D3D46}.Release|x64.Build.0 = Release|Any CPU
 		{71D6F42F-95F7-486D-A659-22503A6D3D46}.Release|x86.ActiveCfg = Release|Any CPU
 		{71D6F42F-95F7-486D-A659-22503A6D3D46}.Release|x86.Build.0 = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|x64.Build.0 = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Debug|x86.Build.0 = Debug|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|Any CPU.Build.0 = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|x64.ActiveCfg = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|x64.Build.0 = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|x86.ActiveCfg = Release|Any CPU
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29}.Release|x86.Build.0 = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|x64.Build.0 = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Debug|x86.Build.0 = Debug|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|Any CPU.Build.0 = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|x64.ActiveCfg = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|x64.Build.0 = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|x86.ActiveCfg = Release|Any CPU
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3}.Release|x86.Build.0 = Release|Any CPU
 		{D117AC96-F53F-47A8-81DF-F4A7CCEDB58E}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{D117AC96-F53F-47A8-81DF-F4A7CCEDB58E}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{D117AC96-F53F-47A8-81DF-F4A7CCEDB58E}.Debug|x64.ActiveCfg = Debug|Any CPU
@@ -687,6 +715,8 @@ Global
 		{70873943-22E2-4254-9CE6-A0186586DCEC} = {3079E458-D0E6-4F99-8CAB-80011D35C7DA}
 		{25E150ED-EAE6-4EB0-BA94-2FE25826EEDE} = {4B000021-5278-4F2A-B734-DE49F55D4024}
 		{71D6F42F-95F7-486D-A659-22503A6D3D46} = {3079E458-D0E6-4F99-8CAB-80011D35C7DA}
+		{A679FE55-5C11-4CE2-A634-B78AC8C76C29} = {4B000021-5278-4F2A-B734-DE49F55D4024}
+		{20F82CD0-49EE-4A2A-945C-22BC34B136D3} = {3079E458-D0E6-4F99-8CAB-80011D35C7DA}
 		{D117AC96-F53F-47A8-81DF-F4A7CCEDB58E} = {4B000021-5278-4F2A-B734-DE49F55D4024}
 		{C58BC591-9C3F-44E8-8408-7C57B1806B21} = {3079E458-D0E6-4F99-8CAB-80011D35C7DA}
 	EndGlobalSection

src/System.Text.ValueBuilder/System.Text.ValueBuilder.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="..\..\tools\common.props" />
+  <PropertyGroup>
+    <Description>Low allocation string building.</Description>
+    <Copyright>Microsoft Corporation, All rights reserved.</Copyright>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <PackageTags>.NET formatting parsing encoding UTF8</PackageTags>
+    <RootNamespace />
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.1" />
+  </ItemGroup>
+</Project>

src/System.Text.ValueBuilder/System/FormatString.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System
+{
+    /// <summary>
+    /// Simple wrapper for a format specifier string. We could potentially use this to indicate to C# that we have a direct
+    /// string formatting method.
+    /// </summary>
+    /// <remarks>
+    /// <see cref="IO.FormattingTextWriter"/> for examples of how this would be used.
+    /// </remarks>
+    public readonly ref struct FormatString
+    {
+        public ReadOnlySpan<char> Format { get; }
+
+        public FormatString(ReadOnlySpan<char> format)
+        {
+            Format = format;
+        }
+
+        public int Length => Format.Length;
+
+        public static implicit operator FormatString(string format) => new FormatString(format);
+        public static implicit operator FormatString(ReadOnlySpan<char> format) => new FormatString(format);
+    }
+}

src/System.Text.ValueBuilder/System/IO/FormattingTextWriter.cs

@@ -0,0 +1,41 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Text;
+
+namespace System.IO
+{
+    public class FormattingTextWriter : TextWriter
+    {
+        private Encoding _encoding;
+
+        public FormattingTextWriter(Encoding encoding)
+        {
+            _encoding = encoding;
+        }
+
+        public override Encoding Encoding => _encoding;
+
+        /// <remarks>
+        /// If you were to call Write($"Some string {value}"), C# would see that we have a Write() that takes
+        /// <see cref="FormatString"/> and would prefer that. It would create a <see cref="Variant"/> around
+        /// value and pass it via <see cref="Variant.ToSpan(in Variant)"/>.
+        /// 
+        /// Additionally, if you call Write($"Some string {value}", CultureInfo.CurrentCulture) it would get
+        /// transformed into a call to this specific method. General rules:
+        /// 
+        ///  1. When invoking a method with an interpolated string...
+        ///  2. If there is an overload that matches with FormatString, ReadOnlySpan(Variant) in the $ position...
+        ///  3. Use it
+        /// </remarks>
+        public void Write(FormatString format, ReadOnlySpan<Variant> args, IFormatProvider formatProvider = null)
+        {
+            ValueStringBuilder vsb = new ValueStringBuilder(format.Length);
+            vsb.Append(format, args, formatProvider);
+
+            Write(vsb.AsSpan());
+            vsb.Dispose();
+        }
+    }
+}