[STJ] Replace module-wide SkipLocalsInit with per-method [SkipLocalsInit]

src/libraries/Common/src/Polyfills/SkipLocalsInitAttributePolyfill.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Used to indicate to the compiler that the <c>.locals init</c> flag should not be set in method headers.
+/// </summary>
+/// <remarks>
+/// Downlevel polyfill of <see href="https://learn.microsoft.com/dotnet/api/system.runtime.compilerservices.skiplocalsinitattribute">System.Runtime.CompilerServices.SkipLocalsInitAttribute</see>
+/// for target frameworks that do not provide it (.NET Standard 2.0, .NET Framework). The C# compiler recognizes the
+/// attribute by full type name, so providing this internal definition is enough to enable the optimization.
+/// </remarks>
+[AttributeUsage(AttributeTargets.Module
+    | AttributeTargets.Class
+    | AttributeTargets.Struct
+    | AttributeTargets.Interface
+    | AttributeTargets.Constructor
+    | AttributeTargets.Method
+    | AttributeTargets.Property
+    | AttributeTargets.Event, Inherited = false)]
+internal sealed class SkipLocalsInitAttribute : Attribute
+{
+    public SkipLocalsInitAttribute()
+    {
+    }
+}

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -13,6 +13,9 @@
 
 The System.Text.Json library is built-in as part of the shared framework in .NET Runtime. The package can be installed when you need to use it in other target frameworks.</PackageDescription>
     <IncludeSpanPolyfills>true</IncludeSpanPolyfills>
+    <!-- SkipLocalsInit is applied per-method via [SkipLocalsInit] on the specific
+         functions that use stackalloc rather than module-wide. -->
+    <SkipLocalsInit>false</SkipLocalsInit>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->
@@ -402,6 +405,7 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
+    <Compile Include="$(CommonPath)Polyfills\SkipLocalsInitAttributePolyfill.cs" Link="Common\Polyfills\SkipLocalsInitAttributePolyfill.cs" />
     <Compile Include="$(CommonPath)Polyfills\StreamMemoryPolyfills.cs" Link="Common\Polyfills\StreamMemoryPolyfills.cs" />
     <Compile Include="$(CommonPath)Polyfills\StringBuilderPolyfills.cs" Link="Common\Polyfills\StringBuilderPolyfills.cs" />
     <Compile Include="$(CommonPath)Polyfills\StackPolyfills.cs" Link="Common\Polyfills\StackPolyfills.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.TryGetProperty.cs

@@ -3,11 +3,13 @@
 
 using System.Buffers;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
 
 namespace System.Text.Json
 {
     public sealed partial class JsonDocument
     {
+        [SkipLocalsInit]
         internal unsafe bool TryGetNamedPropertyValue(int index, ReadOnlySpan<char> propertyName, out JsonElement value)
         {
             CheckNotDisposed();
@@ -132,6 +134,7 @@ internal bool TryGetNamedPropertyValue(int index, ReadOnlySpan<byte> propertyNam
                 out value);
         }
 
+        [SkipLocalsInit]
         private unsafe bool TryGetNamedPropertyValue(
             int startIndex,
             int endIndex,

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Threading;
 
 namespace System.Text.Json
@@ -302,6 +303,7 @@ private ReadOnlyMemory<byte> GetPropertyRawValue(int valueIndex)
                 : JsonReaderHelper.TranscodeHelper(segment);
         }
 
+        [SkipLocalsInit]
         internal unsafe bool TextEquals(int index, ReadOnlySpan<char> otherText, bool isPropertyName)
         {
             CheckNotDisposed();

src/libraries/System.Text.Json/src/System/Text/Json/JsonEncodedText.cs

@@ -4,6 +4,7 @@
 using System.Buffers;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Text.Encodings.Web;
 
 namespace System.Text.Json
@@ -73,6 +74,7 @@ public static JsonEncodedText Encode(ReadOnlySpan<char> value, JavaScriptEncoder
             return TranscodeAndEncode(value, encoder);
         }
 
+        [SkipLocalsInit]
         private static unsafe JsonEncodedText TranscodeAndEncode(ReadOnlySpan<char> value, JavaScriptEncoder? encoder)
         {
             JsonWriterHelper.ValidateValue(value);

src/libraries/System.Text.Json/src/System/Text/Json/JsonHelpers.Escaping.cs

@@ -25,6 +25,7 @@ public static byte[] GetEscapedPropertyNameSection(ReadOnlySpan<byte> utf8Value,
             }
         }
 
+        [SkipLocalsInit]
         public static unsafe byte[] EscapeValue(
             ReadOnlySpan<byte> utf8Value,
             int firstEscapeIndexVal,
@@ -53,6 +54,7 @@ public static unsafe byte[] EscapeValue(
             return escapedString;
         }
 
+        [SkipLocalsInit]
         private static unsafe byte[] GetEscapedPropertyNameSection(
             ReadOnlySpan<byte> utf8Value,
             int firstEscapeIndexVal,

src/libraries/System.Text.Json/src/System/Text/Json/JsonHelpers.cs

@@ -190,6 +190,7 @@ public static bool TrySkipPartial(this ref Utf8JsonReader reader)
             return reader.TrySkipPartial(reader.CurrentDepth);
         }
 
+        [SkipLocalsInit]
         public static unsafe bool TryLookupUtf8Key<TValue>(
             this Dictionary<string, TValue> dictionary,
             ReadOnlySpan<byte> utf8Key,

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonArray.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Text.Json.Serialization.Converters;
 using System.Threading;
 
@@ -238,6 +239,7 @@ private protected override void SetItem(int index, JsonNode? value)
             List[index] = value;
         }
 
+        [SkipLocalsInit]
         internal override unsafe void GetPath(ref ValueStringBuilder path, JsonNode? child)
         {
             Parent?.GetPath(ref path, this);

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonNode.cs

@@ -1,9 +1,10 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Text.Json.Serialization.Converters;
 using System.Text.Json.Serialization.Metadata;
 
@@ -135,6 +136,7 @@ internal set
         ///   Gets the JSON path.
         /// </summary>
         /// <returns>The JSON Path value.</returns>
+        [SkipLocalsInit]
         public unsafe string GetPath()
         {
             if (Parent == null)

src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.Unescaping.cs

@@ -5,12 +5,14 @@
 using System.Buffers.Text;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Text.Unicode;
 
 namespace System.Text.Json
 {
     internal static partial class JsonReaderHelper
     {
+        [SkipLocalsInit]
         public static unsafe bool TryGetUnescapedBase64Bytes(ReadOnlySpan<byte> utf8Source, [NotNullWhen(true)] out byte[]? bytes)
         {
             byte[]? unescapedArray = null;
@@ -39,6 +41,7 @@ public static unsafe bool TryGetUnescapedBase64Bytes(ReadOnlySpan<byte> utf8Sour
         public static readonly UTF8Encoding s_utf8Encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
 
         // TODO: Similar to escaping, replace the unescaping logic with publicly shipping APIs from https://github.com/dotnet/runtime/issues/27919
+        [SkipLocalsInit]
         public static unsafe string GetUnescapedString(ReadOnlySpan<byte> utf8Source)
         {
             // The escaped name is always >= than the unescaped, so it is safe to use escaped name for the buffer length.
@@ -66,6 +69,7 @@ public static unsafe string GetUnescapedString(ReadOnlySpan<byte> utf8Source)
             return utf8String;
         }
 
+        [SkipLocalsInit]
         public static unsafe byte[] GetUnescaped(ReadOnlySpan<byte> utf8Source)
         {
             // The escaped name is always >= than the unescaped, so it is safe to use escaped name for the buffer length.
@@ -91,6 +95,7 @@ public static unsafe byte[] GetUnescaped(ReadOnlySpan<byte> utf8Source)
             return propertyName;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool UnescapeAndCompare(ReadOnlySpan<byte> utf8Source, ReadOnlySpan<byte> other)
         {
             Debug.Assert(utf8Source.Length >= other.Length && utf8Source.Length / JsonConstants.MaxExpansionFactorWhileEscaping <= other.Length);
@@ -118,6 +123,7 @@ public static unsafe bool UnescapeAndCompare(ReadOnlySpan<byte> utf8Source, Read
             return result;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool UnescapeAndCompare(ReadOnlySequence<byte> utf8Source, ReadOnlySpan<byte> other)
         {
             Debug.Assert(!utf8Source.IsSingleSegment);
@@ -159,6 +165,7 @@ public static unsafe bool UnescapeAndCompare(ReadOnlySequence<byte> utf8Source,
             return result;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool UnescapeAndCompareBothInputs(ReadOnlySpan<byte> utf8Source1, ReadOnlySpan<byte> utf8Source2)
         {
             int index1 = utf8Source1.IndexOf(JsonConstants.BackSlash);
@@ -215,6 +222,7 @@ public static bool TryDecodeBase64InPlace(Span<byte> utf8Unescaped, [NotNullWhen
             return true;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool TryDecodeBase64(ReadOnlySpan<byte> utf8Unescaped, [NotNullWhen(true)] out byte[]? bytes)
         {
             byte[]? pooledArray = null;

src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.cs

@@ -176,6 +176,7 @@ public static bool TryGetValue(ReadOnlySpan<byte> segment, bool isEscaped, out D
             return false;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool TryGetEscapedDateTime(ReadOnlySpan<byte> source, out DateTime value)
         {
             Debug.Assert(source.Length <= JsonConstants.MaximumEscapedDateTimeOffsetParseLength);
@@ -224,6 +225,7 @@ public static bool TryGetValue(ReadOnlySpan<byte> segment, bool isEscaped, out D
             return false;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool TryGetEscapedDateTimeOffset(ReadOnlySpan<byte> source, out DateTimeOffset value)
         {
             Debug.Assert(source.Length <= JsonConstants.MaximumEscapedDateTimeOffsetParseLength);
@@ -273,6 +275,7 @@ public static bool TryGetValue(ReadOnlySpan<byte> segment, bool isEscaped, out G
             return false;
         }
 
+        [SkipLocalsInit]
         public static unsafe bool TryGetEscapedGuid(ReadOnlySpan<byte> source, out Guid value)
         {
             Debug.Assert(source.Length <= JsonConstants.MaximumEscapedGuidLength);

src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.MultiSegment.cs

@@ -535,6 +535,7 @@ private bool ConsumeLiteralMultiSegment(ReadOnlySpan<byte> literal, JsonTokenTyp
             return true;
         }
 
+        [SkipLocalsInit]
         private unsafe bool CheckLiteralMultiSegment(ReadOnlySpan<byte> span, ReadOnlySpan<byte> literal, out int consumed)
         {
             Debug.Assert(span.Length > 0 && span[0] == literal[0] && literal.Length <= JsonConstants.MaximumLiteralLength);

src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.TryGet.cs

@@ -140,6 +140,7 @@ public readonly int CopyString(Span<char> destination)
             return CopyValue(destination);
         }
 
+        [SkipLocalsInit]
         internal readonly unsafe int CopyValue(Span<char> destination)
         {
             Debug.Assert(_tokenType is JsonTokenType.String or JsonTokenType.PropertyName or JsonTokenType.Number);
@@ -192,6 +193,7 @@ internal readonly unsafe int CopyValue(Span<char> destination)
             return charsWritten;
         }
 
+        [SkipLocalsInit]
         private readonly unsafe bool TryCopyEscapedString(Span<byte> destination, out int bytesWritten)
         {
             Debug.Assert(_tokenType is JsonTokenType.String or JsonTokenType.PropertyName);
@@ -1241,6 +1243,7 @@ public bool TryGetDateTime(out DateTime value)
             return TryGetDateTimeCore(out value);
         }
 
+        [SkipLocalsInit]
         internal unsafe bool TryGetDateTimeCore(out DateTime value)
         {
             scoped ReadOnlySpan<byte> span;
@@ -1286,6 +1289,7 @@ public bool TryGetDateTimeOffset(out DateTimeOffset value)
             return TryGetDateTimeOffsetCore(out value);
         }
 
+        [SkipLocalsInit]
         internal unsafe bool TryGetDateTimeOffsetCore(out DateTimeOffset value)
         {
             scoped ReadOnlySpan<byte> span;
@@ -1332,6 +1336,7 @@ public bool TryGetGuid(out Guid value)
             return TryGetGuidCore(out value);
         }
 
+        [SkipLocalsInit]
         internal unsafe bool TryGetGuidCore(out Guid value)
         {
             scoped ReadOnlySpan<byte> span;

src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.cs

@@ -521,6 +521,7 @@ private readonly bool TextEqualsHelper(ReadOnlySpan<byte> otherUtf8Text)
         ///     if required. The look up text is matched as is, without any modifications to it.
         ///   </para>
         /// </remarks>
+        [SkipLocalsInit]
         public readonly unsafe bool ValueTextEquals(ReadOnlySpan<char> text)
         {
             if (!IsTokenTypeString(TokenType))

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/CharConverter.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
 using System.Text.Json.Schema;
 
 namespace System.Text.Json.Serialization.Converters
@@ -10,6 +11,7 @@ internal sealed class CharConverter : JsonPrimitiveConverter<char>
     {
         private const int MaxEscapedCharacterLength = JsonConstants.MaxExpansionFactorWhileEscaping;
 
+        [SkipLocalsInit]
         public override unsafe char Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
             if (reader.TokenType is not (JsonTokenType.String or JsonTokenType.PropertyName))

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/DateOnlyConverter.cs

@@ -1,8 +1,9 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// 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.Globalization;
+using System.Runtime.CompilerServices;
 using System.Text.Json.Schema;
 
 namespace System.Text.Json.Serialization.Converters
@@ -28,6 +29,7 @@ internal override DateOnly ReadAsPropertyNameCore(ref Utf8JsonReader reader, Typ
             return ReadCore(ref reader);
         }
 
+        [SkipLocalsInit]
         private static unsafe DateOnly ReadCore(ref Utf8JsonReader reader)
         {
             if (!JsonHelpers.IsInRangeInclusive(reader.ValueLength, FormatLength, MaxEscapedFormatLength))
@@ -62,6 +64,7 @@ private static unsafe DateOnly ReadCore(ref Utf8JsonReader reader)
             return value;
         }
 
+        [SkipLocalsInit]
         public override unsafe void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
         {
             Span<byte> buffer = stackalloc byte[FormatLength];
@@ -70,6 +73,7 @@ public override unsafe void Write(Utf8JsonWriter writer, DateOnly value, JsonSer
             writer.WriteStringValue(buffer);
         }
 
+        [SkipLocalsInit]
         internal override unsafe void WriteAsPropertyNameCore(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options, bool isWritingExtensionDataProperty)
         {
             Span<byte> buffer = stackalloc byte[FormatLength];

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs

@@ -233,6 +233,7 @@ internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, T value, J
             }
         }
 
+        [SkipLocalsInit]
         private unsafe bool TryParseEnumFromString(ref Utf8JsonReader reader, out T result)
         {
             Debug.Assert(reader.TokenType is JsonTokenType.String or JsonTokenType.PropertyName);
@@ -393,6 +394,7 @@ private static T ConvertFromUInt64(ulong value)
         /// <summary>
         /// Attempt to format the enum value as a comma-separated string of flag values, or returns false if not a valid flag combination.
         /// </summary>
+        [SkipLocalsInit]
         private unsafe string FormatEnumAsString(ulong key, T value, JsonNamingPolicy? dictionaryKeyPolicy)
         {
             Debug.Assert(IsDefinedValueOrCombinationOfValues(key), "must only be invoked against valid enum values.");