Trim some HttpHeader parsing logic on browser-wasm

src/libraries/System.Net.Http/src/ILLink/ILLink.Substitutions.xml

@@ -6,5 +6,8 @@
     <type fullname="System.Net.Http.HttpClient">
       <method signature="System.Boolean IsNativeHandlerEnabled()" body="stub" value="true" feature="System.Net.Http.UseNativeHttpHandler" featurevalue="true" />
     </type>
+    <type fullname="System.Net.Http.Headers.KnownHeaders">
+      <method signature="System.Boolean get_MinimalHeaderValidation()" body="stub" value="true" feature="System.Net.Http.MinimalHeaderValidation" featurevalue="true" />
+    </type>
   </assembly>
 </linker>

src/libraries/System.Net.Http/src/System/Net/Http/Headers/GenericHeaderParser.cs

@@ -18,7 +18,6 @@ internal sealed class GenericHeaderParser : BaseHeaderParser
         internal static readonly GenericHeaderParser MultipleValueNameValueWithParametersParser = new GenericHeaderParser(true, NameValueWithParametersHeaderValue.GetNameValueWithParametersLength);
         internal static readonly GenericHeaderParser SingleValueNameValueParser = new GenericHeaderParser(false, ParseNameValue);
         internal static readonly GenericHeaderParser MultipleValueNameValueParser = new GenericHeaderParser(true, ParseNameValue);
-        internal static readonly GenericHeaderParser MailAddressParser = new GenericHeaderParser(false, ParseMailAddress);
         internal static readonly GenericHeaderParser SingleValueProductParser = new GenericHeaderParser(false, ParseProduct);
         internal static readonly GenericHeaderParser MultipleValueProductParser = new GenericHeaderParser(true, ParseProduct);
         internal static readonly GenericHeaderParser RangeConditionParser = new GenericHeaderParser(false, RangeConditionHeaderValue.GetRangeConditionLength);
@@ -37,6 +36,9 @@ internal sealed class GenericHeaderParser : BaseHeaderParser
         internal static readonly GenericHeaderParser SingleValueWarningParser = new GenericHeaderParser(false, WarningHeaderValue.GetWarningLength);
         internal static readonly GenericHeaderParser MultipleValueWarningParser = new GenericHeaderParser(true, WarningHeaderValue.GetWarningLength);
 
+        // These parsers are not cached in a static field, so they can be trimmed when not needed
+        internal static GenericHeaderParser GetMailAddressParser() => new GenericHeaderParser(false, ParseMailAddress);
+
         #endregion
 
         private readonly GetParsedValueLengthDelegate _getParsedValueLength;

src/libraries/System.Net.Http/src/System/Net/Http/Headers/KnownHeaders.cs

@@ -9,6 +9,9 @@ namespace System.Net.Http.Headers
 {
     internal static class KnownHeaders
     {
+        private static bool MinimalHeaderValidation { get; } =
+            AppContext.TryGetSwitch("System.Net.Http.MinimalHeaderValidation", out bool minimalHeaderValidation) ? minimalHeaderValidation : false;
+
         // If you add a new entry here, you need to add it to TryGetKnownHeader below as well.
 
         public static readonly KnownHeader PseudoStatus = new KnownHeader(":status", HttpHeaderType.Response, parser: null);
@@ -26,7 +29,7 @@ internal static class KnownHeaders
         public static readonly KnownHeader AccessControlMaxAge = new KnownHeader("Access-Control-Max-Age");
         public static readonly KnownHeader Age = new KnownHeader("Age", HttpHeaderType.Response | HttpHeaderType.NonTrailing, TimeSpanHeaderParser.Parser, null, H2StaticTable.Age, H3StaticTable.Age0);
         public static readonly KnownHeader Allow = new KnownHeader("Allow", HttpHeaderType.Content, GenericHeaderParser.TokenListParser, null, H2StaticTable.Allow);
-        public static readonly KnownHeader AltSvc = new KnownHeader("Alt-Svc", HttpHeaderType.Response, AltSvcHeaderParser.Parser, http3StaticTableIndex: H3StaticTable.AltSvcClear);
+        public static readonly KnownHeader AltSvc = new KnownHeader("Alt-Svc", HttpHeaderType.Response, MinimalHeaderValidation ? null : AltSvcHeaderParser.Parser, http3StaticTableIndex: H3StaticTable.AltSvcClear);
         public static readonly KnownHeader AltUsed = new KnownHeader("Alt-Used", HttpHeaderType.Request, parser: null);
         public static readonly KnownHeader Authorization = new KnownHeader("Authorization", HttpHeaderType.Request | HttpHeaderType.NonTrailing, GenericHeaderParser.SingleValueAuthenticationParser, null, H2StaticTable.Authorization, H3StaticTable.Authorization);
         public static readonly KnownHeader CacheControl = new KnownHeader("Cache-Control", HttpHeaderType.General | HttpHeaderType.NonTrailing, CacheControlHeaderParser.Parser, new string[] { "must-revalidate", "no-cache", "no-store", "no-transform", "private", "proxy-revalidate", "public" }, H2StaticTable.CacheControl, H3StaticTable.CacheControlMaxAge0);
@@ -47,7 +50,7 @@ internal static class KnownHeaders
         public static readonly KnownHeader Expect = new KnownHeader("Expect", HttpHeaderType.Request | HttpHeaderType.NonTrailing, GenericHeaderParser.MultipleValueNameValueWithParametersParser, new string[] { "100-continue" }, H2StaticTable.Expect);
         public static readonly KnownHeader ExpectCT = new KnownHeader("Expect-CT");
         public static readonly KnownHeader Expires = new KnownHeader("Expires", HttpHeaderType.Content | HttpHeaderType.NonTrailing, DateHeaderParser.Parser, null, H2StaticTable.Expires);
-        public static readonly KnownHeader From = new KnownHeader("From", HttpHeaderType.Request, GenericHeaderParser.MailAddressParser, null, H2StaticTable.From);
+        public static readonly KnownHeader From = new KnownHeader("From", HttpHeaderType.Request, MinimalHeaderValidation ? null : GenericHeaderParser.GetMailAddressParser(), null, H2StaticTable.From);
         public static readonly KnownHeader GrpcEncoding = new KnownHeader("grpc-encoding", HttpHeaderType.Custom, null, new string[] { "identity", "gzip", "deflate" });
         public static readonly KnownHeader GrpcMessage = new KnownHeader("grpc-message");
         public static readonly KnownHeader GrpcStatus = new KnownHeader("grpc-status", HttpHeaderType.Custom, null, new string[] { "0" });

src/libraries/System.Net.Http/tests/UnitTests/Headers/GenericHeaderParserTest/MailAddressParserTest.cs

@@ -1,12 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Net.Mail;
 using System.Net.Http.Headers;
-using System.Text;
 
 using Xunit;
 
@@ -17,7 +12,7 @@ public class MailAddressParserTest
         [Fact]
         public void Properties_ReadValues_MatchExpectation()
         {
-            HttpHeaderParser parser = GenericHeaderParser.MailAddressParser;
+            HttpHeaderParser parser = GenericHeaderParser.GetMailAddressParser();
             Assert.False(parser.SupportsMultipleValues);
             Assert.Null(parser.Comparer);
         }
@@ -54,7 +49,7 @@ public void TryParse_SetOfInvalidValueStrings_ReturnsFalse()
         private void CheckValidParsedValue(string input, int startIndex, string expectedResult,
             int expectedIndex)
         {
-            HttpHeaderParser parser = GenericHeaderParser.MailAddressParser;
+            HttpHeaderParser parser = GenericHeaderParser.GetMailAddressParser();
             object result = null;
             Assert.True(parser.TryParseValue(input, null, ref startIndex, out result),
                 string.Format("TryParse returned false: {0}", input));
@@ -64,7 +59,7 @@ private void CheckValidParsedValue(string input, int startIndex, string expected
 
         private void CheckInvalidParsedValue(string input, int startIndex)
         {
-            HttpHeaderParser parser = GenericHeaderParser.MailAddressParser;
+            HttpHeaderParser parser = GenericHeaderParser.GetMailAddressParser();
             object result = null;
             int newIndex = startIndex;
             Assert.False(parser.TryParseValue(input, null, ref newIndex, out result),