Avoid url string allocation in WebProxy.IsMatchInBypassList (#73807)

* Avoid url string allocation in WebProxy.IsMatchInBypassList

* Update src/libraries/System.Net.WebProxy/src/System/Net/WebProxy.cs

Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>

Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>

Author: stephentoub

src/libraries/System.Net.WebProxy/src/System.Net.WebProxy.csproj

@@ -10,6 +10,7 @@
     <Compile Condition="'$(TargetPlatformIdentifier)' != 'Browser'" Include="System\Net\WebProxy.NonBrowser.cs" />
   </ItemGroup>
   <ItemGroup>
+    <Reference Include="System.Memory" />
     <Reference Include="System.Net.NameResolution" />
     <Reference Include="System.Net.NetworkInformation" />
     <Reference Include="System.Net.Primitives" />

src/libraries/System.Net.WebProxy/src/System/Net/WebProxy.cs

@@ -1,7 +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.Buffers;
 using System.Collections;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Runtime.Serialization;
@@ -154,16 +156,26 @@ private bool IsMatchInBypassList(Uri input)
         {
             UpdateRegexList(canThrow: false);
 
-            if (_regexBypassList != null)
+            if (_regexBypassList is Regex[] bypassList)
             {
-                Span<char> stackBuffer = stackalloc char[128];
-                string matchUriString = input.IsDefaultPort ?
-                    string.Create(null, stackBuffer, $"{input.Scheme}://{input.Host}") :
-                    string.Create(null, stackBuffer, $"{input.Scheme}://{input.Host}:{(uint)input.Port}");
+                bool isDefaultPort = input.IsDefaultPort;
+                int lengthRequired = input.Scheme.Length + 3 + input.Host.Length;
+                if (!isDefaultPort)
+                {
+                    lengthRequired += 1 + 5; // 1 for ':' and 5 for max formatted length of a port (16 bit value)
+                }
+
+                int charsWritten;
+                Span<char> url = lengthRequired <= 256 ? stackalloc char[256] : new char[lengthRequired];
+                bool formatted = isDefaultPort ?
+                    url.TryWrite($"{input.Scheme}://{input.Host}", out charsWritten) :
+                    url.TryWrite($"{input.Scheme}://{input.Host}:{(uint)input.Port}", out charsWritten);
+                Debug.Assert(formatted);
+                url = url.Slice(0, charsWritten);
 
-                foreach (Regex r in _regexBypassList)
+                foreach (Regex r in bypassList)
                 {
-                    if (r.IsMatch(matchUriString))
+                    if (r.IsMatch(url))
                     {
                         return true;
                     }