Make non-file URL support optional in XmlUrlResolver

This adds [removable feature annotation](dotnet/designs#42) to `XmlDownloadManager`.

If the user specifies that they would like to remove support for this at publish/native compilation time, the linker/compiler is going to replace the method body of `CreateWebRequestOrThrowIfRemoved` with a throwing method body. The exception message is going to inform the user that the feature has been removed because they opted into the removal.

Contributes to dotnet/corefx#30597. Saves 1.2 MB of the size of the Windows UWP People app. This is a size on disk regression that came with NetStandard 2.0.


Commit migrated from dotnet/corefx@5898cdd

src/libraries/System.Private.Xml/src/System.Private.Xml.csproj

@@ -740,6 +740,7 @@
     <Compile Include="$(CommonPath)\System\LocalAppContext.cs" />
     <Compile Include="System\Xml\Core\LocalAppContextSwitches.cs" />
     <Compile Include="$(CommonPath)\System\CSharpHelpers.cs" />
+    <Compile Include="$(CommonPath)\System\Runtime\CompilerServices\RemovableFeatureAttribute.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsWindows)'=='true'">
     <Compile Include="System\Xml\Xsl\Runtime\XmlCollation.Windows.cs" />

src/libraries/System.Private.Xml/src/System/Xml/XmlDownloadManager.cs

@@ -10,6 +10,7 @@ namespace System.Xml
     using System.Collections;
     using System.Net;
     using System.Net.Cache;
+    using System.Runtime.CompilerServices;
     using System.Runtime.Versioning;
     using System.Net.Http;
 
@@ -33,6 +34,27 @@ internal Stream GetStream(Uri uri, ICredentials credentials, IWebProxy proxy,
 
         private Stream GetNonFileStream(Uri uri, ICredentials credentials, IWebProxy proxy,
             RequestCachePolicy cachePolicy)
+        {
+            WebRequest req = CreateWebRequestOrThrowIfRemoved(uri, credentials, proxy, cachePolicy);
+
+            using (WebResponse resp = req.GetResponse())
+            using (Stream respStream = resp.GetResponseStream())
+            {
+                var result = new MemoryStream();
+                respStream.CopyTo(result);
+                result.Position = 0;
+                return result;
+            }
+        }
+
+        // This method is marked Removable because WebRequest has a lot of dependencies that will bloat
+        // self-contained distributions of .NET Apps.
+        // This code is statically reachable from any place that uses XmlReaderSettings (i.e. every app that
+        // does something XML related is going to have this in their transitive call graph). People rarely need
+        // this functionality though.
+        [RemovableFeature("System.Xml.XmlUrlResolver.NonFileUrlSupport")] 
+        private static WebRequest CreateWebRequestOrThrowIfRemoved(Uri uri, ICredentials credentials, IWebProxy proxy,
+            RequestCachePolicy cachePolicy)
         {
             WebRequest req = WebRequest.Create(uri);
             if (credentials != null)
@@ -48,14 +70,7 @@ private Stream GetNonFileStream(Uri uri, ICredentials credentials, IWebProxy pro
                 req.CachePolicy = cachePolicy;
             }
 
-            using (WebResponse resp = req.GetResponse())
-            using (Stream respStream = resp.GetResponseStream())
-            {
-                var result = new MemoryStream();
-                respStream.CopyTo(result);
-                result.Position = 0;
-                return result;
-            }
+            return req;
         }
     }
 }

src/libraries/System.Private.Xml/src/System/Xml/XmlDownloadManagerAsync.cs

@@ -34,19 +34,7 @@ internal Task<Stream> GetStreamAsync(Uri uri, ICredentials credentials, IWebProx
         private async Task<Stream> GetNonFileStreamAsync(Uri uri, ICredentials credentials, IWebProxy proxy,
             RequestCachePolicy cachePolicy)
         {
-            WebRequest req = WebRequest.Create(uri);
-            if (credentials != null)
-            {
-                req.Credentials = credentials;
-            }
-            if (proxy != null)
-            {
-                req.Proxy = proxy;
-            }
-            if (cachePolicy != null)
-            {
-                req.CachePolicy = cachePolicy;
-            }
+            WebRequest req = CreateWebRequestOrThrowIfRemoved(uri, credentials, proxy, cachePolicy);
 
             using (WebResponse resp = await req.GetResponseAsync().ConfigureAwait(false))
             using (Stream respStream = resp.GetResponseStream())