Use a task to launch the linker

src/Components/Blazor/Build/src/Tasks/BlazorCreateRootDescriptorFile.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Xml;
+using System.Xml.Linq;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.AspNetCore.Blazor.Build
+{
+    // Based on https://github.com/mono/linker/blob/3b329b9481e300bcf4fb88a2eebf8cb5ef8b323b/src/ILLink.Tasks/CreateRootDescriptorFile.cs
+    public class BlazorCreateRootDescriptorFile : Task
+    {
+        [Required]
+        public ITaskItem[] AssemblyNames { get; set; }
+
+        [Required]
+        public ITaskItem RootDescriptorFilePath { get; set; }
+
+        public override bool Execute()
+        {
+            using var fileStream = File.Create(RootDescriptorFilePath.ItemSpec);
+            var assemblyNames = AssemblyNames.Select(a => a.ItemSpec);
+
+            WriteRootDescriptor(fileStream, assemblyNames);
+            return true;
+        }
+
+        internal static void WriteRootDescriptor(Stream stream, IEnumerable<string> assemblyNames)
+        {
+            var roots = new XElement("linker");
+            foreach (var assemblyName in assemblyNames)
+            {
+                roots.Add(new XElement("assembly",
+                    new XAttribute("fullname", assemblyName),
+                    new XElement("type",
+                        new XAttribute("fullname", "*"),
+                        new XAttribute("required", "true"))));
+            }
+
+            var xmlWriterSettings = new XmlWriterSettings
+            {
+                Indent = true,
+                OmitXmlDeclaration = true
+            };
+
+            using var writer = XmlWriter.Create(stream, xmlWriterSettings);
+            var xDocument = new XDocument(roots);
+
+            xDocument.Save(writer);
+        }
+    }
+}

src/Components/Blazor/Build/src/Tasks/BlazorILLink.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.AspNetCore.Blazor.Build.Tasks
+{
+    // Based on https://github.com/mono/linker/blob/3b329b9481e300bcf4fb88a2eebf8cb5ef8b323b/src/ILLink.Tasks/LinkTask.cs
+    public class BlazorILLink : ToolTask
+    {
+        private const string DotNetHostPathEnvironmentName = "DOTNET_HOST_PATH";
+
+        [Required]
+        public string ILLinkPath { get; set; }
+
+        [Required]
+        public ITaskItem[] AssemblyPaths { get; set; }
+
+        public ITaskItem[] ReferenceAssemblyPaths { get; set; }
+
+        [Required]
+        public ITaskItem[] RootAssemblyNames { get; set; }
+
+        [Required]
+        public ITaskItem OutputDirectory { get; set; }
+
+        public ITaskItem[] RootDescriptorFiles { get; set; }
+
+        public bool ClearInitLocals { get; set; }
+
+        public string ClearInitLocalsAssemblies { get; set; }
+
+        public string ExtraArgs { get; set; }
+
+        public bool DumpDependencies { get; set; }
+
+        private string _dotnetPath;
+
+        private string DotNetPath
+        {
+            get
+            {
+                if (!string.IsNullOrEmpty(_dotnetPath))
+                {
+                    return _dotnetPath;
+                }
+
+                _dotnetPath = Environment.GetEnvironmentVariable(DotNetHostPathEnvironmentName);
+                if (string.IsNullOrEmpty(_dotnetPath))
+                {
+                    throw new InvalidOperationException($"{DotNetHostPathEnvironmentName} is not set");
+                }
+
+                return _dotnetPath;
+            }
+        }
+
+        protected override MessageImportance StandardErrorLoggingImportance => MessageImportance.High;
+
+        protected override string ToolName => Path.GetFileName(DotNetPath);
+
+        protected override string GenerateFullPathToTool() => DotNetPath;
+
+        protected override string GenerateCommandLineCommands() => ILLinkPath;
+
+        private static string Quote(string path)
+        {
+            return $"\"{path.TrimEnd('\\')}\"";
+        }
+
+        protected override string GenerateResponseFileCommands()
+        {
+            var args = new StringBuilder();
+
+            if (RootDescriptorFiles != null)
+            {
+                foreach (var rootFile in RootDescriptorFiles)
+                {
+                    args.Append("-x ").AppendLine(Quote(rootFile.ItemSpec));
+                }
+            }
+
+            foreach (var assemblyItem in RootAssemblyNames)
+            {
+                args.Append("-a ").AppendLine(Quote(assemblyItem.ItemSpec));
+            }
+
+            var assemblyNames = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            foreach (var assembly in AssemblyPaths)
+            {
+                var assemblyPath = assembly.ItemSpec;
+                var assemblyName = Path.GetFileNameWithoutExtension(assemblyPath);
+
+                // If there are multiple paths with the same assembly name, only use the first one.
+                if (!assemblyNames.Add(assemblyName))
+                {
+                    continue;
+                }
+
+                args.Append("-reference ")
+                    .AppendLine(Quote(assemblyPath));
+
+                var action = assembly.GetMetadata("action");
+                if ((action != null) && (action.Length > 0))
+                {
+                    args.Append("-p ");
+                    args.Append(action);
+                    args.Append(" ").AppendLine(Quote(assemblyName));
+                }
+            }
+
+            if (ReferenceAssemblyPaths != null)
+            {
+                foreach (var assembly in ReferenceAssemblyPaths)
+                {
+                    var assemblyPath = assembly.ItemSpec;
+                    var assemblyName = Path.GetFileNameWithoutExtension(assemblyPath);
+
+                    // Don't process references for which we already have
+                    // implementation assemblies.
+                    if (assemblyNames.Contains(assemblyName))
+                    {
+                        continue;
+                    }
+
+                    args.Append("-reference ").AppendLine(Quote(assemblyPath));
+
+                    // Treat reference assemblies as "skip". Ideally we
+                    // would not even look at the IL, but only use them to
+                    // resolve surface area.
+                    args.Append("-p skip ").AppendLine(Quote(assemblyName));
+                }
+            }
+
+            if (OutputDirectory != null)
+            {
+                args.Append("-out ").AppendLine(Quote(OutputDirectory.ItemSpec));
+            }
+
+            if (ClearInitLocals)
+            {
+                args.AppendLine("--enable-opt clearinitlocals");
+                if ((ClearInitLocalsAssemblies != null) && (ClearInitLocalsAssemblies.Length > 0))
+                {
+                    args.Append("-m ClearInitLocalsAssemblies ");
+                    args.AppendLine(ClearInitLocalsAssemblies);
+                }
+            }
+
+            if (ExtraArgs != null)
+            {
+                args.AppendLine(ExtraArgs);
+            }
+
+            if (DumpDependencies)
+            {
+                args.AppendLine("--dump-dependencies");
+            }
+
+            return args.ToString();
+        }
+
+        protected override bool HandleTaskExecutionErrors()
+        {
+            // Show a slightly better error than the standard ToolTask message that says "dotnet" failed.
+            Log.LogError($"ILLink failed with exited code {ExitCode}.");
+            return false;
+        }
+
+        protected override void LogEventsFromTextOutput(string singleLine, MessageImportance messageImportance)
+        {
+            if (!string.IsNullOrEmpty(singleLine) && singleLine.StartsWith("Unhandled exception.", StringComparison.Ordinal))
+            {
+                // The Mono linker currently prints out an entire stack trace when the linker fails.
+                // We want to show something actionable in the VS Error window.
+                Log.LogError(singleLine);
+            }
+            else
+            {
+                base.LogEventsFromTextOutput(singleLine, messageImportance);
+            }
+        }
+    }
+}