Tell dotnet restore to install nuget.props and nuget.targets files using an environment variable, so the SDK .targets are installed into the user's project.

src/dotnet/ForwardingApp.cs

@@ -1,38 +1,34 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using Microsoft.DotNet.Cli.Utils;
-using Microsoft.DotNet.ProjectModel;
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
-using System.Runtime.InteropServices;
-using System.Threading.Tasks;
+using Microsoft.DotNet.Cli.Utils;
+using Microsoft.DotNet.ProjectModel;
 
 namespace Microsoft.DotNet.Cli
 {
-   /// <summary>
-   /// A class which encapsulates logic needed to forward arguments from the current process to another process
-   /// invoked with the dotnet.exe host.
-   /// </summary>
+    /// <summary>
+    /// A class which encapsulates logic needed to forward arguments from the current process to another process
+    /// invoked with the dotnet.exe host.
+    /// </summary>
     public class ForwardingApp
     {
         private const string s_hostExe = "dotnet";
 
         private readonly string _forwardApplicationPath;
-        private readonly string[] _argsToForward;
+        private readonly IEnumerable<string> _argsToForward;
         private readonly string _depsFile;
         private readonly string _runtimeConfig;
         private readonly string _additionalProbingPath;
-        private readonly Dictionary<string, string> _environmentVariables;
+        private Dictionary<string, string> _environmentVariables;
 
         private readonly string[] _allArgs;
-        
+
         public ForwardingApp(
-            string forwardApplicationPath, 
-            string[] argsToForward, 
-            string depsFile = null, 
+            string forwardApplicationPath,
+            IEnumerable<string> argsToForward,
+            string depsFile = null,
             string runtimeConfig = null,
             string additionalProbingPath = null,
             Dictionary<string, string> environmentVariables = null)
@@ -43,7 +39,7 @@ namespace Microsoft.DotNet.Cli
             _runtimeConfig = runtimeConfig;
             _additionalProbingPath = additionalProbingPath;
             _environmentVariables = environmentVariables;
-            
+
             var allArgs = new List<string>();
             allArgs.Add("exec");
 
@@ -99,6 +95,15 @@ namespace Microsoft.DotNet.Cli
             return process.ExitCode;
         }
 
+        public ForwardingApp WithEnvironmentVariable(string name, string value)
+        {
+            _environmentVariables = _environmentVariables ?? new Dictionary<string, string>();
+
+            _environmentVariables.Add(name, value);
+
+            return this;
+        }
+
         private string GetHostExeName()
         {
             return $"{s_hostExe}{FileNameSuffixes.CurrentPlatform.Exe}";

src/dotnet/commands/dotnet-restore/NuGet3.cs

@@ -1,9 +1,6 @@
 using System;
 using System.Collections.Generic;
-using System.IO;
 using System.Linq;
-using System.Reflection;
-using Microsoft.DotNet.Cli.Utils;
 
 namespace Microsoft.DotNet.Tools.Restore
 {
@@ -19,11 +16,18 @@ namespace Microsoft.DotNet.Tools.Restore
             }
             prefixArgs.Add("restore");
 
-            var result = new NuGetForwardingApp(Enumerable.Concat(
-                    prefixArgs,
-                    args).ToArray()).Execute();
+            var nugetApp = new NuGetForwardingApp(Enumerable.Concat(prefixArgs, args));
 
-            return result;
+            // setting NUGET_XPROJ_WRITE_TARGETS will tell nuget restore to install .props and .targets files
+            // coming from NuGet packages
+            const string nugetXProjWriteTargets = "NUGET_XPROJ_WRITE_TARGETS";
+            bool setXProjWriteTargets = Environment.GetEnvironmentVariable(nugetXProjWriteTargets) == null;
+            if (setXProjWriteTargets)
+            {
+                nugetApp.WithEnvironmentVariable(nugetXProjWriteTargets, "true");
+            }
+
+            return nugetApp.Execute();
         }
     }
 }

src/dotnet/commands/dotnet-restore/NuGetForwardingApp.cs

@@ -1,14 +1,10 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using Microsoft.DotNet.Cli;
-using Microsoft.DotNet.Cli.Utils;
-using Microsoft.DotNet.InternalAbstractions;
 using System;
 using System.Collections.Generic;
 using System.IO;
-using System.Linq;
-using System.Threading.Tasks;
+using Microsoft.DotNet.Cli;
 
 namespace Microsoft.DotNet.Tools.Restore
 {
@@ -17,7 +13,7 @@ namespace Microsoft.DotNet.Tools.Restore
         private const string s_nugetExeName = "NuGet.CommandLine.XPlat.dll";
         private readonly ForwardingApp _forwardingApp;
 
-        public NuGetForwardingApp(string[] argsToForward)
+        public NuGetForwardingApp(IEnumerable<string> argsToForward)
         {
             _forwardingApp = new ForwardingApp(
                 GetNuGetExePath(),
@@ -29,6 +25,13 @@ namespace Microsoft.DotNet.Tools.Restore
             return _forwardingApp.Execute();
         }
 
+        public NuGetForwardingApp WithEnvironmentVariable(string name, string value)
+        {
+            _forwardingApp.WithEnvironmentVariable(name, value);
+
+            return this;
+        }
+
         private static string GetNuGetExePath()
         {
             return Path.Combine(