Pass ExcludeRestorePackageImports during restore to avoid redundant evaluations

documentation/wiki/ChangeWaves.md

@@ -29,6 +29,9 @@ Change wave checks around features will be removed in the release that accompani
 
 ## Current Rotation of Change Waves
 
+### 18.10
+- [Restore passes ExcludeRestorePackageImports=true as a global property so NuGet's restore no longer triggers a second evaluation of every project.](https://github.com/dotnet/msbuild/issues/14273)
+
 ### 18.9
 - [GenerateResource: typed ResX data/metadata entries in Mark-of-the-Web files are now treated as untrusted and blocked with MSB3821; unblock the file (or set MSBUILDDISABLEFEATURESFROMVERSION=18.9) to restore prior behavior. ResXFileRef entries are always blocked regardless of this wave.](https://github.com/dotnet/msbuild/pull/14015)
 - [TaskHost named-pipe buffers default to 1 MB (was 128 KB), reducing send backpressure for large TaskHostConfiguration packets. Tunable via MSBUILDNODECONNECTIONBUFFERSIZE](https://github.com/dotnet/msbuild/pull/14094)

src/Framework/ChangeWaves.cs

@@ -37,7 +37,8 @@ internal static class ChangeWaves
         internal static readonly Version Wave18_7 = new Version(18, 7);
         internal static readonly Version Wave18_8 = new Version(18, 8);
         internal static readonly Version Wave18_9 = new Version(18, 9);
-        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3, Wave18_4, Wave18_5, Wave18_6, Wave18_7, Wave18_8, Wave18_9];
+        internal static readonly Version Wave18_10 = new Version(18, 10);
+        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3, Wave18_4, Wave18_5, Wave18_6, Wave18_7, Wave18_8, Wave18_9, Wave18_10];
 
         /// <summary>
         /// Special value indicating that all features behind all Change Waves should be enabled.

src/Framework/MSBuildConstants.cs

@@ -80,6 +80,20 @@ internal static class MSBuildConstants
         /// </summary>
         internal const string MSBuildIsRestoring = nameof(MSBuildIsRestoring);
 
+        /// <summary>
+        /// A property set during an implicit restore (/restore) or explicit restore (/t:restore) to instruct projects to exclude imports that are
+        /// provided by NuGet packages. NuGet passes this same global property when it re-invokes the build, so setting it up front ensures the
+        /// initial evaluation is reused rather than being discarded and re-evaluated.
+        /// </summary>
+        internal const string ExcludeRestorePackageImports = nameof(ExcludeRestorePackageImports);
+
+        /// <summary>
+        /// The value MSBuild assigns to <see cref="ExcludeRestorePackageImports"/> during restore. It must match the literal value NuGet passes
+        /// (lowercase "true" from NuGet.targets) because global property values are compared case-sensitively when matching build configurations
+        /// for evaluation reuse.
+        /// </summary>
+        internal const string ExcludeRestorePackageImportsValue = "true";
+
         /// <summary>
         /// The most current VSGeneralAssemblyVersion known to this version of MSBuild.
         /// </summary>

src/MSBuild.UnitTests/XMake_Tests.cs

@@ -2615,6 +2615,49 @@ public void RestoreRunsInitialTargets()
             logContents.ShouldContain("Restore target ran");
         }
 
+        /// <summary>
+        /// Verifies restore sets the ExcludeRestorePackageImports global property so that NuGet's restore reuses the initial
+        /// evaluation instead of forcing a second evaluation of every project.
+        /// </summary>
+        [Fact]
+        public void RestoreSetsExcludeRestorePackageImports()
+        {
+            string projectContents = ObjectModelHelpers.CleanupFileContents(
+                @"<Project>
+  <Target Name=""Restore"">
+    <Message Text=""ExcludeRestorePackageImports=[$(ExcludeRestorePackageImports)]"" Importance=""High"" />
+  </Target>
+</Project>");
+
+            string logContents = ExecuteMSBuildExeExpectSuccess(projectContents, arguments: "/t:restore");
+
+            // The value must be the literal lowercase "true" to exactly match the global property NuGet's restore passes
+            // (NuGet.targets sets ExcludeRestorePackageImports=true), otherwise evaluation reuse would not occur.
+            logContents.ShouldContain($"ExcludeRestorePackageImports=[{MSBuildConstants.ExcludeRestorePackageImportsValue}]");
+        }
+
+        /// <summary>
+        /// Verifies restore does not set the ExcludeRestorePackageImports global property when change wave 18.10 is disabled,
+        /// preserving the prior behavior for repositories that opt out.
+        /// </summary>
+        [Fact]
+        public void RestoreDoesNotSetExcludeRestorePackageImportsWhenWaveDisabled()
+        {
+            string projectContents = ObjectModelHelpers.CleanupFileContents(
+                @"<Project>
+  <Target Name=""Restore"">
+    <Message Text=""ExcludeRestorePackageImports=[$(ExcludeRestorePackageImports)]"" Importance=""High"" />
+  </Target>
+</Project>");
+
+            Dictionary<string, string> envVars = new() { { "MSBUILDDISABLEFEATURESFROMVERSION", ChangeWaves.Wave18_10.ToString() } };
+
+            string logContents = ExecuteMSBuildExeExpectSuccess(projectContents, envsToCreate: envVars, arguments: "/t:restore");
+
+            // When the wave is disabled, the property must not be injected.
+            logContents.ShouldContain("ExcludeRestorePackageImports=[]");
+        }
+
         /// <summary>
         /// We check if there is only one target name specified and this logic caused a regression: https://github.com/dotnet/msbuild/issues/3317
         /// </summary>

src/MSBuild/XMake.cs

@@ -2064,6 +2064,19 @@ private static BuildResult ExecuteRestore(string projectFile, string toolsVersio
             // Add a property to indicate that a Restore is executing
             restoreGlobalProperties[MSBuildConstants.MSBuildIsRestoring] = bool.TrueString;
 
+            // Add the property that NuGet passes when it re-invokes the build during restore. NuGet's restore targets pass
+            // ExcludeRestorePackageImports=true as a global property, which normally forces a second evaluation of every project.
+            // Setting it up front here means the initial evaluation already matches, so it is reused instead of being discarded.
+            // The value must match NuGet's exactly (the literal lowercase "true" from NuGet.targets) because global property values
+            // are compared case-sensitively when matching build configurations for evaluation reuse.
+            // Only set it when the user has not explicitly supplied a value so that an explicit opt-out (e.g.
+            // /p:ExcludeRestorePackageImports=false) is respected instead of being silently overwritten.
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_10)
+                && !globalProperties.ContainsKey(MSBuildConstants.ExcludeRestorePackageImports))
+            {
+                restoreGlobalProperties[MSBuildConstants.ExcludeRestorePackageImports] = MSBuildConstants.ExcludeRestorePackageImportsValue;
+            }
+
             // Create a new request with a Restore target only and specify:
             //  - BuildRequestDataFlags.ClearCachesAfterBuild to ensure the projects will be reloaded from disk for subsequent builds
             //  - BuildRequestDataFlags.SkipNonexistentTargets to ignore missing targets since Restore does not require that all targets exist