Avoid dropping compilations that can be reused

src/EditorFeatures/CSharpTest/Workspaces/WorkspaceTests.cs

@@ -484,6 +484,7 @@ public async Task TestGetCompilationOnCrossLanguageDependentProjectChanged()
             var solutionZ = workspace.CurrentSolution;
             var docZ = solutionZ.GetDocument(document1.Id);
             var docZText = await docZ.GetTextAsync();
+            Assert.Equal("public class X { }", docZText.ToString());
 
             var compilation2Z = await solutionZ.GetProject(id2).GetCompilationAsync();
             var classDz = compilation2Z.SourceModule.GlobalNamespace.GetTypeMembers("D").Single();

src/Workspaces/Core/Portable/Workspace/Solution/ProjectDependencyGraph.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
@@ -652,6 +653,21 @@ private ImmutableHashSet<ProjectId> GetProjectsThatDirectlyDependOnThisProject_N
                 .ToImmutableDictionary();
         }
 
+        /// <summary>
+        /// Gets the list of projects that directly or transitively this project depends on, if it has already been
+        /// cached.
+        /// </summary>
+        internal ImmutableHashSet<ProjectId>? TryGetProjectsThatThisProjectTransitivelyDependsOn(ProjectId projectId)
+        {
+            if (projectId is null)
+            {
+                throw new ArgumentNullException(nameof(projectId));
+            }
+
+            _transitiveReferencesMap.TryGetValue(projectId, out var projects);
+            return projects;
+        }
+
         /// <summary>
         /// Gets the list of projects that directly or transitively this project depends on
         /// </summary>

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs

@@ -1812,20 +1812,32 @@ public ImmutableArray<DocumentId> GetDocumentIdsWithFilePath(string? filePath)
         private ImmutableDictionary<ProjectId, CompilationTracker> CreateCompilationTrackerMap(ProjectId projectId, ProjectDependencyGraph dependencyGraph)
         {
             var builder = ImmutableDictionary.CreateBuilder<ProjectId, CompilationTracker>();
-            var dependencies = dependencyGraph.TryGetProjectsThatTransitivelyDependOnThisProject(projectId);
+            IEnumerable<ProjectId>? dependencies = null;
 
-            foreach (var projectIdAndTracker in _projectIdToTrackerMap)
+            foreach (var (id, tracker) in _projectIdToTrackerMap)
             {
-                var id = projectIdAndTracker.Key;
-                var tracker = projectIdAndTracker.Value;
-
                 if (!tracker.HasCompilation)
                 {
                     continue;
                 }
 
-                var canReuse = id == projectId || (dependencies is object && !dependencies.Contains(id));
-                builder.Add(id, canReuse ? tracker : tracker.Fork(tracker.ProjectState));
+                builder.Add(id, CanReuse() ? tracker : tracker.Fork(tracker.ProjectState));
+
+                // Returns true if 'tracker' can be reused for project 'id'
+                bool CanReuse()
+                {
+                    if (id == projectId)
+                        return true;
+
+                    var forwardDependencies = dependencyGraph.TryGetProjectsThatThisProjectTransitivelyDependsOn(id);
+                    if (forwardDependencies is object && !forwardDependencies.Contains(projectId))
+                    {
+                        return true;
+                    }
+
+                    dependencies ??= dependencyGraph.GetProjectsThatTransitivelyDependOnThisProject(projectId);
+                    return !dependencies.Contains(id);
+                }
             }
 
             return builder.ToImmutable();