Make diagnostic checksum computation into an extension method.

src/Features/Core/Portable/Diagnostics/DiagnosticAnalyzerExtensions.cs

@@ -8,8 +8,6 @@
 using System.Linq;
 using System.Reflection;
 using Microsoft.CodeAnalysis.Diagnostics.Telemetry;
-using Microsoft.CodeAnalysis.Simplification;
-using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Diagnostics;
 

src/LanguageServer/Protocol/Features/Diagnostics/EngineV2/DiagnosticIncrementalAnalyzer.CompilationManager.cs

@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections.Immutable;
-using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -37,7 +36,7 @@ internal partial class DiagnosticAnalyzerService
             return null;
 
         var projectState = project.State;
-        var checksum = await project.GetDependentChecksumAsync(cancellationToken).ConfigureAwait(false);
+        var checksum = await project.GetDiagnosticChecksumAsync(cancellationToken).ConfigureAwait(false);
 
         // Make sure the cached pair was computed with at least the same state sets we're asking about.  if not,
         // recompute and cache with the new state sets.

src/LanguageServer/Protocol/Features/Diagnostics/EngineV2/DiagnosticIncrementalAnalyzer_GetDiagnostics.cs

@@ -121,7 +121,7 @@ async Task<ImmutableDictionary<DiagnosticAnalyzer, DiagnosticAnalysisResult>> Ge
                 if (s_projectToForceAnalysisData.TryGetValue(project.State, out var box) &&
                     analyzers.IsSubsetOf(box.Value.analyzers))
                 {
-                    var checksum = await project.GetDependentChecksumAsync(cancellationToken).ConfigureAwait(false);
+                    var checksum = await project.GetDiagnosticChecksumAsync(cancellationToken).ConfigureAwait(false);
                     if (box.Value.checksum == checksum)
                         return box.Value.diagnosticAnalysisResults;
                 }

src/LanguageServer/Protocol/Features/Diagnostics/EngineV2/DiagnosticIncrementalAnalyzer_IncrementalAnalyzer.cs

@@ -37,7 +37,7 @@ private partial class DiagnosticIncrementalAnalyzer
         public async Task<ImmutableArray<DiagnosticData>> ForceAnalyzeProjectAsync(Project project, CancellationToken cancellationToken)
         {
             var projectState = project.State;
-            var checksum = await project.GetDependentChecksumAsync(cancellationToken).ConfigureAwait(false);
+            var checksum = await project.GetDiagnosticChecksumAsync(cancellationToken).ConfigureAwait(false);
 
             try
             {

src/LanguageServer/Protocol/Handler/Diagnostics/DiagnosticsPullCache.cs

@@ -18,11 +18,12 @@ internal abstract partial class AbstractPullDiagnosticHandler<TDiagnosticsParams
     internal readonly record struct DiagnosticsRequestState(Project Project, int GlobalStateVersion, RequestContext Context, IDiagnosticSource DiagnosticSource);
 
     /// <summary>
-    /// Cache where we store the data produced by prior requests so that they can be returned if nothing of significance 
-    /// changed. The <see cref="VersionStamp"/> is produced by <see cref="Project.GetDependentVersionAsync(CancellationToken)"/> while the 
-    /// <see cref="Checksum"/> is produced by <see cref="Project.GetDependentChecksumAsync(CancellationToken)"/>.  The former is faster
-    /// and works well for us in the normal case.  The latter still allows us to reuse diagnostics when changes happen that
-    /// update the version stamp but not the content (for example, forking LSP text).
+    /// Cache where we store the data produced by prior requests so that they can be returned if nothing of significance
+    /// changed. The <see cref="VersionStamp"/> is produced by <see
+    /// cref="Project.GetDependentVersionAsync(CancellationToken)"/> while the <see cref="Checksum"/> is produced by
+    /// <see cref="CodeAnalysis.Diagnostics.Extensions.GetDiagnosticChecksumAsync"/>.  The former is faster and works
+    /// well for us in the normal case.  The latter still allows us to reuse diagnostics when changes happen that update
+    /// the version stamp but not the content (for example, forking LSP text).
     /// </summary>
     private sealed class DiagnosticsPullCache(string uniqueKey) : VersionedPullCache<(int globalStateVersion, VersionStamp? dependentVersion), (int globalStateVersion, Checksum dependentChecksum), DiagnosticsRequestState, ImmutableArray<DiagnosticData>>(uniqueKey)
     {
@@ -33,7 +34,7 @@ private sealed class DiagnosticsPullCache(string uniqueKey) : VersionedPullCache
 
         public override async Task<(int globalStateVersion, Checksum dependentChecksum)> ComputeExpensiveVersionAsync(DiagnosticsRequestState state, CancellationToken cancellationToken)
         {
-            return (state.GlobalStateVersion, await state.Project.GetDependentChecksumAsync(cancellationToken).ConfigureAwait(false));
+            return (state.GlobalStateVersion, await state.Project.GetDiagnosticChecksumAsync(cancellationToken).ConfigureAwait(false));
         }
 
         /// <inheritdoc cref="VersionedPullCache{TCheapVersion, TExpensiveVersion, TState, TComputedData}.ComputeDataAsync(TState, CancellationToken)"/>

src/Workspaces/Core/Portable/Diagnostics/Extensions.cs

@@ -7,6 +7,7 @@
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Collections;
@@ -21,6 +22,8 @@ namespace Microsoft.CodeAnalysis.Diagnostics;
 
 internal static partial class Extensions
 {
+    private static readonly ConditionalWeakTable<Project, AsyncLazy<Checksum>> s_projectToDiagnosticChecksum = new();
+
     public static async Task<ImmutableArray<Diagnostic>> ToDiagnosticsAsync(this IEnumerable<DiagnosticData> diagnostics, Project project, CancellationToken cancellationToken)
     {
         var result = ArrayBuilder<Diagnostic>.GetInstance();
@@ -429,4 +432,76 @@ await suppressionAnalyzer.AnalyzeAsync(
                 semanticModel, span, hostCompilationWithAnalyzers, analyzerInfoCache.GetDiagnosticDescriptors, reportDiagnostic, cancellationToken).ConfigureAwait(false);
         }
     }
+
+    /// <summary>
+    /// Calculates a checksum that contains a project's checksum along with a checksum for each of the project's 
+    /// transitive dependencies.
+    /// </summary>
+    /// <remarks>
+    /// This checksum calculation can be used for cases where a feature needs to know if the semantics in this project
+    /// changed.  For example, for diagnostics or caching computed semantic data. The goal is to ensure that changes to
+    /// <list type="bullet">
+    ///    <item>Files inside the current project</item>
+    ///    <item>Project properties of the current project</item>
+    ///    <item>Visible files in referenced projects</item>
+    ///    <item>Project properties in referenced projects</item>
+    /// </list>
+    /// are reflected in the metadata we keep so that comparing solutions accurately tells us when we need to recompute
+    /// semantic work.   
+    /// 
+    /// <para>This method of checking for changes has a few important properties that differentiate it from other methods of determining project version.
+    /// <list type="bullet">
+    ///    <item>Changes to methods inside the current project will be reflected to compute updated diagnostics.
+    ///        <see cref="Project.GetDependentSemanticVersionAsync(CancellationToken)"/> does not change as it only returns top level changes.</item>
+    ///    <item>Reloading a project without making any changes will re-use cached diagnostics.
+    ///        <see cref="Project.GetDependentSemanticVersionAsync(CancellationToken)"/> changes as the project is removed, then added resulting in a version change.</item>
+    /// </list>   
+    /// </para>
+    /// This checksum is also affected by the <see cref="SourceGeneratorExecutionVersion"/> for this project.
+    /// As such, it is not usable across different sessions of a particular host.
+    /// </remarks>
+    public static Task<Checksum> GetDiagnosticChecksumAsync(this Project? project, CancellationToken cancellationToken)
+    {
+        if (project is null)
+            return SpecializedTasks.Default<Checksum>();
+
+        var lazyChecksum = s_projectToDiagnosticChecksum.GetValue(
+            project,
+            static project => AsyncLazy.Create(
+                static (project, cancellationToken) => ComputeDiagnosticChecksumAsync(project, cancellationToken),
+                project));
+
+        return lazyChecksum.GetValueAsync(cancellationToken);
+
+        static async Task<Checksum> ComputeDiagnosticChecksumAsync(Project project, CancellationToken cancellationToken)
+        {
+            var solution = project.Solution;
+
+            using var _ = ArrayBuilder<Checksum>.GetInstance(out var tempChecksumArray);
+
+            // Mix in the SG information for this project.  That way if it changes, we will have a different
+            // checksum (since semantics could have changed because of this).
+            if (solution.CompilationState.SourceGeneratorExecutionVersionMap.Map.TryGetValue(project.Id, out var executionVersion))
+                tempChecksumArray.Add(executionVersion.Checksum);
+
+            // Get the checksum for the project itself.  Note: this will normally be cached.  As such, even if we
+            // have a different Project instance (due to a change in an unrelated project), this will be fast to
+            // compute and return.
+            var projectChecksum = await project.State.GetChecksumAsync(cancellationToken).ConfigureAwait(false);
+            tempChecksumArray.Add(projectChecksum);
+
+            // Calculate a checksum this project and for each dependent project that could affect semantics for this
+            // project. We order the projects guid so that we are resilient to the underlying in-memory graph structure
+            // changing this arbitrarily.
+            foreach (var projectRef in project.ProjectReferences.OrderBy(r => r.ProjectId.Id))
+            {
+                // Note that these checksums should only actually be calculated once, if the project is unchanged
+                // the same checksum will be returned.
+                tempChecksumArray.Add(await GetDiagnosticChecksumAsync(
+                    solution.GetProject(projectRef.ProjectId), cancellationToken).ConfigureAwait(false));
+            }
+
+            return Checksum.Create(tempChecksumArray);
+        }
+    }
 }

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

@@ -544,36 +544,6 @@ public Task<VersionStamp> GetDependentSemanticVersionAsync(CancellationToken can
     public Task<VersionStamp> GetSemanticVersionAsync(CancellationToken cancellationToken = default)
         => State.GetSemanticVersionAsync(cancellationToken);
 
-    /// <summary>
-    /// Calculates a checksum that contains a project's checksum along with a checksum for each of the project's 
-    /// transitive dependencies.
-    /// </summary>
-    /// <remarks>
-    /// This checksum calculation can be used for cases where a feature needs to know if the semantics in this project
-    /// changed.  For example, for diagnostics or caching computed semantic data. The goal is to ensure that changes to
-    /// <list type="bullet">
-    ///    <item>Files inside the current project</item>
-    ///    <item>Project properties of the current project</item>
-    ///    <item>Visible files in referenced projects</item>
-    ///    <item>Project properties in referenced projects</item>
-    /// </list>
-    /// are reflected in the metadata we keep so that comparing solutions accurately tells us when we need to recompute
-    /// semantic work.   
-    /// 
-    /// <para>This method of checking for changes has a few important properties that differentiate it from other methods of determining project version.
-    /// <list type="bullet">
-    ///    <item>Changes to methods inside the current project will be reflected to compute updated diagnostics.
-    ///        <see cref="Project.GetDependentSemanticVersionAsync(CancellationToken)"/> does not change as it only returns top level changes.</item>
-    ///    <item>Reloading a project without making any changes will re-use cached diagnostics.
-    ///        <see cref="Project.GetDependentSemanticVersionAsync(CancellationToken)"/> changes as the project is removed, then added resulting in a version change.</item>
-    /// </list>   
-    /// </para>
-    /// This checksum is also affected by the <see cref="SourceGeneratorExecutionVersion"/> for this project.
-    /// As such, it is not usable across different sessions of a particular host.
-    /// </remarks>
-    internal Task<Checksum> GetDependentChecksumAsync(CancellationToken cancellationToken)
-        => Solution.CompilationState.GetDependentChecksumAsync(this.Id, cancellationToken);
-
     /// <summary>
     /// Creates a new instance of this project updated to have the new assembly name.
     /// </summary>

src/Workspaces/Core/Portable/Workspace/Solution/SolutionCompilationState.ICompilationTracker.cs

@@ -53,7 +53,6 @@ bool ContainsAssemblyOrModuleOrDynamic(
 
         Task<VersionStamp> GetDependentVersionAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken);
         Task<VersionStamp> GetDependentSemanticVersionAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken);
-        Task<Checksum> GetDependentChecksumAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken);
 
         /// <summary>
         /// Gets the source generator files generated by this <see cref="ICompilationTracker"/>. <paramref

src/Workspaces/Core/Portable/Workspace/Solution/SolutionCompilationState.RegularCompilationTracker.cs

@@ -1008,7 +1008,6 @@ public CompilationTrackerValidationException(string message, Exception inner) :
 
             private AsyncLazy<VersionStamp>? _lazyDependentVersion;
             private AsyncLazy<VersionStamp>? _lazyDependentSemanticVersion;
-            private AsyncLazy<Checksum>? _lazyDependentChecksum;
 
             public Task<VersionStamp> GetDependentVersionAsync(
                 SolutionCompilationState compilationState, CancellationToken cancellationToken)
@@ -1087,66 +1086,6 @@ private async Task<VersionStamp> ComputeDependentSemanticVersionAsync(
                 return version;
             }
 
-            public Task<Checksum> GetDependentChecksumAsync(
-                SolutionCompilationState compilationState, CancellationToken cancellationToken)
-            {
-                if (_lazyDependentChecksum == null)
-                {
-                    // note: solution is captured here, but it will go away once GetValueAsync executes.
-                    Interlocked.CompareExchange(
-                        ref _lazyDependentChecksum,
-                        AsyncLazy.Create(static (arg, c) =>
-                            arg.self.ComputeDependentChecksumAsync(arg.compilationState, c),
-                            arg: (self: this, compilationState)),
-                        null);
-                }
-
-                return _lazyDependentChecksum.GetValueAsync(cancellationToken);
-            }
-
-            private async Task<Checksum> ComputeDependentChecksumAsync(
-                SolutionCompilationState solution, CancellationToken cancellationToken)
-            {
-                using var _ = ArrayBuilder<Checksum>.GetInstance(out var tempChecksumArray);
-
-                // Mix in the SG information for this project.  That way if it changes, we will have a different
-                // checksum (since semantics could have changed because of this).
-                if (solution.SourceGeneratorExecutionVersionMap.Map.TryGetValue(this.ProjectState.Id, out var executionVersion))
-                    tempChecksumArray.Add(executionVersion.Checksum);
-
-                // Get the checksum for the project itself.
-                var projectChecksum = await this.ProjectState.GetChecksumAsync(cancellationToken).ConfigureAwait(false);
-                tempChecksumArray.Add(projectChecksum);
-
-                // Calculate a checksum this project and for each dependent project that could affect semantics for this
-                // project. We order the projects so that we are resilient to the underlying in-memory graph structure
-                // changing this arbitrarily.  We do not want that to cause us to change our semantic version.. Note: we
-                // use the project filepath+name as a unique way to reference a project.  This matches the logic in our
-                // persistence-service implementation as to how information is associated with a project.
-                var transitiveDependencies = solution.SolutionState.GetProjectDependencyGraph().GetProjectsThatThisProjectTransitivelyDependsOn(this.ProjectState.Id);
-                var orderedProjectIds = transitiveDependencies.OrderBy(id =>
-                {
-                    var depProject = solution.SolutionState.GetRequiredProjectState(id);
-                    return (depProject.FilePath, depProject.Name);
-                });
-
-                foreach (var projectId in orderedProjectIds)
-                {
-                    // Mix in the SG information for the dependent project.  That way if it changes, we will have a
-                    // different checksum (since semantics could have changed because of this).
-                    if (solution.SourceGeneratorExecutionVersionMap.Map.TryGetValue(projectId, out executionVersion))
-                        tempChecksumArray.Add(executionVersion.Checksum);
-
-                    // Note that these checksums should only actually be calculated once, if the project is unchanged
-                    // the same checksum will be returned.
-                    var referencedProject = solution.SolutionState.GetRequiredProjectState(projectId);
-                    var referencedProjectChecksum = await referencedProject.GetChecksumAsync(cancellationToken).ConfigureAwait(false);
-                    tempChecksumArray.Add(referencedProjectChecksum);
-                }
-
-                return Checksum.Create(tempChecksumArray);
-            }
-
             #endregion
         }
     }

src/Workspaces/Core/Portable/Workspace/Solution/SolutionCompilationState.WithFrozenSourceGeneratedDocumentsCompilationTracker.cs

@@ -28,8 +28,6 @@ private sealed class WithFrozenSourceGeneratedDocumentsCompilationTracker : ICom
     {
         private readonly TextDocumentStates<SourceGeneratedDocumentState> _replacementDocumentStates;
 
-        private AsyncLazy<Checksum>? _lazyDependentChecksum;
-
         /// <summary>
         /// The lazily-produced compilation that has the generated document updated. This is initialized by call to
         /// <see cref="GetCompilationAsync"/>.
@@ -145,28 +143,6 @@ public Task<VersionStamp> GetDependentVersionAsync(SolutionCompilationState comp
         public Task<VersionStamp> GetDependentSemanticVersionAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken)
             => UnderlyingTracker.GetDependentSemanticVersionAsync(compilationState, cancellationToken);
 
-        public Task<Checksum> GetDependentChecksumAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken)
-        {
-            if (_lazyDependentChecksum == null)
-            {
-                var tmp = compilationState; // temp. local to avoid a closure allocation for the fast path
-                // note: solution is captured here, but it will go away once GetValueAsync executes.
-                Interlocked.CompareExchange(
-                    ref _lazyDependentChecksum,
-                    AsyncLazy.Create(static (arg, c) =>
-                        arg.self.ComputeDependentChecksumAsync(arg.tmp, c),
-                        arg: (self: this, tmp)),
-                    null);
-            }
-
-            return _lazyDependentChecksum.GetValueAsync(cancellationToken);
-        }
-
-        private async Task<Checksum> ComputeDependentChecksumAsync(SolutionCompilationState compilationState, CancellationToken cancellationToken)
-            => Checksum.Create(
-                await UnderlyingTracker.GetDependentChecksumAsync(compilationState, cancellationToken).ConfigureAwait(false),
-                (await _replacementDocumentStates.GetDocumentChecksumsAndIdsAsync(cancellationToken).ConfigureAwait(false)).Checksum);
-
         public async ValueTask<TextDocumentStates<SourceGeneratedDocumentState>> GetSourceGeneratedDocumentStatesAsync(
             SolutionCompilationState compilationState, bool withFrozenSourceGeneratedDocuments, CancellationToken cancellationToken)
         {