Avoid cancellableTask CE in DocumentCache to preserve static-state-machine optimization

docs/release-notes/.VisualStudio/18.vNext.md

@@ -7,6 +7,7 @@
 
 * Fixed Rename incorrectly renaming `get` and `set` keywords for properties with explicit accessors. ([Issue #18270](https://github.com/dotnet/fsharp/issues/18270), [PR #19252](https://github.com/dotnet/fsharp/pull/19252))
 * Fixed Find All References crash when F# project contains non-F# files like `.cshtml`. ([Issue #16394](https://github.com/dotnet/fsharp/issues/16394), [PR #19252](https://github.com/dotnet/fsharp/pull/19252))
+* Avoid using `cancellableTask` in `DocumentCache`; the editor cache now uses direct `CancellationToken`-aware `task` wrappers, avoiding the background `Task.Run` offload and a larger wrapper closure from the `cancellableTask` builder. ([Issue #20268](https://github.com/dotnet/fsharp/issues/20268))
 * Find All References for external DLL symbols now only searches projects that reference the specific assembly. ([Issue #10227](https://github.com/dotnet/fsharp/issues/10227), [PR #19252](https://github.com/dotnet/fsharp/pull/19252))
 * Improve static compilation of state machines. ([PR #19297](https://github.com/dotnet/fsharp/pull/19297))
 * Make Alt+F1 (momentary toggle) work for inlay hints. ([PR #19421](https://github.com/dotnet/fsharp/pull/19421))

vsintegration/src/FSharp.Editor/Common/DocumentCache.fs

@@ -2,6 +2,8 @@ namespace Microsoft.VisualStudio.FSharp.Editor
 
 open System
 open System.Runtime.Caching
+open System.Threading
+open System.Threading.Tasks
 open Microsoft.CodeAnalysis
 open CancellableTasks
 
@@ -16,30 +18,40 @@ type DocumentCache<'Value when 'Value: not struct>(name: string, ?cacheItemPolic
     let policy =
         defaultArg cacheItemPolicy (CacheItemPolicy(SlidingExpiration = (TimeSpan.FromSeconds defaultSlidingExpiration)))
 
+    static let tryGetCachedValueAsync (doc: Document, cache: MemoryCache, ct: CancellationToken) =
+        if ct.IsCancellationRequested then
+            Task.FromCanceled<'Value voption>(ct)
+        else
+            task {
+                let! currentVersion = doc.GetTextVersionAsync ct
+
+                match cache.Get(doc.Id.ToString()) with
+                | null -> return ValueNone
+                | :? (VersionStamp * 'Value) as value ->
+                    if fst value = currentVersion then
+                        return ValueSome(snd value)
+                    else
+                        return ValueNone
+                | _ -> return ValueNone
+            }
+
+    static let setCacheValueAsync (doc: Document, value: 'Value, cache: MemoryCache, policy: CacheItemPolicy, ct: CancellationToken) =
+        if ct.IsCancellationRequested then
+            Task.FromCanceled<unit>(ct)
+        else
+            task {
+                let! currentVersion = doc.GetTextVersionAsync ct
+                do cache.Set(doc.Id.ToString(), (currentVersion, value), policy)
+            }
+
     new(name: string, slidingExpirationSeconds: float) =
         new DocumentCache<'Value>(name, CacheItemPolicy(SlidingExpiration = (TimeSpan.FromSeconds slidingExpirationSeconds)))
 
-    member _.TryGetValueAsync(doc: Document) =
-        cancellableTask {
-            let! ct = CancellableTask.getCancellationToken ()
-            let! currentVersion = doc.GetTextVersionAsync ct
-
-            match cache.Get(doc.Id.ToString()) with
-            | null -> return ValueNone
-            | :? (VersionStamp * 'Value) as value ->
-                if fst value = currentVersion then
-                    return ValueSome(snd value)
-                else
-                    return ValueNone
-            | _ -> return ValueNone
-        }
-
-    member _.SetAsync(doc: Document, value: 'Value) =
-        cancellableTask {
-            let! ct = CancellableTask.getCancellationToken ()
-            let! currentVersion = doc.GetTextVersionAsync ct
-            do cache.Set(doc.Id.ToString(), (currentVersion, value), policy)
-        }
+    member _.TryGetValueAsync(doc: Document) : CancellableTask<'Value voption> =
+        fun ct -> tryGetCachedValueAsync (doc, cache, ct)
+
+    member _.SetAsync(doc: Document, value: 'Value) : CancellableTask<unit> =
+        fun ct -> setCacheValueAsync (doc, value, cache, policy, ct)
 
     interface IDisposable with
         member _.Dispose() = cache.Dispose()