Merge pull request #381 from fsprojects/repo-assist/perf-toResizeArray-direct-loop-20260408-116ad7cb53aa926e

[Repo Assist] perf: direct loop in toResizeArrayAsync; simplify tryItem loop

Author: dsyme

release-notes.txt

@@ -3,6 +3,8 @@ Release notes:
 
 1.0.0
     - adds taskSeqDynamic computation expression and TaskSeqDynamic/TaskSeqDynamicInfo types for dynamic (FSI-compatible) resumable code, fixing issue where taskSeq would raise NotImplementedException in F# Interactive, #246
+    - perf: toResizeArrayAsync (and therefore toArrayAsync, toListAsync, toResizeArrayAsync, toIListAsync) uses a direct loop instead of going through iter, avoiding a lambda and DU allocation per call
+    - perf: tryItem uses a simpler loop that skips the redundant inner index check on every iteration
     - perf: TaskSeq.chunkBy and chunkByAsync reuse the ResizeArray buffer between chunks, reducing allocations on sequences with many chunk boundaries
     - fixes: TaskSeq.insertAt, insertManyAt, removeAt, removeManyAt, updateAt now raise ArgumentNullException (not NullReferenceException) when given a null source; insertManyAt also validates the values argument
     - refactor: simplify lengthBy and lengthBeforeMax to use while! and remove the redundant mutable 'go' and initial MoveNextAsync

src/FSharp.Control.TaskSeq/TaskSeqInternal.fs

@@ -532,12 +532,16 @@ module internal TaskSeqInternal =
                 yield result
         }
 
-    let toResizeArrayAsync source =
+    let toResizeArrayAsync (source: TaskSeq<'T>) =
         checkNonNull (nameof source) source
 
         task {
-            let res = ResizeArray()
-            do! source |> iter (SimpleAction(fun item -> res.Add item))
+            let res = ResizeArray<'T>()
+            use e = source.GetAsyncEnumerator CancellationToken.None
+
+            while! e.MoveNextAsync() do
+                res.Add e.Current
+
             return res
         }
 
@@ -915,14 +919,14 @@ module internal TaskSeqInternal =
                 let! step = e.MoveNextAsync()
                 go <- step
 
-                while go && idx <= index do
-                    if idx = index then
-                        foundItem <- Some e.Current
-                        go <- false
-                    else
-                        let! step = e.MoveNextAsync()
-                        go <- step
-                        idx <- idx + 1
+                // advance past the first `index` elements, then capture the current element
+                while go && idx < index do
+                    let! step = e.MoveNextAsync()
+                    go <- step
+                    idx <- idx + 1
+
+                if go then
+                    foundItem <- Some e.Current
 
                 return foundItem
         }