[repo-assist] Improve: replace ref cell with mutable in mergeAll

RELEASE_NOTES.md

@@ -3,6 +3,7 @@
 * Test coverage: Added tests for previously-untested public API functions `AsyncSeq.tryFirst`, `AsyncSeq.firstOrDefault`, `AsyncSeq.zipWithParallel`, `AsyncSeq.combineLatestWithAsync`, and `AsyncSeq.toObservable`. No functional changes.
 * Fixed Fable CI build: `Microsoft.Bcl.AsyncInterfaces` was pinned to a specific version (`10.0.7`) that was older than the version resolved transitively via `System.Threading.Channels`, causing a `NU1605` package downgrade error that made Fable's project cracker fail during `dotnet fable`. The reference now uses `Version="*"` (matching `System.Threading.Channels`) so both resolve consistently. (#334)
 * Tests: Added comprehensive tests for `AsyncSeq.zapp`, `AsyncSeq.zappAsync`, and `AsyncSeq.compareWithAsync`, which previously had no dedicated test coverage.
+* Performance: Replaced the last remaining `ref` cell in `AsyncSeq.mergeAll`'s internal counter with a direct `mutable` local, matching the `ref`-to-`mutable` modernisation already applied elsewhere in the codebase. Eliminates a per-call heap allocation of a `Ref<int>` wrapper; no behavioral change.
 
 ### 4.17.0
 

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -2682,8 +2682,8 @@
         for i in 0 .. ss.Length - 1 do
             let! task = Async.StartChildAsTask (ies.[i].MoveNext())
             do tasks.[i] <- task
-        let fin = ref n
-        while fin.Value > 0 do
+        let mutable fin = n
+        while fin > 0 do
             let! ti = Task.WhenAny (tasks) |> Async.AwaitTask
             let i  = Array.IndexOf (tasks, ti)
             let v = ti.Result
@@ -2697,7 +2697,7 @@
             | None ->
                 let t = System.Threading.Tasks.TaskCompletionSource()
                 tasks.[i] <- t.Task // result never gets set
-                fin.Value <- fin.Value - 1
+                fin <- fin - 1
     }
 
   let combineLatestWithAsync (f:'a -> 'b -> Async<'c>) (source1:AsyncSeq<'a>) (source2:AsyncSeq<'b>) : AsyncSeq<'c> =
@@ -2900,7 +2900,7 @@
 
   [<CompilerMessage("The result of groupBy must be consumed with a parallel combinator such as AsyncSeq.mapAsyncParallel. Sequential consumption will deadlock because sub-sequence completion depends on other sub-sequences being consumed concurrently.", 9999)>]
   let groupBy (p:'a -> 'k) (s:AsyncSeq<'a>) : AsyncSeq<'k * AsyncSeq<'a>> =
     groupByAsync (p >> async.Return) s
   #endif
   #endif