[repo-assist] Perf: eliminate per-element allocation in distinctUntilChangedWithAsync

RELEASE_NOTES.md

@@ -2,6 +2,7 @@
 
 * 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)
+* Performance: Optimised `AsyncSeq.distinctUntilChangedWithAsync` (and thus `distinctUntilChangedWith` / `distinctUntilChanged`) to track the previous element with a `hasPrev` flag and a direct `mutable` field instead of wrapping it in a `'T option`. Previously each iteration heap-allocated a new `Some` box; the new implementation eliminates that allocation, matching the pattern already used by `pairwise`.
 
 ### 4.17.0
 

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -2740,16 +2740,18 @@ module AsyncSeq =
       use ie = source.GetEnumerator()
       let! move = ie.MoveNext()
       let mutable b = move
-      let mutable prev = None
+      // Use a flag + mutable field instead of Option to avoid per-element heap allocation
+      let mutable hasPrev = false
+      let mutable prev = Unchecked.defaultof<'T>
       while b.IsSome do
           let v = b.Value
-          match prev with
-          | None ->
+          if not hasPrev then
               yield v
-          | Some p ->
-              let! changed = f p v
+          else
+              let! changed = f prev v
               if not changed then yield v
-          prev <- Some v
+          hasPrev <- true
+          prev <- v
           let! moven = ie.MoveNext()
           b <- moven }