diff --git a/.fantomasignore b/.fantomasignore
index 9c12ffa8e2c..f5798fbdf00 100644
--- a/.fantomasignore
+++ b/.fantomasignore
@@ -41,6 +41,11 @@ src/Compiler/Checking/TypeHierarchy.fs
 src/Compiler/Checking/TypeRelations.fs
 
 # nullness-related problems
+src/Compiler/Utilities/lib.fsi
+src/Compiler/Utilities/Cancellable.fsi
+src/FSharp.Core/tasks.fsi
+src/FSharp.Core/tasks.fs
+src/FSharp.Core/resumable.fs
 src/Compiler/DependencyManager/DependencyProvider.fs
 src/FSharp.Core/fslib-extra-pervasives.fs
 src/FSharp.Core/fslib-extra-pervasives.fsi
diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
index f41b4ca9289..c766a1804f0 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
@@ -19,6 +19,7 @@
 * FSharpCheckFileResults.ProjectContext.ProjectOptions will not be available when using the experimental Transparent Compiler feature. ([PR #18205](https://github.com/dotnet/fsharp/pull/18205))
 * Update `Obsolete` attribute checking to account for `DiagnosticId` and `UrlFormat` properties. ([PR #18224](https://github.com/dotnet/fsharp/pull/18224))
 * Remove `Cancellable.UsingToken` from tests ([PR #18276](https://github.com/dotnet/fsharp/pull/18276))
+* Added nullability annotations to `.Using` builder method for `async`, `task` and compiler-internal builders ([PR #18292](https://github.com/dotnet/fsharp/pull/18292))
 
 ### Breaking Changes
 * Struct unions with overlapping fields now generate mappings needed for reading via reflection ([Issue #18121](https://github.com/dotnet/fsharp/issues/17797), [PR #18274](https://github.com/dotnet/fsharp/pull/17877))
\ No newline at end of file
diff --git a/docs/release-notes/.FSharp.Core/9.0.300.md b/docs/release-notes/.FSharp.Core/9.0.300.md
index 87d3502539c..5e6903f3fa4 100644
--- a/docs/release-notes/.FSharp.Core/9.0.300.md
+++ b/docs/release-notes/.FSharp.Core/9.0.300.md
@@ -1,8 +1,9 @@
 ### Fixed
 
 ### Added
+* Added nullability annotations to `.Using` builder method for `async` and `task` builders ([PR #18292](https://github.com/dotnet/fsharp/pull/18292))
 
 ### Changed
 
 ### Breaking Changes
-* Struct unions with overlapping fields now generate mappings needed for reading via reflection ([Issue #18121](https://github.com/dotnet/fsharp/issues/17797), [PR #18274](https://github.com/dotnet/fsharp/pull/17877)). Previous versions of FSharp.Core returned incomplete mapping between fields and cases, these older fslib versions will now report an exception.
+* Struct unions with overlapping fields now generate mappings needed for reading via reflection ([Issue #18121](https://github.com/dotnet/fsharp/issues/17797), [PR #18274](https://github.com/dotnet/fsharp/pull/18274)). Previous versions of FSharp.Core returned incomplete mapping between fields and cases, these older fslib versions will now report an exception.
diff --git a/src/Compiler/Utilities/Activity.fs b/src/Compiler/Utilities/Activity.fs
index 2403f00d3de..e2d7e78fcb6 100644
--- a/src/Compiler/Utilities/Activity.fs
+++ b/src/Compiler/Utilities/Activity.fs
@@ -89,18 +89,18 @@ module internal Activity =
 
     let private activitySource = new ActivitySource(ActivityNames.FscSourceName)
 
-    let start (name: string) (tags: (string * string) seq) : IDisposable =
+    let start (name: string) (tags: (string * string) seq) : IDisposable MaybeNull =
         let activity = activitySource.CreateActivity(name, ActivityKind.Internal)
 
         match activity with
-        | null -> !!activity //TODO change retTy to |null after PR #18262 is merged!!
+        | null -> activity 
         | activity ->
             for key, value in tags do
                 activity.AddTag(key, value) |> ignore
 
             activity.Start()
 
-    let startNoTags (name: string) : IDisposable = !! (activitySource.StartActivity name) //TODO change retTy to |null after PR #18262 is merged!!
+    let startNoTags (name: string) : IDisposable MaybeNull = activitySource.StartActivity name
 
     let addEvent name =
         match Activity.Current with
@@ -122,7 +122,7 @@ module internal Activity =
 
         let private profiledSource = new ActivitySource(ActivityNames.ProfiledSourceName)
 
-        let startAndMeasureEnvironmentStats (name: string) : IDisposable = !!(profiledSource.StartActivity(name)) //TODO change retTy to |null after PR #18262 is merged!!
+        let startAndMeasureEnvironmentStats (name: string) : IDisposable MaybeNull = profiledSource.StartActivity(name)
 
         type private GCStats = int[]
 
diff --git a/src/Compiler/Utilities/Activity.fsi b/src/Compiler/Utilities/Activity.fsi
index ec6a9fbf6f8..6e7244f5e59 100644
--- a/src/Compiler/Utilities/Activity.fsi
+++ b/src/Compiler/Utilities/Activity.fsi
@@ -2,6 +2,7 @@
 namespace FSharp.Compiler.Diagnostics
 
 open System
+open Internal.Utilities.Library
 
 /// For activities following the dotnet distributed tracing concept
 /// https://learn.microsoft.com/dotnet/core/diagnostics/distributed-tracing-concepts?source=recommendations
@@ -39,14 +40,14 @@ module internal Activity =
     module Events =
         val cacheHit: string
 
-    val startNoTags: name: string -> IDisposable
+    val startNoTags: name: string -> IDisposable MaybeNull
 
-    val start: name: string -> tags: (string * string) seq -> IDisposable
+    val start: name: string -> tags: (string * string) seq -> IDisposable MaybeNull
 
     val addEvent: name: string -> unit
 
     module Profiling =
-        val startAndMeasureEnvironmentStats: name: string -> IDisposable
+        val startAndMeasureEnvironmentStats: name: string -> IDisposable MaybeNull
         val addConsoleListener: unit -> IDisposable
 
     module CsvExport =
diff --git a/src/Compiler/Utilities/Cancellable.fs b/src/Compiler/Utilities/Cancellable.fs
index ad739b5039e..4d6c42cd6f0 100644
--- a/src/Compiler/Utilities/Cancellable.fs
+++ b/src/Compiler/Utilities/Cancellable.fs
@@ -162,7 +162,7 @@ type CancellableBuilder() =
                 | Choice2Of2 err -> Cancellable.run ct (handler err)
             | ValueOrCancelled.Cancelled err1 -> ValueOrCancelled.Cancelled err1)
 
-    member inline _.Using(resource, [<InlineIfLambda>] comp) =
+    member inline _.Using(resource: _ MaybeNull, [<InlineIfLambda>] comp) =
         Cancellable(fun ct ->
 #if !FSHARPCORE_USE_PACKAGE
             __debugPoint ""
diff --git a/src/Compiler/Utilities/Cancellable.fsi b/src/Compiler/Utilities/Cancellable.fsi
index aba96859491..737a27a4f2a 100644
--- a/src/Compiler/Utilities/Cancellable.fsi
+++ b/src/Compiler/Utilities/Cancellable.fsi
@@ -71,8 +71,12 @@ type internal CancellableBuilder =
         comp: Cancellable<'T> * [<InlineIfLambda>] handler: (exn -> Cancellable<'T>) -> Cancellable<'T>
 
     member inline Using:
-        resource: 'Resource * [<InlineIfLambda>] comp: ('Resource -> Cancellable<'T>) -> Cancellable<'T>
+        resource: 'Resource MaybeNull * [<InlineIfLambda>] comp: ('Resource MaybeNull -> Cancellable<'T>) -> Cancellable<'T>
             when 'Resource :> IDisposable
+            and 'Resource:not struct
+#if !(NO_CHECKNULLS || BUILDING_WITH_LKG)
+            and 'Resource:not null 
+#endif
 
     member inline Zero: unit -> Cancellable<unit>
 
diff --git a/src/Compiler/Utilities/lib.fs b/src/Compiler/Utilities/lib.fs
index 921b0a6dba3..609245f636e 100755
--- a/src/Compiler/Utilities/lib.fs
+++ b/src/Compiler/Utilities/lib.fs
@@ -403,7 +403,10 @@ type DisposablesTracker() =
     let items = Stack<IDisposable>()
 
     /// Register some items to dispose
-    member _.Register i = items.Push i
+    member _.Register (i:#IDisposable MaybeNull) = 
+        match box i with
+        | null -> ()
+        | _ -> items.Push (!!i)
 
     interface IDisposable with
 
diff --git a/src/Compiler/Utilities/lib.fsi b/src/Compiler/Utilities/lib.fsi
index 08b834d17c6..ea0669bdf4a 100644
--- a/src/Compiler/Utilities/lib.fsi
+++ b/src/Compiler/Utilities/lib.fsi
@@ -263,7 +263,12 @@ type DisposablesTracker =
     new: unit -> DisposablesTracker
 
     /// Register some items to dispose
-    member Register: i: System.IDisposable -> unit
+    member Register: i:'a MaybeNull -> unit 
+        when 'a:>System.IDisposable 
+#if !(NO_CHECKNULLS || BUILDING_WITH_LKG)
+        and 'a:not null
+#endif
+        and 'a:not struct
 
     interface System.IDisposable
 
diff --git a/src/FSharp.Core/async.fs b/src/FSharp.Core/async.fs
index 959d47d0aab..2b117be0b16 100644
--- a/src/FSharp.Core/async.fs
+++ b/src/FSharp.Core/async.fs
@@ -753,7 +753,7 @@ module AsyncPrimitives =
     ///   - Cancellation check after 'entering' the implied try/finally and before running the body  (see CreateTryFinallyAsync)
     ///   - Hijack check after 'entering' the implied try/finally and before running the body  (see CreateTryFinallyAsync)
     ///   - Run 'disposeFunction' with exception protection (see CreateTryFinallyAsync)
-    let CreateUsingAsync (resource: 'T :> IDisposable) (computation: 'T -> Async<'a>) : Async<'a> =
+    let CreateUsingAsync (resource: 'T :> IDisposable | null) (computation: 'T -> Async<'a>) : Async<'a> =
         let disposeFunction () =
             Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions.Dispose resource
 
diff --git a/src/FSharp.Core/async.fsi b/src/FSharp.Core/async.fsi
index e01ba0d8d75..d4227d292f5 100644
--- a/src/FSharp.Core/async.fsi
+++ b/src/FSharp.Core/async.fsi
@@ -1325,7 +1325,7 @@ namespace Microsoft.FSharp.Control
         /// <returns>An asynchronous computation that binds and eventually disposes <c>resource</c>.</returns>
         ///
         /// <example-tbd></example-tbd>
-        member Using: resource:'T * binder:('T -> Async<'U>) -> Async<'U> when 'T :> System.IDisposable
+        member Using: resource:'T * binder:('T -> Async<'U>) -> Async<'U> when 'T :> System.IDisposable|null
 
         /// <summary>Creates an asynchronous computation that runs <c>computation</c>, and when 
         /// <c>computation</c> generates a result <c>T</c>, runs <c>binder res</c>.</summary>
diff --git a/src/FSharp.Core/prim-types.fs b/src/FSharp.Core/prim-types.fs
index 1fc9e799ab4..4f4849eb849 100644
--- a/src/FSharp.Core/prim-types.fs
+++ b/src/FSharp.Core/prim-types.fs
@@ -739,8 +739,11 @@ namespace Microsoft.FSharp.Core
             let inline TypeTestFast<'T>(source: objnull) = 
                 //assert not(TypeInfo<'T>.TypeInfo = TypeNullnessSemantics_NullTrueValue)
                 notnullPrim(isinstPrim<'T>(source)) 
-
+#if !BUILDING_WITH_LKG && !NO_NULLCHECKING_LIB_SUPPORT
+            let Dispose<'T when 'T :> IDisposable >(resource:'T|null) = 
+#else
             let Dispose<'T when 'T :> IDisposable >(resource:'T) = 
+#endif
                 match box resource with 
                 | null -> ()
                 | _ -> resource.Dispose()
diff --git a/src/FSharp.Core/prim-types.fsi b/src/FSharp.Core/prim-types.fsi
index 17f5a6418b0..c56fe5375e4 100644
--- a/src/FSharp.Core/prim-types.fsi
+++ b/src/FSharp.Core/prim-types.fsi
@@ -1796,7 +1796,11 @@ namespace Microsoft.FSharp.Core
 
             /// <summary>A compiler intrinsic for the efficient compilation of sequence expressions</summary>
             [<CompilerMessage("This function is for use by compiled F# code and should not be used directly", 1204, IsHidden=true)>]
+#if !BUILDING_WITH_LKG && !NO_NULLCHECKING_LIB_SUPPORT
+            val Dispose<'T when 'T :> System.IDisposable> : resource: 'T|null -> unit
+#else
             val Dispose<'T when 'T :> System.IDisposable> : resource: 'T -> unit
+#endif
 
             /// <summary>A compiler intrinsic for checking initialization soundness of recursive bindings</summary>
             [<CompilerMessage("This function is for use by compiled F# code and should not be used directly", 1204, IsHidden=true)>]
diff --git a/src/FSharp.Core/resumable.fs b/src/FSharp.Core/resumable.fs
index 27e91a93256..1488ab220cf 100644
--- a/src/FSharp.Core/resumable.fs
+++ b/src/FSharp.Core/resumable.fs
@@ -399,7 +399,7 @@ module ResumableCode =
         (
             resource: 'Resource,
             body: 'Resource -> ResumableCode<'Data, 'T>
-        ) : ResumableCode<'Data, 'T> when 'Resource :> IDisposable =
+        ) : ResumableCode<'Data, 'T> when 'Resource :> IDisposable|null =
         // A using statement is just a try/finally with the finally block disposing if non-null.
         TryFinally(
             ResumableCode<'Data, 'T>(fun sm -> (body resource).Invoke(&sm)),
diff --git a/src/FSharp.Core/resumable.fsi b/src/FSharp.Core/resumable.fsi
index 466301dc36b..2835b75a4dc 100644
--- a/src/FSharp.Core/resumable.fsi
+++ b/src/FSharp.Core/resumable.fsi
@@ -90,7 +90,7 @@ module ResumableCode =
     val inline TryWith: body: ResumableCode<'Data, 'T> * catch: (exn -> ResumableCode<'Data, 'T>) -> ResumableCode<'Data, 'T>
 
     /// Specifies resumable code which executes with 'use' semantics
-    val inline Using: resource: 'Resource * body: ('Resource -> ResumableCode<'Data, 'T>) -> ResumableCode<'Data, 'T> when 'Resource :> IDisposable
+    val inline Using: resource: 'Resource * body: ('Resource -> ResumableCode<'Data, 'T>) -> ResumableCode<'Data, 'T> when 'Resource :> IDisposable|null
 
     /// Specifies resumable code which executes a loop
     val inline While: [<InlineIfLambda>] condition: (unit -> bool) * body: ResumableCode<'Data, unit> -> ResumableCode<'Data, unit>
diff --git a/src/FSharp.Core/tasks.fs b/src/FSharp.Core/tasks.fs
index faaa920cb15..dfe7f4ba624 100644
--- a/src/FSharp.Core/tasks.fs
+++ b/src/FSharp.Core/tasks.fs
@@ -138,7 +138,7 @@ type TaskBuilderBase() =
                         false)
         )
 
-    member inline this.Using<'Resource, 'TOverall, 'T when 'Resource :> IAsyncDisposable>
+    member inline this.Using<'Resource, 'TOverall, 'T when 'Resource :> IAsyncDisposable|null>
         (
             resource: 'Resource,
             body: 'Resource -> TaskCode<'TOverall, 'T>
@@ -382,7 +382,7 @@ module LowPriority =
 
             this.Bind(task, this.Return)
 
-        member inline _.Using<'Resource, 'TOverall, 'T when 'Resource :> IDisposable>
+        member inline _.Using<'Resource, 'TOverall, 'T when 'Resource :> IDisposable|null>
             (
                 resource: 'Resource,
                 body: 'Resource -> TaskCode<'TOverall, 'T>
diff --git a/src/FSharp.Core/tasks.fsi b/src/FSharp.Core/tasks.fsi
index 47713a60a3f..c626cb4bb7b 100644
--- a/src/FSharp.Core/tasks.fsi
+++ b/src/FSharp.Core/tasks.fsi
@@ -237,7 +237,7 @@ module LowPriority =
         /// </summary>
         member inline Using:
             resource: 'Resource * body: ('Resource -> TaskCode<'TOverall, 'T>) -> TaskCode<'TOverall, 'T>
-                when 'Resource :> IDisposable
+                when 'Resource :> IDisposable|null
 
 /// <summary>
 /// Contains medium-priority overloads for the `task` computation expression builder.