Remove ActivityDisposable conditional directives and replace with System.IDisposable | null (#18707)

Author: Copilot

src/Compiler/Utilities/Activity.fs

@@ -89,7 +89,7 @@ module internal Activity =
 
     let private activitySource = new ActivitySource(ActivityNames.FscSourceName)
 
-    let start (name: string) (tags: (string * string) seq) : ActivityDisposable =
+    let start (name: string) (tags: (string * string) seq) : System.IDisposable | null =
         let activity = activitySource.CreateActivity(name, ActivityKind.Internal)
 
         match activity with
@@ -100,7 +100,7 @@ module internal Activity =
 
             activity.Start()
 
-    let startNoTags (name: string) : ActivityDisposable = activitySource.StartActivity name
+    let startNoTags (name: string) : System.IDisposable | null = activitySource.StartActivity name
 
     let addEventWithTags name (tags: (string * objnull) seq) =
         match Activity.Current with
@@ -127,7 +127,7 @@ module internal Activity =
 
         let private profiledSource = new ActivitySource(ActivityNames.ProfiledSourceName)
 
-        let startAndMeasureEnvironmentStats (name: string) : ActivityDisposable = profiledSource.StartActivity(name)
+        let startAndMeasureEnvironmentStats (name: string) : System.IDisposable | null = profiledSource.StartActivity(name)
 
         type private GCStats = int[]
 

src/Compiler/Utilities/Activity.fsi

@@ -40,16 +40,16 @@ module internal Activity =
     module Events =
         val cacheHit: string
 
-    val startNoTags: name: string -> ActivityDisposable
+    val startNoTags: name: string -> System.IDisposable | null
 
-    val start: name: string -> tags: (string * string) seq -> ActivityDisposable
+    val start: name: string -> tags: (string * string) seq -> System.IDisposable | null
 
     val addEvent: name: string -> unit
 
     val addEventWithTags: name: string -> tags: (string * objnull) seq -> unit
 
     module Profiling =
-        val startAndMeasureEnvironmentStats: name: string -> ActivityDisposable
+        val startAndMeasureEnvironmentStats: name: string -> System.IDisposable | null
         val addConsoleListener: unit -> IDisposable
 
     module CsvExport =

src/Compiler/Utilities/NullnessShims.fs

@@ -23,12 +23,6 @@ module internal NullnessShims =
         | _, null -> false
         | x, y -> nonNullEqualityFunc !!x !!y
 
-#if BUILDING_WITH_LKG
-    type ActivityDisposable = System.IDisposable
-#else
-    type ActivityDisposable = System.IDisposable | null
-#endif
-
 #if NET5_0_OR_GREATER
     // Argument type for overriding System.Object.Equals(arg)
     // Desktop frameworks as well as netstandard need plain 'obj' and are not annotated, NET5 and higher can use (obj|null)