From 3666cc1b32737d47bb19cc82a433206df03b6442 Mon Sep 17 00:00:00 2001
From: dimension-zero <zero@ditech.ai>
Date: Fri, 22 May 2026 18:56:35 +0100
Subject: [PATCH] refactor: Replace ref-cell late-binding with let mutable
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The two parent-link slots in `PreCompute.fs` used `let current = ref None`
plus `!current` / `current := …`, which:

- relies on the deprecated bang/colon-equals ref operator syntax (the
  source of the prior file-level `#nowarn 44` is unrelated, but
  `ref`-based mutation is no longer idiomatic);
- forces the reader to recognise the pattern before understanding
  what's being done.

Replace each with `let mutable current : … = …` plus a one-paragraph
comment explaining the invariant — written exactly once after the
parent record is built, read by children via the closure. Semantics
unchanged.
---
 src/Argu/PreCompute.fs | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/Argu/PreCompute.fs b/src/Argu/PreCompute.fs
index df61e8b..ce8eb17 100644
--- a/src/Argu/PreCompute.fs
+++ b/src/Argu/PreCompute.fs
@@ -290,9 +290,13 @@ let rec private preComputeUnionCaseArgInfo (stack : Type list) (helpParam : Help
         let dummyFields = types |> Array.map Unchecked.UntypedDefaultOf
         caseCtor.Value dummyFields :?> IArgParserTemplate)
 
-    // use ref cell for late binding of parent argInfo
-    let current = ref None
-    let tryGetCurrent = fun () -> !current
+    // Late binding of parent argInfo: children captured during construction
+    // need a back-reference to this `UnionCaseArgInfo`, but we don't have it
+    // until after we've built `uai` below. Invariant: written exactly once,
+    // immediately after `uai` is constructed; children read it via the
+    // `tryGetCurrent` closure.
+    let mutable current : UnionCaseArgInfo option = None
+    let tryGetCurrent = fun () -> current
 
     let attributes = lazy uci.GetCustomAttributes()
     let declaringTypeAttributes = lazy uci.DeclaringType.GetCustomAttributes(true)
@@ -540,7 +544,7 @@ let rec private preComputeUnionCaseArgInfo (stack : Type list) (helpParam : Help
         IsHidden = isHidden
     }
 
-    current := Some uai // assign result to children
+    current <- Some uai // assign result to children
     uai
 
 and private preComputeUnionArgInfoInner (stack : Type list) (helpParam : HelpParam option) (tryGetParent : unit -> UnionCaseArgInfo option) (t : Type) : UnionArgInfo =
@@ -569,9 +573,11 @@ and private preComputeUnionArgInfoInner (stack : Type list) (helpParam : HelpPar
 
             { Flags = helpSwitches ; Description = description }
 
-    // use ref cell for late binding of parent argInfo
-    let current = ref Unchecked.defaultof<_>
-    let getCurrent = fun () -> !current
+    // Late binding of parent argInfo: see comment on the equivalent slot in
+    // preComputeUnionCaseArgInfo. Invariant: written exactly once at the end
+    // of this function; children read it via the `getCurrent` closure.
+    let mutable current : UnionArgInfo = Unchecked.defaultof<_>
+    let getCurrent = fun () -> current
 
     let caseInfo = lazy(
         FSharpType.GetUnionCases(t, allBindings)
@@ -639,7 +645,7 @@ and private preComputeUnionArgInfoInner (stack : Type list) (helpParam : HelpPar
         MainCommandParam = mainCommandParam
     }
 
-    current := result // assign result to children
+    current <- result // assign result to children
     result
 
 and preComputeUnionArgInfo<'Template when 'Template :> IArgParserTemplate> () =