From 7987490043ba32f329b8df94517a763d423858c7 Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 1/7] refactor: centralize process environment serialization

---
 otherlibs/stdune/src/env.ml  | 2 ++
 otherlibs/stdune/src/env.mli | 5 +++++
 src/dune_engine/dtemp.ml     | 4 ++--
 src/dune_engine/dtemp.mli    | 5 ++---
 src/dune_engine/process.ml   | 5 +----
 5 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/otherlibs/stdune/src/env.ml b/otherlibs/stdune/src/env.ml
index bd74e92ee8a..cb8ab1d2052 100644
--- a/otherlibs/stdune/src/env.ml
+++ b/otherlibs/stdune/src/env.ml
@@ -61,6 +61,8 @@ let to_unix t =
     res
 ;;
 
+let to_unix_with_override t ~var ~value = of_map (Map.set t.vars var value) |> to_unix
+
 let of_unix arr =
   Array.to_list arr
   |> List.map ~f:(fun s ->
diff --git a/otherlibs/stdune/src/env.mli b/otherlibs/stdune/src/env.mli
index 1835646c8d5..f1d8b0189e4 100644
--- a/otherlibs/stdune/src/env.mli
+++ b/otherlibs/stdune/src/env.mli
@@ -27,6 +27,11 @@ val vars : t -> Var.Set.t
 val initial : t
 
 val to_unix : t -> string list
+
+(** [to_unix_with_override t ~var ~value] serializes [t] with [var] set to
+    [value]. *)
+val to_unix_with_override : t -> var:Var.t -> value:string -> string list
+
 val of_unix : string array -> t
 val get : t -> Var.t -> string option
 
diff --git a/src/dune_engine/dtemp.ml b/src/dune_engine/dtemp.ml
index 146d8a63c7a..36d58b483fe 100644
--- a/src/dune_engine/dtemp.ml
+++ b/src/dune_engine/dtemp.ml
@@ -7,9 +7,9 @@ let file ~prefix ~suffix =
   Temp.temp_in_dir File ~dir:(Lazy.force temp_dir) ~suffix ~prefix
 ;;
 
-let add_to_env env =
+let to_unix env =
   let value = Lazy.force temp_dir_value in
-  Env.add env ~var:Env.Var.temp_dir ~value
+  Env.to_unix_with_override env ~var:Env.Var.temp_dir ~value
 ;;
 
 let destroy = Temp.destroy
diff --git a/src/dune_engine/dtemp.mli b/src/dune_engine/dtemp.mli
index 313a6c636dc..d19b15eb134 100644
--- a/src/dune_engine/dtemp.mli
+++ b/src/dune_engine/dtemp.mli
@@ -5,9 +5,8 @@ open Import
 (** This returns a build path, but we don't rely on that *)
 val file : prefix:string -> suffix:string -> Path.t
 
-(** Add the temp env var to the environment passed or return the initial
-    environment with the temp var added. *)
-val add_to_env : Env.t -> Env.t
+(** Serialize an environment with Dune's temp directory set. *)
+val to_unix : Env.t -> string list
 
 (** Destroy the temporary file or directory *)
 val destroy : Temp.what -> Path.t -> unit
diff --git a/src/dune_engine/process.ml b/src/dune_engine/process.ml
index 789639c4df8..461a2e69ad2 100644
--- a/src/dune_engine/process.ml
+++ b/src/dune_engine/process.ml
@@ -982,10 +982,7 @@ let spawn
     Time.now ()
   in
   let pid =
-    let env =
-      let env = Dtemp.add_to_env env in
-      Env.to_unix env |> Spawn.Env.of_list
-    in
+    let env = Dtemp.to_unix env |> Spawn.Env.of_list in
     let stdout = Io.fd stdout |> Fd.unsafe_to_unix_file_descr in
     let stderr = Io.fd stderr |> Fd.unsafe_to_unix_file_descr in
     let stdin = Io.fd stdin |> Fd.unsafe_to_unix_file_descr in

From eea63c812ade4632cfc055b7588fb2e162b1e9d1 Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 2/7] bench: measure process environment overrides

---
 bench/micro/dune_bench/env_bench.ml | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
 create mode 100644 bench/micro/dune_bench/env_bench.ml

diff --git a/bench/micro/dune_bench/env_bench.ml b/bench/micro/dune_bench/env_bench.ml
new file mode 100644
index 00000000000..a7fc441a74b
--- /dev/null
+++ b/bench/micro/dune_bench/env_bench.ml
@@ -0,0 +1,17 @@
+open Stdune
+
+let make_env size =
+  List.init size ~f:(fun i ->
+    let prefix = if i mod 2 = 0 then 'A' else 'Z' in
+    Printf.sprintf "%c_DUNE_BENCH_%04d=value-%04d" prefix i i)
+  |> Array.of_list
+  |> Env.of_unix
+  |> Env.add ~var:Env.Var.temp_dir ~value:"original-temp-directory"
+;;
+
+let%bench_fun ("render environment override" [@indexed variables = [ 10; 100; 1_000 ]]) =
+  let env = make_env variables in
+  fun () ->
+    Env.to_unix_with_override env ~var:Env.Var.temp_dir ~value:"dune-temp-directory"
+    |> ignore
+;;

From 037ad61a869ff3f1891276bd5283180c8d7cf432 Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 3/7] test(stdune): cover serialized environment overrides

---
 otherlibs/stdune/test/dune         | 10 ++++++++++
 otherlibs/stdune/test/env_tests.ml | 21 +++++++++++++++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 otherlibs/stdune/test/env_tests.ml

diff --git a/otherlibs/stdune/test/dune b/otherlibs/stdune/test/dune
index 509c2c2c3f0..b5ae4cef349 100644
--- a/otherlibs/stdune/test/dune
+++ b/otherlibs/stdune/test/dune
@@ -3,6 +3,7 @@
  (modules
   :standard
   \
+  env_tests
   path_external_build_tests
   path_tests
   proc_linux_tests
@@ -24,6 +25,14 @@
  (preprocess
   (pps ppx_expect)))
 
+(library
+ (name stdune_env_tests)
+ (modules env_tests)
+ (inline_tests)
+ (libraries stdune ppx_inline_test.config)
+ (preprocess
+  (pps ppx_inline_test)))
+
 (library
  (name stdune_path_tests)
  (modules path_tests)
@@ -88,4 +97,5 @@
 (alias
  (name runtest-windows)
  (deps
+  (alias runtest-stdune_env_tests)
   (alias runtest-stdune_external_build_tests)))
diff --git a/otherlibs/stdune/test/env_tests.ml b/otherlibs/stdune/test/env_tests.ml
new file mode 100644
index 00000000000..a266c9e413b
--- /dev/null
+++ b/otherlibs/stdune/test/env_tests.ml
@@ -0,0 +1,21 @@
+open Stdune
+
+let test_override env ~var ~value =
+  let expected = Env.add env ~var ~value in
+  let actual =
+    Env.to_unix_with_override env ~var ~value |> Array.of_list |> Env.of_unix
+  in
+  if not (Env.equal expected actual)
+  then
+    Code_error.raise
+      "Env.to_unix_with_override returned the wrong environment"
+      [ "expected", Env.to_dyn expected; "actual", Env.to_dyn actual ]
+;;
+
+let%test_unit "to_unix_with_override" =
+  let env = Env.of_unix [| "A=one"; "B=two"; "C=three" |] in
+  test_override env ~var:"A" ~value:"one";
+  test_override env ~var:"B" ~value:"changed";
+  test_override env ~var:"D" ~value:"new";
+  test_override env ~var:"C" ~value:"value=with=equals"
+;;

From 4b0dd10620e5e7775adb7c0b6cea71cd96b626e0 Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 4/7] perf: reuse environment entries across process spawns

---
 otherlibs/stdune/src/env.ml  | 22 +++++++++++++++++++++-
 otherlibs/stdune/src/env.mli |  2 +-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/otherlibs/stdune/src/env.ml b/otherlibs/stdune/src/env.ml
index cb8ab1d2052..d08e43237f0 100644
--- a/otherlibs/stdune/src/env.ml
+++ b/otherlibs/stdune/src/env.ml
@@ -61,7 +61,27 @@ let to_unix t =
     res
 ;;
 
-let to_unix_with_override t ~var ~value = of_map (Map.set t.vars var value) |> to_unix
+let to_unix_with_override t ~var ~value =
+  match get t var with
+  | None -> of_map (Map.set t.vars var value) |> to_unix
+  | Some old_value when String.equal old_value value -> to_unix t
+  | Some old_value ->
+    let old_entry = binding var old_value in
+    let new_entry = binding var value in
+    let rec replace = function
+      | [] -> None
+      | entry :: rest ->
+        if String.equal entry old_entry
+        then Some (new_entry :: rest)
+        else (
+          match replace rest with
+          | None -> None
+          | Some rest -> Some (entry :: rest))
+    in
+    (match replace (to_unix t) with
+     | Some result -> result
+     | None -> of_map (Map.set t.vars var value) |> to_unix)
+;;
 
 let of_unix arr =
   Array.to_list arr
diff --git a/otherlibs/stdune/src/env.mli b/otherlibs/stdune/src/env.mli
index f1d8b0189e4..a737ee7dbf6 100644
--- a/otherlibs/stdune/src/env.mli
+++ b/otherlibs/stdune/src/env.mli
@@ -29,7 +29,7 @@ val initial : t
 val to_unix : t -> string list
 
 (** [to_unix_with_override t ~var ~value] serializes [t] with [var] set to
-    [value]. *)
+    [value]. It reuses the cached serialization of variables that are unchanged. *)
 val to_unix_with_override : t -> var:Var.t -> value:string -> string list
 
 val of_unix : string array -> t

From 69c51652fc704aad3d75a8961ff1c0a3cb3c57fc Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 5/7] bench: cover absent temporary directory variable

---
 bench/micro/dune_bench/env_bench.ml | 34 +++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/bench/micro/dune_bench/env_bench.ml b/bench/micro/dune_bench/env_bench.ml
index a7fc441a74b..a3511866131 100644
--- a/bench/micro/dune_bench/env_bench.ml
+++ b/bench/micro/dune_bench/env_bench.ml
@@ -1,17 +1,33 @@
 open Stdune
 
-let make_env size =
-  List.init size ~f:(fun i ->
-    let prefix = if i mod 2 = 0 then 'A' else 'Z' in
-    Printf.sprintf "%c_DUNE_BENCH_%04d=value-%04d" prefix i i)
-  |> Array.of_list
-  |> Env.of_unix
-  |> Env.add ~var:Env.Var.temp_dir ~value:"original-temp-directory"
+let make_env size ~temp_dir =
+  let env =
+    List.init size ~f:(fun i ->
+      let prefix = if i mod 2 = 0 then 'A' else 'Z' in
+      Printf.sprintf "%c_DUNE_BENCH_%04d=value-%04d" prefix i i)
+    |> Array.of_list
+    |> Env.of_unix
+  in
+  if temp_dir
+  then Env.add env ~var:Env.Var.temp_dir ~value:"original-temp-directory"
+  else env
 ;;
 
-let%bench_fun ("render environment override" [@indexed variables = [ 10; 100; 1_000 ]]) =
-  let env = make_env variables in
+let render_environment_override ~temp_dir variables =
+  let env = make_env variables ~temp_dir in
   fun () ->
     Env.to_unix_with_override env ~var:Env.Var.temp_dir ~value:"dune-temp-directory"
     |> ignore
 ;;
+
+let%bench_fun
+    ("render environment override: present" [@indexed variables = [ 10; 100; 1_000 ]])
+  =
+  render_environment_override ~temp_dir:true variables
+;;
+
+let%bench_fun
+    ("render environment override: absent" [@indexed variables = [ 10; 100; 1_000 ]])
+  =
+  render_environment_override ~temp_dir:false variables
+;;

From 5841651ccb36f954373c740464bf0490b0c8161a Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 6/7] test(stdune): cover case-insensitive environment
 overrides

---
 otherlibs/stdune/test/env_tests.ml | 34 ++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/otherlibs/stdune/test/env_tests.ml b/otherlibs/stdune/test/env_tests.ml
index a266c9e413b..29be044d4dd 100644
--- a/otherlibs/stdune/test/env_tests.ml
+++ b/otherlibs/stdune/test/env_tests.ml
@@ -2,9 +2,29 @@ open Stdune
 
 let test_override env ~var ~value =
   let expected = Env.add env ~var ~value in
-  let actual =
-    Env.to_unix_with_override env ~var ~value |> Array.of_list |> Env.of_unix
-  in
+  let actual_unix = Env.to_unix_with_override env ~var ~value in
+  let actual = actual_unix |> Array.of_list |> Env.of_unix in
+  if Sys.win32
+  then (
+    let expected_unix = Env.to_unix expected in
+    if not (List.equal String.equal expected_unix actual_unix)
+    then
+      Code_error.raise
+        "Env.to_unix_with_override changed the serialized environment"
+        [ "expected", Dyn.list Dyn.string expected_unix
+        ; "actual", Dyn.list Dyn.string actual_unix
+        ];
+    let matching_entries =
+      List.filter actual_unix ~f:(fun entry ->
+        match String.lsplit2 entry ~on:'=' with
+        | None -> false
+        | Some (entry_var, _) -> Ordering.is_eq (Env.Var.compare entry_var var))
+    in
+    if List.length matching_entries <> 1
+    then
+      Code_error.raise
+        "Env.to_unix_with_override returned duplicate variables"
+        [ "var", Dyn.string var; "environment", Dyn.list Dyn.string actual_unix ]);
   if not (Env.equal expected actual)
   then
     Code_error.raise
@@ -17,5 +37,11 @@ let%test_unit "to_unix_with_override" =
   test_override env ~var:"A" ~value:"one";
   test_override env ~var:"B" ~value:"changed";
   test_override env ~var:"D" ~value:"new";
-  test_override env ~var:"C" ~value:"value=with=equals"
+  test_override env ~var:"C" ~value:"value=with=equals";
+  if Sys.win32
+  then (
+    let mixed_case = Env.of_unix [| "A=one"; "Temp=first" |] in
+    test_override mixed_case ~var:"TEMP" ~value:"changed";
+    let duplicate_case = Env.of_unix [| "A=one"; "Temp=first"; "TEMP=second" |] in
+    test_override duplicate_case ~var:"temp" ~value:"changed")
 ;;

From 9e1b253acb866c2a6d8aa3552b6a66631ce60013 Mon Sep 17 00:00:00 2001
From: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Date: Sun, 2 Aug 2026 21:21:40 -0700
Subject: [PATCH 7/7] perf: reuse serialized environment for missing variables

---
 otherlibs/stdune/src/env.ml | 51 +++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 11 deletions(-)

diff --git a/otherlibs/stdune/src/env.ml b/otherlibs/stdune/src/env.ml
index d08e43237f0..e18c1d312f6 100644
--- a/otherlibs/stdune/src/env.ml
+++ b/otherlibs/stdune/src/env.ml
@@ -52,6 +52,15 @@ let binding var value =
   Bytes.unsafe_to_string result
 ;;
 
+let unix_entry_has_var_caseless entry var =
+  let var_length = String.length var in
+  let rec loop i =
+    i = var_length
+    || (Char.lowercase_ascii entry.[i] = Char.lowercase_ascii var.[i] && loop (i + 1))
+  in
+  String.length entry > var_length && entry.[var_length] = '=' && loop 0
+;;
+
 let to_unix t =
   match t.unix with
   | Some v -> v
@@ -63,20 +72,40 @@ let to_unix t =
 
 let to_unix_with_override t ~var ~value =
   match get t var with
-  | None -> of_map (Map.set t.vars var value) |> to_unix
+  | None ->
+    if Sys.win32
+    then of_map (Map.set t.vars var value) |> to_unix
+    else binding var value :: to_unix t
   | Some old_value when String.equal old_value value -> to_unix t
   | Some old_value ->
-    let old_entry = binding var old_value in
     let new_entry = binding var value in
-    let rec replace = function
-      | [] -> None
-      | entry :: rest ->
-        if String.equal entry old_entry
-        then Some (new_entry :: rest)
-        else (
-          match replace rest with
-          | None -> None
-          | Some rest -> Some (entry :: rest))
+    let replace =
+      if Sys.win32
+      then (
+        let rec loop = function
+          | [] -> None
+          | entry :: rest ->
+            if unix_entry_has_var_caseless entry var
+            then Some (new_entry :: rest)
+            else (
+              match loop rest with
+              | None -> None
+              | Some rest -> Some (entry :: rest))
+        in
+        loop)
+      else (
+        let old_entry = binding var old_value in
+        let rec loop = function
+          | [] -> None
+          | entry :: rest ->
+            if String.equal entry old_entry
+            then Some (new_entry :: rest)
+            else (
+              match loop rest with
+              | None -> None
+              | Some rest -> Some (entry :: rest))
+        in
+        loop)
     in
     (match replace (to_unix t) with
      | Some result -> result