Normalize case of context root under Win32

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

CHANGES.md

@@ -7,6 +7,10 @@ Unreleased
 - Fixed wrong workspace discovery from `dune ocaml-merlin` (#4127, fixes #4125,
   @voodoos)
 
+- Fixed a bug that could result in needless recompilation under Windows due to
+  case differences in the result of `Sys.getcwd` (observed under `emacs`).
+  (#3966, @nojb).
+
 2.8.1 (14/01/2021)
 ------------------
 

bin/common.ml

@@ -86,9 +86,18 @@ let prefix_target common s = common.target_prefix ^ s
 
 let instrument_with t = t.instrument_with
 
+(* To avoid needless recompilations under Windows, where the case of
+   [Sys.getcwd] can vary between different invocations of [dune], normalize to
+   lowercase. *)
+let normalize_path p =
+  if Sys.win32 then
+    Path.External.lowercase_ascii p
+  else
+    p
+
 let set_dirs c =
   if c.root.dir <> Filename.current_dir_name then Sys.chdir c.root.dir;
-  Path.set_root (Path.External.cwd ());
+  Path.set_root (normalize_path (Path.External.cwd ()));
   Path.Build.set_build_dir (Path.Build.Kind.of_string c.build_dir)
 
 let set_common_other ?log_file c ~targets =

src/stdune/path.ml

@@ -50,6 +50,8 @@ module External : sig
   val cwd : unit -> t
 
   val as_local : t -> string
+
+  val lowercase_ascii : t -> t
 end = struct
   module T =
     Interned.No_interning
@@ -164,6 +166,8 @@ end = struct
     else
       String.is_prefix ~prefix:(to_string a ^ "/") (to_string b)
 
+  let lowercase_ascii t = make (String.lowercase_ascii (to_string t))
+
   module Set = struct
     include T.Set
 

src/stdune/path.mli

@@ -111,6 +111,8 @@ module External : sig
   val relative : t -> string -> t
 
   val mkdir_p : ?perms:int -> t -> unit
+
+  val lowercase_ascii : t -> t
 end
 
 module Build : sig