diff --git a/master_changes.md b/master_changes.md
index 84a08da01a4..dfd9ed3b1e3 100644
--- a/master_changes.md
+++ b/master_changes.md
@@ -46,6 +46,8 @@ Possibly scripts breaking changes are prefixed with ✘
 
 ## Internal
   * Disable chrono when timestamps are disables [#4206 @rjbou]
+  * Expose some functionality in the `OpamAction`, `OpamPath` and `OpamSwitchState`
+    modules for use without a `switch` value [#4147]
 
 ## Test
   * Add show cram test [#4206 @rjbou]
diff --git a/src/client/opamAction.ml b/src/client/opamAction.ml
index 61af4913fd0..6ba26423b78 100644
--- a/src/client/opamAction.ml
+++ b/src/client/opamAction.ml
@@ -240,15 +240,13 @@ let download_package st nv =
 (* Prepare the package build:
    * apply the patches
    * substitute the files *)
-let prepare_package_build st nv dir =
-  let opam = OpamSwitchState.opam st nv in
-
+let prepare_package_build env opam nv dir =
   let patches = OpamFile.OPAM.patches opam in
 
   let rec iter_patches f = function
     | [] -> Done []
     | (patchname,filter)::rest ->
-      if OpamFilter.opt_eval_to_bool (OpamPackageVar.resolve ~opam st) filter
+      if OpamFilter.opt_eval_to_bool env filter
       then
         OpamFilename.patch (dir // OpamFilename.Base.to_string patchname) dir
         @@+ function
@@ -278,8 +276,7 @@ let prepare_package_build st nv dir =
     OpamFilename.in_dir dir  @@ fun () ->
     List.fold_left (fun errs f ->
         try
-          OpamFilter.expand_interpolations_in_file
-            (OpamPackageVar.resolve ~opam st) f;
+          OpamFilter.expand_interpolations_in_file env f;
           errs
         with e -> (f, e)::errs)
       [] subst_patches
@@ -306,8 +303,7 @@ let prepare_package_build st nv dir =
     OpamFilename.in_dir dir @@ fun () ->
     List.fold_left (fun errs f ->
         try
-          OpamFilter.expand_interpolations_in_file
-            (OpamPackageVar.resolve ~opam st) f;
+          OpamFilter.expand_interpolations_in_file env f;
           errs
         with e -> (f, e)::errs)
       subst_errs subst_others
@@ -404,7 +400,8 @@ let prepare_package_source st nv dir =
   OpamFilename.mkdir dir;
   get_extra_sources_job @@+ function Some _ as err -> Done err | None ->
     check_extra_files |> function Some _ as err -> Done err | None ->
-      prepare_package_build st nv dir
+      let opam = OpamSwitchState.opam st nv in
+      prepare_package_build (OpamPackageVar.resolve ~opam st) opam nv dir
 
 let compilation_env t opam =
   OpamEnv.get_full ~force_path:true t ~updates:([
diff --git a/src/client/opamAction.mli b/src/client/opamAction.mli
index ebf847fadbe..fc4ce0fdfe6 100644
--- a/src/client/opamAction.mli
+++ b/src/client/opamAction.mli
@@ -29,6 +29,12 @@ val download_package:
 val prepare_package_source:
   rw switch_state -> package -> dirname -> exn option OpamProcess.job
 
+(** [prepare_package_build env opam pkg dir] is a lower level version
+    of `prepare_package_source`, without requiring a switch and
+    without handling extra downloads. *)
+val prepare_package_build:
+  OpamFilter.env -> OpamFile.OPAM.t -> package -> dirname -> exn option OpamProcess.job
+
 (** [build_package t build_dir pkg] builds the package [pkg] within [build_dir].
     Returns [None] on success, [Some exn] on error.
     See {!download_package} and {!prepare_package_source} for the previous
diff --git a/src/format/opamPath.ml b/src/format/opamPath.ml
index fc8ce0addfa..dd04ab870c8 100644
--- a/src/format/opamPath.ml
+++ b/src/format/opamPath.ml
@@ -74,6 +74,12 @@ let plugin t name =
   assert (sname <> "bin");
   plugins t / sname
 
+module type LAYOUT = sig
+  type ctx
+  val root : dirname -> ctx -> dirname
+  val lib_dir : dirname -> ctx -> dirname
+end
+
 module Switch = struct
 
   let root t a = OpamSwitch.get_root t a
@@ -142,39 +148,45 @@ module Switch = struct
   let installed_opam_files_dir t a nv =
     installed_package_dir t a nv / "files"
 
-  module Default = struct
-
-    (** Visible files that can be redirected using
-        [config/global-config.config] *)
+  module DefaultF(L:LAYOUT) = struct
+    let lib_dir = L.lib_dir
 
-    let lib_dir t a = root t a / "lib"
+    let lib t a n = L.lib_dir t a / OpamPackage.Name.to_string n
 
-    let lib t a n = lib_dir t a / OpamPackage.Name.to_string n
+    let stublibs t a = L.lib_dir t a / "stublibs"
 
-    let stublibs t a = lib_dir t a / "stublibs"
+    let toplevel t a = L.lib_dir t a / "toplevel"
 
-    let toplevel t a = lib_dir t a / "toplevel"
-
-    let doc_dir t a = root t a / "doc"
+    let doc_dir t a = L.root t a / "doc"
 
     let man_dir ?num t a =
       match num with
-      | None -> root t a / "man"
-      | Some n -> root t a / "man" / ("man" ^ n)
+      | None -> L.root t a / "man"
+      | Some n -> L.root t a / "man" / ("man" ^ n)
 
-    let share_dir t a = root t a / "share"
+    let share_dir t a = L.root t a / "share"
 
     let share t a n = share_dir t a / OpamPackage.Name.to_string n
 
-    let etc_dir t a = root t a / "etc"
+    let etc_dir t a = L.root t a / "etc"
 
     let etc t a n = etc_dir t a / OpamPackage.Name.to_string n
 
     let doc t a n = doc_dir t a / OpamPackage.Name.to_string n
 
-    let bin t a = root t a / "bin"
+    let bin t a = L.root t a / "bin"
+
+    let sbin t a = L.root t a / "sbin"
+  end
 
-    let sbin t a = root t a / "sbin"
+  (** Visible files that can be redirected using
+      [config/global-config.config] *)
+  module Default = struct
+    include DefaultF(struct
+      type ctx = switch
+      let root = root
+      let lib_dir t a = root t a / "lib"
+    end)
 
   end
 
diff --git a/src/format/opamPath.mli b/src/format/opamPath.mli
index c6e74362245..1c06fc41a1f 100644
--- a/src/format/opamPath.mli
+++ b/src/format/opamPath.mli
@@ -78,6 +78,12 @@ val plugin_bin: t -> name -> filename
     forbidden. *)
 val plugin: t -> name -> dirname
 
+module type LAYOUT = sig
+  type ctx
+  val root : dirname -> ctx -> dirname
+  val lib_dir : dirname -> ctx -> dirname
+end
+
 (** Switch related paths *)
 module Switch: sig
 
@@ -237,6 +243,37 @@ module Switch: sig
     val sbin: t -> switch -> dirname
   end
 
+  (** Fuctorised version of Default, for replicating
+      a switch's layout in non-switch contexts *)
+  module DefaultF : functor (L:LAYOUT) -> sig
+    val lib: t -> L.ctx -> name -> dirname
+  
+    val lib_dir: t -> L.ctx -> dirname
+  
+    val stublibs: t -> L.ctx -> dirname
+  
+    val toplevel: t -> L.ctx -> dirname
+  
+    val doc: t -> L.ctx -> name -> dirname
+  
+    val doc_dir: t -> L.ctx -> dirname
+  
+    val share_dir: t -> L.ctx -> dirname
+  
+    val share: t -> L.ctx -> name -> dirname
+  
+    val etc_dir: t -> L.ctx -> dirname
+  
+    val etc: t -> L.ctx -> name -> dirname
+  
+    val man_dir: ?num:string -> t -> L.ctx -> dirname
+  
+    val bin: t -> L.ctx -> dirname
+  
+    val sbin: t -> L.ctx -> dirname
+  end
+  
+
   (** Actual config handling the global-config.config indirections *)
 
   (** Package-independent dirs *)
diff --git a/src/state/opamSwitchState.ml b/src/state/opamSwitchState.ml
index aaa68bb8c13..4d92fb0ab1f 100644
--- a/src/state/opamSwitchState.ml
+++ b/src/state/opamSwitchState.ml
@@ -723,7 +723,7 @@ let conflicts_with st subset =
 let remove_conflicts st subset pkgs =
   pkgs -- conflicts_with st subset pkgs
 
-let get_conflicts st packages opams_map =
+let get_conflicts_t env packages opams_map =
   let conflict_classes =
     OpamPackage.Map.fold (fun nv opam acc ->
         List.fold_left (fun acc cc ->
@@ -750,7 +750,7 @@ let get_conflicts st packages opams_map =
   OpamPackage.Map.fold (fun nv opam acc ->
       let conflicts =
         OpamFilter.filter_formula ~default:false
-          (OpamPackageVar.resolve_switch ~package:nv st)
+          (env nv)
           (OpamFile.OPAM.conflicts opam)
       in
       let conflicts =
@@ -770,6 +770,11 @@ let get_conflicts st packages opams_map =
     opams_map
     OpamPackage.Map.empty
 
+let get_conflicts st packages opams_map =
+  get_conflicts_t
+    (fun package -> OpamPackageVar.resolve_switch ~package st)
+    packages opams_map
+
 let universe st
     ?(test=OpamStateConfig.(!r.build_test))
     ?(doc=OpamStateConfig.(!r.build_doc))
diff --git a/src/state/opamSwitchState.mli b/src/state/opamSwitchState.mli
index 2dacefb867b..a1c903966b4 100644
--- a/src/state/opamSwitchState.mli
+++ b/src/state/opamSwitchState.mli
@@ -54,6 +54,14 @@ val compute_available_packages:
   pinned:package_set -> opams:OpamFile.OPAM.t package_map ->
   package_set
 
+(** Raw function to compute the conflicts for all packages, given
+    the set of available packages and the corresponding opam files.
+    This is useful to populate the `u_conflicts` field when building
+    a universe manually. *)
+val get_conflicts_t:
+  (package -> OpamFilter.env) -> package_set ->
+  OpamFile.OPAM.t package_map -> formula package_map
+
 (** Infer a switch invariant from a switch state with compiler_packages and
     roots set, using some heuristics. Useful for migration from pre-2.1 opam *)
 val infer_switch_invariant: 'a switch_state -> OpamFormula.t