fix(rules): cross compilation bug

CHANGES.md

@@ -1,6 +1,9 @@
 Unreleased
 ----------
 
+- Fix cross compilation configuration when a context with targets is itself a
+  host of another context (#6958, fixes #6843, @rgrinberg)
+
 - Fix parsing of the `<=` operator in *blang* expressions of `dune` files.
   Previously, the operator would be interpreted as `,`. (#6928, @tatchi)
 

src/dune_rules/context.ml

@@ -346,6 +346,11 @@ let check_fdo_support has_native ocfg ~name =
             version_string
         ]
 
+type instance =
+  { native : t
+  ; targets : t list
+  }
+
 let create ~(kind : Kind.t) ~path ~env ~env_nodes ~name ~merlin ~targets
     ~host_context ~host_toolchain ~profile ~fdo_target_exe
     ~dynamically_linked_foreign_archives ~instrument_with =
@@ -680,7 +685,7 @@ let create ~(kind : Kind.t) ~path ~env ~env_nodes ~name ~merlin ~targets
           ~findlib_toolchain:(Some findlib_toolchain)
         >>| Option.some)
   in
-  native :: List.filter_opt others
+  { native; targets = List.filter_opt others }
 
 let which t fname = Program.which ~path:t.path fname
 
@@ -735,9 +740,9 @@ let create_for_opam ~loc ~root ~env ~env_nodes ~targets ~profile ~switch ~name
     ~instrument_with
 
 module rec Instantiate : sig
-  val instantiate : Context_name.t -> t list Memo.t
+  val instantiate : Context_name.t -> instance Memo.t
 end = struct
-  let instantiate_impl name : t list Memo.t =
+  let instantiate_impl name : instance Memo.t =
     let env = Global.env () in
     let* workspace = Workspace.workspace () in
     let context =
@@ -747,13 +752,9 @@ end = struct
     let* host_context =
       match Workspace.Context.host_context context with
       | None -> Memo.return None
-      | Some context_name -> (
-        let+ contexts = Instantiate.instantiate context_name in
-        match contexts with
-        | [ x ] -> Some x
-        | [] -> assert false (* checked by workspace *)
-        | _ :: _ -> assert false)
-      (* target cannot be host *)
+      | Some context_name ->
+        let+ { native; targets = _ } = Instantiate.instantiate context_name in
+        Some native
     in
     let env_nodes =
       let context = Workspace.Context.env context in
@@ -826,7 +827,10 @@ module DB = struct
       let* workspace = Workspace.workspace () in
       let+ contexts =
         Memo.parallel_map workspace.contexts ~f:(fun c ->
-            Instantiate.instantiate (Workspace.Context.name c))
+            let+ { native; targets } =
+              Instantiate.instantiate (Workspace.Context.name c)
+            in
+            native :: targets)
       in
       let all = List.concat contexts in
       List.iter all ~f:(fun t ->

test/blackbox-tests/test-cases/custom-cross-compilation/github6843.t

@@ -21,9 +21,9 @@ Cross compilation setup that causes dune to crash
   $ export OCAMLFIND_CONF=$PWD/etc/findlib.conf
   $ mkdir -p etc/findlib.conf.d
   $ touch etc/findlib.conf etc/findlib.conf.d/esperanto.conf
-  $ dune build -x esperanto ./cat.exe 2>&1 | awk '/I must not/,/Only I will remain/'
-  I must not crash.  Uncertainty is the mind-killer. Exceptions are the
-  little-death that brings total obliteration.  I will fully express my cases. 
-  Execution will pass over me and through me.  And when it has gone past, I
-  will unwind the stack along its path.  Where the cases are handled there will
-  be nothing.  Only I will remain.
+  $ dune build -x esperanto ./cat.exe
+  File "dune", line 1, characters 18-21:
+  1 | (executable (name cat))
+                        ^^^
+  Error: Module "Cat" doesn't exist.
+  [1]