[WIP] Fix alias hint generation to use selected build context

bin/alias.ml

@@ -106,8 +106,17 @@ let dep_on_alias_rec_multi_contexts ~dir:src_dir ~name ~contexts =
   | true -> Action_builder.return ()
   | false ->
     let* load_dir =
+      let selected = !Dune_engine.Clflags.selected_context in
+      let hint_contexts =
+        match List.filter contexts ~f:(Context_name.equal selected) with
+        | [] ->
+          (match contexts with
+           | [] -> []
+           | ctx :: _ -> [ ctx ])
+        | ctxs -> ctxs
+      in
       Action_builder.all
-      @@ List.map contexts ~f:(fun ctx ->
+      @@ List.map hint_contexts ~f:(fun ctx ->
         let dir =
           Source_tree.Dir.path dir
           |> Path.Build.append_source (Context_name.build_dir ctx)

src/dune_engine/clflags.ml

@@ -32,3 +32,4 @@ let display = ref Display.Quiet
 let can_go_in_shared_cache_default = ref false
 let diff_command = ref None
 let wait_for_filesystem_clock = ref false
+let selected_context = ref Context_name.default

src/dune_engine/clflags.mli

@@ -44,3 +44,6 @@ val diff_command : string option ref
 (** Wait for the filesystem clock to advance rather than dropping cached digest
     entries *)
 val wait_for_filesystem_clock : bool ref
+
+(** The build context selected by the user on the command line *)
+val selected_context : Context_name.t ref

test/blackbox-tests/test-cases/alias/alias-candidates-multi-context.t

@@ -0,0 +1,34 @@
+Alias hint generation should only use the selected build context in a
+multi-context workspace. Aliases from other build contexts should not appear
+as hints.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.17)
+  > EOF
+
+  $ cat > dune-workspace << EOF
+  > (lang dune 3.17)
+  > (context default)
+  > (context (default (name other)))
+  > EOF
+
+  $ cat > dune << EOF
+  > (rule
+  >  (alias foo)
+  >  (action (echo "foo"))
+  >  (enabled_if (= %{context_name} "default")))
+  > (rule
+  >  (alias foe)
+  >  (action (echo "foe"))
+  >  (enabled_if (= %{context_name} "other")))
+  > EOF
+
+We have "foo" in the default context and "foe" in the "other" context. When
+building a misspelled alias "@fou", hints should only come from the default
+context, so only "foo" (not "foe") should be suggested.
+
+  $ dune build @fou
+  Error: Alias "fou" specified on the command line is empty.
+  It is not defined in . or any of its descendants.
+  Hint: did you mean fmt or foo?
+  [1]