Add %{toolchain} variable

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

CHANGES.md

@@ -1,6 +1,8 @@
 Unreleased
 ----------
 
+- Add a `%{toolchain}` expansion variable (#4899, fixes #3949, @rgrinberg)
+
 - Include dependencies of executables when creating toplevels (either `dune
   top` or `dune utop`) (#4882, fixes #4872, @Gopiancode)
 

src/dune_engine/pform.ml

@@ -44,6 +44,7 @@ module Var = struct
     | Test
     | Corrected_suffix
     | Inline_tests
+    | Toolchain
 
   let compare : t -> t -> Ordering.t = Poly.compare
 
@@ -94,6 +95,7 @@ module Var = struct
         | Test -> "Test"
         | Corrected_suffix -> "Corrected_suffix"
         | Inline_tests -> "Inline_tests"
+        | Toolchain -> "Toolchain"
       in
       Dyn.Variant (cstr, [])
 end
@@ -285,6 +287,7 @@ let encode_to_latest_dune_lang_version t =
       | Test -> Some "test"
       | Corrected_suffix -> Some "corrected-suffix"
       | Inline_tests -> Some "inline_tests"
+      | Toolchain -> Some "toolchain"
     with
     | None -> Pform_was_deleted
     | Some name -> Success { name; payload = None })
@@ -469,6 +472,7 @@ module Env = struct
         ; ("input-file", since ~version:(1, 0) Var.Input_file)
         ; ("corrected-suffix", No_info Corrected_suffix)
         ; ("inline_tests", No_info Inline_tests)
+        ; ("toolchains", since ~version:(3, 0) Var.Toolchain)
         ]
       in
       String.Map.of_list_exn (List.concat [ lowercased; uppercased; other ])

src/dune_engine/pform.mli

@@ -44,6 +44,7 @@ module Var : sig
     | Test
     | Corrected_suffix
     | Inline_tests
+    | Toolchain
 
   val compare : t -> t -> Ordering.t
 

src/dune_rules/expander.ml

@@ -371,7 +371,16 @@ let expand_pform_gen ~(context : Context.t) ~bindings ~dir ~source
       | Ccomp_type ->
         static
           (string
-             (Ocaml_config.Ccomp_type.to_string context.lib_config.ccomp_type)))
+             (Ocaml_config.Ccomp_type.to_string context.lib_config.ccomp_type))
+      | Toolchain ->
+        static
+          (string
+             (match context.findlib_toolchain with
+             | Some toolchain -> Context_name.to_string toolchain
+             | None ->
+               let loc = Dune_lang.Template.Pform.loc source in
+               User_error.raise ~loc
+                 [ Pp.text "No toolchain defined for this context" ])))
     | Macro (macro, s) -> (
       match macro with
       | Ocaml_config ->