Use [Config.Toggle] for toolchains (ocaml#10810)

This changes the variable that enables toolchains from
DUNE_CONFIG__TOOLCHAINS_ENABLED=[true|false] to
DUNE_CONFIG__TOOLCHAINS=[enabled|disabled], following the established
convention for such variables in dune, and for compatibility with
upcoming support for compile-time configuration.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

src/dune_config/config.ml

@@ -85,6 +85,7 @@ let make ~name ~of_string ~default =
   t
 ;;
 
+let make_toggle ~name ~default = make ~name ~default ~of_string:Toggle.of_string
 let global_lock = make ~name:"global_lock" ~of_string:Toggle.of_string ~default:`Enabled
 
 let cutoffs_that_reduce_concurrency_in_watch_mode =

src/dune_config/config.mli

@@ -26,6 +26,8 @@ end
     parsed using [of_string], defaulting to [default]. *)
 val make : name:string -> of_string:(string -> ('a, string) result) -> default:'a -> 'a t
 
+val make_toggle : name:string -> default:Toggle.t -> Toggle.t t
+
 (** [get t] return the value of the configuration for [t] *)
 val get : 'a t -> 'a
 

src/dune_rules/pkg_toolchain.ml

@@ -18,17 +18,7 @@ let base_dir () =
   path
 ;;
 
-let make_bool ~name ~default =
-  let of_string s =
-    match Bool.of_string s with
-    | Some b -> Ok b
-    | None ->
-      Error (sprintf "%s is not a bool (must be \"true\" or \"false\")" (String.quoted s))
-  in
-  Config.make ~name ~of_string ~default
-;;
-
-let enabled = make_bool ~name:"toolchains_enabled" ~default:false
+let enabled = Config.make_toggle ~name:"toolchains" ~default:`Disabled
 
 let pkg_dir (pkg : Dune_pkg.Lock_dir.Pkg.t) =
   (* The name of this package's directory within the toolchains
@@ -53,16 +43,17 @@ let pkg_dir (pkg : Dune_pkg.Lock_dir.Pkg.t) =
 let installation_prefix ~pkg_dir = Path.Outside_build_dir.relative pkg_dir "target"
 
 let is_compiler_and_toolchains_enabled name =
-  Config.get enabled
-  &&
-  let module Package_name = Dune_pkg.Package_name in
-  let compiler_package_names =
-    (* TODO don't hardcode these names here *)
-    [ Package_name.of_string "ocaml-base-compiler"
-    ; Package_name.of_string "ocaml-variants"
-    ]
-  in
-  List.mem compiler_package_names name ~equal:Package_name.equal
+  match Config.get enabled with
+  | `Enabled ->
+    let module Package_name = Dune_pkg.Package_name in
+    let compiler_package_names =
+      (* TODO don't hardcode these names here *)
+      [ Package_name.of_string "ocaml-base-compiler"
+      ; Package_name.of_string "ocaml-variants"
+      ]
+    in
+    List.mem compiler_package_names name ~equal:Package_name.equal
+  | `Disabled -> false
 ;;
 
 let files ~bin_dir =

test/blackbox-tests/test-cases/pkg/toolchain-installation.t

@@ -78,7 +78,7 @@ name so the output is consistent across test runs.
 
 Attempt to build the project. This will fail due to the fake compiler
 but the fake compiler will end up installed as a toolchain package.
-  $ XDG_CACHE_HOME=$PWD/fake-cache DUNE_CONFIG__TOOLCHAINS_ENABLED=true dune build 2>&1 | remove_hash
+  $ XDG_CACHE_HOME=$PWD/fake-cache DUNE_CONFIG__TOOLCHAINS=enabled dune build 2>&1 | remove_hash
   Error: Failed to parse the output of
   '$TESTCASE_ROOT/fake-cache/dune/toolchains/ocaml-base-compiler.1-HASH/target/bin/ocamlc
   -config':