dialects: allow embedded dots in extensions

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

CHANGES.md

@@ -4,6 +4,9 @@ Unreleased
 - Add additional metadata to the traces provided by `--trace-file` whenever
   `--trace-extended` is passed (#7778, @rleshchinskiy)
 
+- Extensions used in `(dialect)` can include embedded dots (e.g., `cppo.ml`) for
+  versions of the Dune language 3.8 and later. (#7782, @nojb)
+
 3.8.0 (2023-05-23)
 ------------------
 

doc/dune-files.rst

@@ -75,9 +75,10 @@ dialect
 
          Specify the file extension used for this dialect.
 
-         The extension string must not contain any dots and be unique in a given
+         The extension string must not start with a dot and be unique in a given
          project (so that a given extension can be mapped back to a
-         corresponding dialect).
+         corresponding dialect). In Dune 3.8 and later, the extension string may
+         contain embedded dots (e.g., `cppo.ml`).
 
          This field is required.
 

src/dune_rules/dialect.ml

@@ -70,19 +70,19 @@ let encode { name; file_kinds } =
 let decode =
   let open Dune_lang.Decoder in
   let kind kind =
-    let+ loc, extension = field "extension" (located string)
+    let+ loc, extension = field "extension" (located extension)
     and+ preprocess = field_o "preprocess" (located Action.decode_dune_file)
     and+ format =
       field_o "format"
         (map
            ~f:(fun (loc, x) -> (loc, x, []))
            (located Action.decode_dune_file))
-    in
-    let extension =
-      if String.contains extension '.' then
-        User_error.raise ~loc [ Pp.textf "extension must not contain '.'" ];
-      "." ^ extension
-    in
+    and+ syntax_ver = Syntax.get_exn Stanza.syntax in
+    let ver = (3, 9) in
+    (if syntax_ver < ver && Option.is_some (String.index_from extension 1 '.')
+    then
+     let what = "the possibility of defining extensions with embedded dots" in
+     Syntax.Error.since loc Stanza.syntax ver ~what);
     { File_kind.kind; extension; preprocess; format }
   in
   fields

src/dune_rules/melange/melange_stanzas.ml

@@ -24,12 +24,7 @@ module Emit = struct
   let implicit_alias = Alias.Name.of_string "melange"
 
   let decode =
-    let extension_field =
-      let+ loc, extension = located string in
-      if String.is_prefix ~prefix:"." extension then
-        User_error.raise ~loc [ Pp.textf "extension must not start with '.'" ];
-      "." ^ extension
-    in
+    let extension_field = extension in
     let module_systems =
       let module_system =
         enum [ ("es6", Melange.Module_system.Es6); ("commonjs", CommonJs) ]

src/dune_sexp/decoder.ml

@@ -343,6 +343,12 @@ let filename =
           [ Pp.textf "'.' and '..' are not valid filenames" ]
       | fn -> fn)
 
+let extension =
+  plain_string (fun ~loc s ->
+      if String.is_prefix ~prefix:"." s then
+        User_error.raise ~loc [ Pp.textf "extension must not start with '.'" ];
+      "." ^ s)
+
 let relative_file =
   plain_string (fun ~loc fn ->
       if Filename.is_relative fn then fn

src/dune_sexp/decoder.mli

@@ -189,6 +189,10 @@ val plain_string : (loc:Loc.t -> string -> 'a) -> 'a t
 (** A valid filename, i.e. a string other than "." or ".." *)
 val filename : Filename.t t
 
+(** An extension: a string not starting with ".". The value returned by the
+    parser is prefixed with ".". *)
+val extension : string t
+
 (** A relative filename *)
 val relative_file : string t
 

test/blackbox-tests/test-cases/dialects/bad3.t/run.t

@@ -2,5 +2,5 @@
   File "dune-project", line 5, characters 28-32:
   5 |  (implementation (extension .foo))
                                   ^^^^
-  Error: extension must not contain '.'
+  Error: extension must not start with '.'
   [1]

test/blackbox-tests/test-cases/dialects/dots.t/cppo.ml

@@ -0,0 +1,2 @@
+let () =
+  print_endline {|print_endline "Hello, World"|}

test/blackbox-tests/test-cases/dialects/dots.t/dune

@@ -0,0 +1,11 @@
+(executable
+ (name main)
+ (modules main))
+
+(executable
+ (name cppo)
+ (modules cppo))
+
+(rule
+ (alias show)
+ (action (cat main.cppo.ml.ml)))

test/blackbox-tests/test-cases/dialects/dots.t/dune-project.in

@@ -0,0 +1,8 @@
+(dialect
+ (name cppo)
+ (implementation
+  (extension cppo.ml)
+  (preprocess (run ./cppo.exe %{input-file})))
+ (interface
+  (extension cppo.mli)
+  (preprocess (run ./cppo.exe %{input-file}))))

test/blackbox-tests/test-cases/dialects/dots.t/run.t

@@ -0,0 +1,26 @@
+Test the (dialect ...) stanza inside the `dune-project` file.
+
+  $ { echo '(lang dune 3.8)'; cat dune-project.in; } >dune-project
+
+  $ dune build --display short
+  File "dune-project", line 5, characters 13-20:
+  5 |   (extension cppo.ml)
+                   ^^^^^^^
+  Error: the possibility of defining extensions with embedded dots is only
+  available since version 3.9 of the dune language. Please update your
+  dune-project file to have (lang dune 3.9).
+  [1]
+
+  $ { echo '(lang dune 3.9)'; cat dune-project.in; } >dune-project
+
+  $ dune build --display short
+        ocamlc .cppo.eobjs/byte/dune__exe__Cppo.{cmi,cmti}
+        ocamlc .main.eobjs/byte/dune__exe__Main.{cmi,cmti}
+      ocamlopt .cppo.eobjs/native/dune__exe__Cppo.{cmx,o}
+      ocamlopt cppo.exe
+          cppo main.cppo.ml.ml
+      ocamlopt .main.eobjs/native/dune__exe__Main.{cmx,o}
+      ocamlopt main.exe
+
+  $ dune build @show
+  print_endline "Hello, World"