Allow building FLambda parser in Dune (#495)

Unfortunately, I couldn't find a nice way to regenerate
`flambda_parser.ml`, etc. conditionally, which is important because it
requires not just Menhir but precisely the correct version of
Menhir (the same one the library was built with). So one has to change
the mode from `fallback` to `promote` manually. But after that, you get
all the benefits of `dune build -w` when editing the parser or lexer.

.gitignore

@@ -94,8 +94,9 @@ _build
 /manual/manual/cmds/warnings-help.etex
 /manual/manual/warnings-help.etex
 
-middle_end/flambda/parser/flambda_parser.ml.in
-middle_end/flambda/parser/flambda_parser.mli.in
+middle_end/flambda/parser/flambda_parser_in.ml
+middle_end/flambda/parser/flambda_parser_in.mli
+middle_end/flambda/parser/flambda_parser.new-messages
 middle_end/flambda/simplify/simplify.ml
 middle_end/flambda/terms/flambda.ml
 middle_end/flambda/types/flambda_type.ml

middle_end/flambda/dune

@@ -5,6 +5,7 @@
   (wrapped false)
   (flags (:standard -principal -nostdlib))
   (ocamlopt_flags (:standard -O3))
+  (modules (:standard \ flambda_parser_in)) ; ignore inputs to sed
   (modules_without_implementation flambda_backend_intf)
   (libraries stdlib ocamlcommon ocamlbytecomp flambdadeps)
 )

middle_end/flambda/parser/Makefile

@@ -4,17 +4,13 @@ OCAMLLEX=ocamllex
 flambda_lex.ml: flambda_lex.mll
 	${OCAMLLEX} flambda_lex.mll
 
-# Avoid creating intermediate files with .ml or .mli extensions so Dune
-# doesn't complain
-flambda_parser.ml.in flambda_parser.mli.in: flambda_parser.mly
-	${MENHIR} --table flambda_parser.mly --base flambda_parser-in
-	mv -f flambda_parser-in.ml flambda_parser.ml.in
-	mv -f flambda_parser-in.mli flambda_parser.mli.in
-
-flambda_parser.ml: flambda_parser.ml.in
+flambda_parser_in.ml flambda_parser_in.mli: flambda_parser.mly
+	${MENHIR} --table flambda_parser.mly --base flambda_parser_in
+
+flambda_parser.ml: flambda_parser_in.ml
 	sed "s/MenhirLib/CamlinternalMenhirLib/g" $< > $@
 
-flambda_parser.mli: flambda_parser.mli.in
+flambda_parser.mli: flambda_parser_in.mli
 	sed "s/MenhirLib/CamlinternalMenhirLib/g" $< > $@
 
 flambda_parser_messages.ml: flambda_parser.mly flambda_parser.messages

middle_end/flambda/parser/dune

@@ -0,0 +1,70 @@
+; Using user actions rather than (ocamllex) and (menhir) because the Makefile
+; runs ocamllex and menhir from this directory rather than the workspace root,
+; meaning the generated source is annotated with the bare filename.
+
+; These rules are disabled by default because you need precisely the correct
+; version of Menhir installed. Comment out (mode fallback) and uncomment (mode
+; promote) in each stanza if you make changes to the Flambda
+; parser.
+
+(rule
+  (mode    fallback)
+  ;(mode    promote)
+  (target  flambda_lex.ml)
+  (deps    flambda_lex.mll)
+  (action  (run ocamllex -q %{deps})))
+
+(rule
+  (mode   fallback)
+  ;(mode   promote)
+  (target flambda_parser.ml)
+  (deps   flambda_parser_in.ml)
+  (action (with-stdin-from %{deps}
+            (with-stdout-to %{target}
+              (run sed "s/MenhirLib/CamlinternalMenhirLib/g")))))
+
+(rule
+  (mode   fallback)
+  ;(mode   promote)
+  (target flambda_parser.mli)
+  (deps   flambda_parser_in.mli)
+  (action (with-stdin-from %{deps}
+            (with-stdout-to %{target}
+              (run sed "s/MenhirLib/CamlinternalMenhirLib/g")))))
+
+(rule
+  (targets flambda_parser_in.ml
+           flambda_parser_in.mli)
+  (deps    flambda_parser.mly)
+  (action  (run menhir --table %{deps} --base flambda_parser_in)))
+
+; NOTE: flambda_parser.messages is massively out of date. No need to activate
+; this rule unless you've fixed it (thanks!).
+(rule
+  (mode   fallback)
+  ;(mode   promote)
+  (target flambda_parser_messages.ml)
+  (deps   flambda_parser.messages flambda_parser.mly)
+  (action (with-stdout-to %{target}
+            (run menhir --compile-errors %{deps}))))
+
+(alias (name list-errors))
+
+(rule
+  (alias  list-errors)
+  (target flambda_parser.new-messages)
+  (deps   flambda_parser.mly)
+  (mode   promote)
+  (action (with-stdout-to %{target}
+            (run menhir --list-errors %{deps}))))
+
+(alias (name check-errors))
+
+(rule
+  (alias  check-errors)
+  (deps   flambda_parser.mly
+          flambda_parser.messages
+          flambda_parser.new-messages)
+  (action (run menhir flambda_parser.mly
+            --compare-errors flambda_parser.messages
+            --compare-errors flambda_parser.new-messages)))