Auto-detect bnfc tree-sitter support

lib/fe/dune

@@ -45,3 +45,26 @@
  (flags
   (:standard -open BasilIR))
  (libraries bincaml.basilir))
+
+(rule
+ (target has-bnfc-tree-sitter)
+ (enabled_if
+  (not %{bin-available:bnfc}))
+ (action
+  (with-stdout-to
+   %{target}
+   (echo "false"))))
+
+(rule
+ (target has-bnfc-tree-sitter)
+ (enabled_if %{bin-available:bnfc})
+ (action
+  (with-stdout-to
+   %{target}
+   (pipe-stdout
+    (run bnfc --help)
+    (bash
+     "if grep -q -- --tree-sitter-word; then printf true; else printf false; fi")))))
+
+; if `bnfc` exits with an error, we report that error and fail.
+; "false" is only written if bnfc succeeds but tree-sitter was not found.

tree-sitter/.gitignore

@@ -1,3 +1,5 @@
+grammar.js.new
+
 # Rust artifacts
 target/
 

tree-sitter/dune

@@ -10,10 +10,14 @@
 ; check that grammar.js is up to date.
 ; requires `bnfc-treesitter` from pac-nix.
 
+; bnfc tree-sitter backend is only run if the fixed tree-sitter fork is
+; detected in the output of bnfc --help. this lets us use upstream
+; bnfc for the main ocaml parser.
+
 (rule
  (alias runtest)
  (deps %{project_root}/lib/fe/BasilIR.cf)
- (enabled_if %{bin-available:bnfc})
+ (enabled_if %{read:../lib/fe/has-bnfc-tree-sitter})
  (action
   (progn
    (copy %{deps} basilir.cf)
@@ -25,3 +29,13 @@
     basilir.cf
     --makefile)
    (diff? grammar.js tree-sitter-basilir/grammar.js))))
+
+(rule
+ (alias runtest)
+ (deps (universe))
+ (enabled_if
+  (not %{read:../lib/fe/has-bnfc-tree-sitter}))
+ (action
+  (echo
+   "tree-sitter/dune: bnfc tree-sitter generation skipped,"
+   "because the installed bnfc doesn't support it.\n")))