flambda-backend: Add Make/Dune target for debug printers (#1289)

Makefile.common-jst

@@ -357,3 +357,10 @@ hacking-emacs-poller: _build/_bootinstall
 hacking-emacs-builder: _build/_bootinstall
 	$(dune) rpc build $(ws_boot) -w $(boot_targets)
 	$(dune) diagnostics
+
+.PHONY: debug-printers
+debug-printers: runtime-stdlib # required for $(ws_main) to work
+	$(dune) build $(ws_main) $(ocamldir)/tools/debug_printers
+	@echo
+	@echo To load into ocamldebug, use:
+	@echo source \"$$(realpath _build/main/$(ocamldir)/tools/debug_printers)\"

tools/dune

@@ -85,5 +85,22 @@
   (section lib)
   (package ocaml))
 
+(library
+  (name debug_printers)
+  (modes byte)
+  (libraries ocamlcommon)
+  (modules debug_printers))
+
+(rule
+  (target debug_printers)
+  (deps debug_printers.ml %{cmo:debug_printers})
+  (action
+    (with-stdout-to %{target}
+      (progn
+        ; Resorting to Bash instead of the built-in [echo] action because I
+        ; couldn't find a better way to get an absolute path out of Dune
+        (bash "echo load_printer \\\"$(realpath %{cmo:debug_printers})\\\"")
+        (with-stdin-from debug_printers.ml
+          (run awk "{ print \"install_printer Debug_printers.\" $2 }"))))))
 
 ; ocamlcp, ocamloptp and ocamlprof are not currently supported.