erlang: add xref checks to compile step

erlang/Makefile

@@ -26,7 +26,7 @@ PROGRAMS=8to9 ar as nm od readelf
 default: compile link
 
 compile:	$(REBAR3)
-	$(REBAR3) compile
+	$(REBAR3) do compile, xref
 
 link:	$(REBAR3)
 	for PROG in $(PROGRAMS); do \

erlang/rebar.config

@@ -29,6 +29,40 @@
 
 {deps, []}.
 
+{xref_checks,
+  [ undefined_function_calls
+  , undefined_functions
+  , locals_not_used
+  , exports_not_used
+  , deprecated_function_calls
+  , deprecated_functions
+  ]}.
+
+%% silence xref unsed export warnings
+{xref_ignores,
+  [ %% escript entry points (rebar ought to know about these)
+    {'8to9', main, 1}
+  , {ar, main, 1}
+  , {as, main, 1}
+  , {nm, main, 1}
+  , {od, main, 1}
+  , {readelf, main, 1}
+    %% indirect calls to Module:format_error/1
+  , {assemble, format_error, 1}
+  , {getopt, format_error, 1}
+  , {input, format_error, 1}
+  , {output, format_error, 1}
+  , {parse, format_error, 1}
+  , {pdp10_elf36, format_error, 1}
+  , {pdp10_stdio, format_error, 1}
+  , {scan, format_error, 1}
+  , {scan_state, format_error, 1}
+  , {strtol, format_error, 1}
+    %% actual unused exports
+  , {pdp10_opcodes, cpu_device_from_name, 2}
+  , {pdp10_opcodes, models_from_name, 1}
+  ]}.
+
 {escript_emu_args, "%%! +sbtu +A1 +Bd -noshell -smp auto\n"}.
 
 {profiles,