compiler: implement unused warning

* autoload.c (compiler_set_entries): Register slot symbol
"unused".

* stdlib/compiler.tl (compile-opts): New slot, unused.
(%warning-syms%): List unused symbol.
(env lookup-var): Support optional mark-used parameter,
just like lookup-fun.
(env unused-check): New method.
(compiler comp-var): Pass t to mark-used parameter of
lookup-var to register the use.
(compiler (comp-let, comp-var)): Call unused-check
method after sub-compilations are done to dump
diagnostics about unused variables.

1 files changed, 2 insertions, 1 deletions

diff --git a/autoload.c b/autoload.c
index accead94..bb293022 100644
--- a/autoload.c
+++ b/autoload.c
@@ -668,7 +668,8 @@ static val compiler_set_entries(val fun)
     nil
   };
   val slname[] = {
-    lit("shadow-fun"), lit("shadow-var"), lit("shadow-cross"), nil
+    lit("shadow-fun"), lit("shadow-var"), lit("shadow-cross"),
+    lit("unused"), nil
   };
   autoload_sys_set(al_struct, sys_name, fun);
   autoload_set(al_struct, sname, fun);