Make flychecker handle more kinds of defs.

This should help when redefining certain forms. Will also
not do functional arity checking against nil forms, as that
is the default value when a def doesn't evaluate.

src/boot/boot.janet

@@ -2225,6 +2225,29 @@
 ###
 ###
 
+(defn- no-side-effects
+  "Check if form may have side effects. If returns true, then the src
+  must not have side effects, such as calling a C function."
+  [src]
+  (cond
+    (tuple? src)
+    (if (= (tuple/type src) :brackets)
+      (all no-side-effects src))
+    (array? src)
+    (all no-side-effects src)
+    (dictionary? src)
+    (and (all no-side-effects (keys src))
+         (all no-side-effects (values src)))
+    true))
+
+(defn- is-safe-def [x] (no-side-effects (last x)))
+
+(def- safe-forms {'defn true 'defn- true 'defmacro true 'defmacro- true
+                  'def is-safe-def 'var is-safe-def 'def- is-safe-def 'var- is-safe-def
+                  'defglobal is-safe-def 'varglobal is-safe-def})
+
+(def- importers {'import true 'import* true 'use true 'dofile true 'require true})
+
 (defn cli-main
   "Entrance for the Janet CLI tool. Call this functions with the command line
   arguments as an array or tuple of strings to invoke the CLI interface."
@@ -2292,15 +2315,21 @@
     (def h (in handlers n))
     (if h (h i) (do (print "unknown flag -" n) ((in handlers "h")))))
 
-  (def- safe-forms {'defn true 'defn- true 'defmacro true 'defmacro- true})
-  (def- importers {'import true 'import* true 'use true 'dofile true 'require true})
   (defn- evaluator
     [thunk source env where]
     (if *compile-only*
       (when (tuple? source)
+        (def head (source 0))
+        (def safe-check (safe-forms head))
         (cond
-          (safe-forms (source 0)) (thunk)
+          # Sometimes safe form
-          (importers (source 0))
+          (function? safe-check)
+          (if (safe-check source) (thunk))
+          # Always safe form
+          safe-check
+          (thunk)
+          # Import-like form
+          (importers head)
           (do
             (let [[l c] (tuple/sourcemap source)
                   newtup (tuple/setmap (tuple ;source :evaluator evaluator) l c)]
@@ -2347,6 +2376,11 @@
     (setdyn :err-color (if *colorize* true))
     (repl getchunk onsig env)))
 
+(put _env 'no-side-effects nil)
+(put _env 'is-safe-def nil)
+(put _env 'safe-forms nil)
+(put _env 'importers nil)
+
 
 ###
 ###

src/core/compile.c

@@ -455,6 +455,7 @@ static JanetSlot janetc_call(JanetFopts opts, JanetSlot *slots, JanetSlot fun) {
                 break;
                 case JANET_CFUNCTION:
                 case JANET_ABSTRACT:
+                case JANET_NIL:
                     break;
                 case JANET_KEYWORD:
                     if (min_arity == 0) {