Allow destructuring in formal arguments.

e.g.
```
def m(a,(b,c),d); p [a,b,c,d]; end
m(1,[2,3],4)  # => [1,2,3,4]
```

mruby limitation:
Destructured arguments (`b` and `c` in above example) cannot be accessed
from the default expression of optional arguments and keyword arguments,
since actual assignment is done after the evaluation of those default
expressions. Thus:

```
def f(a,(b,c),d=b)
  p [a,b,c,d]
end
f(1,[2,3])
```

raises `NoMethodError` for `b` in mruby.

mrbgems/mruby-compiler/core/codegen.c

@@ -734,15 +734,13 @@ lambda_body(codegen_scope *s, node *tree, int blk)
     mrb_aspec a;
     int ma, oa, ra, pa, ka, kd, ba;
     int pos, i;
-    node *n, *opt;
+    node *opt;
+    node *margs, *pargs;
     node *tail;
 
     /* mandatory arguments */
     ma = node_len(tree->car->car);
-    n = tree->car->car;
-    while (n) {
-      n = n->cdr;
-    }
+    margs = tree->car->car;
     tail = tree->car->cdr->cdr->cdr->cdr;
 
     /* optional arguments */
@@ -751,6 +749,7 @@ lambda_body(codegen_scope *s, node *tree, int blk)
     ra = tree->car->cdr->cdr->car ? 1 : 0;
     /* mandatory arugments after rest argument */
     pa = node_len(tree->car->cdr->cdr->cdr->car);
+    pargs = tree->car->cdr->cdr->cdr->car;
     /* keyword arguments */
     ka = tail? node_len(tail->cdr->car) : 0;
     /* keyword dictionary? */
@@ -798,6 +797,7 @@ lambda_body(codegen_scope *s, node *tree, int blk)
       dispatch(s, pos+i*3+1);
     }
 
+    /* keyword arguments */
     if (tail) {
       node *kwds = tail->cdr->car;
       int kwrest = 0;
@@ -836,7 +836,34 @@ lambda_body(codegen_scope *s, node *tree, int blk)
         genop_0(s, OP_KEYEND);
       }
     }
+
+    /* argument destructuring */
+    if (margs) {
+      node *n = margs;
+
+      pos = 1;
+      while (n) {
+        if (nint(n->car->car) == NODE_MASGN) {
+          gen_vmassignment(s, n->car->cdr->car, pos, NOVAL);
+        }
+        pos++;
+        n = n->cdr;
+      }
+    }
+    if (pargs) {
+      node *n = margs;
+
+      pos = ma+oa+ra+1;
+      while (n) {
+        if (nint(n->car->car) == NODE_MASGN) {
+          gen_vmassignment(s, n->car->cdr->car, pos, NOVAL);
+        }
+        pos++;
+        n = n->cdr;
+      }
+    }
   }
+
   codegen(s, tree->cdr->car, VAL);
   pop();
   if (s->pc > 0) {
@@ -1066,6 +1093,7 @@ gen_assignment(codegen_scope *s, node *tree, int sp, int val)
     idx = new_sym(s, nsym(tree));
     genop_2(s, OP_SETGV, sp, idx);
     break;
+  case NODE_ARG:
   case NODE_LVAR:
     idx = lv_idx(s, nsym(tree));
     if (idx > 0) {
@@ -1173,7 +1201,7 @@ gen_vmassignment(codegen_scope *s, node *tree, int rhs, int val)
     pop_n(post+1);
     genop_3(s, OP_APOST, cursp(), n, post);
     n = 1;
-    if (t->car) {               /* rest */
+    if (t->car && t->car != (node*)-1) { /* rest */
       gen_assignment(s, t->car, cursp(), NOVAL);
     }
     if (t->cdr && t->cdr->car) {

mrbgems/mruby-compiler/core/parse.y

@@ -682,6 +682,25 @@ new_arg(parser_state *p, mrb_sym sym)
   return cons((node*)NODE_ARG, nsym(sym));
 }
 
+static void
+local_add_margs(parser_state *p, node *n)
+{
+  while (n) {
+    if (n->car->car == (node*)NODE_MASGN) {
+      node *t = n->car->cdr->cdr;
+
+      n->car->cdr->cdr = NULL;
+      while (t) {
+        local_add_f(p, sym(t->car));
+        t = t->cdr;
+      }
+      local_add_margs(p, n->car->cdr->car->car);
+      local_add_margs(p, n->car->cdr->car->cdr->cdr->car);
+    }
+    n = n->cdr;
+  }
+}
+
 /* (m o r m2 tail) */
 /* m: (a b c) */
 /* o: ((a . e1) (b . e2)) */
@@ -693,6 +712,8 @@ new_args(parser_state *p, node *m, node *opt, mrb_sym rest, node *m2, node *tail
 {
   node *n;
 
+  local_add_margs(p, m);
+  local_add_margs(p, m2);
   n = cons(m2, tail);
   n = cons(nsym(rest), n);
   n = cons(opt, n);
@@ -3275,9 +3296,15 @@ f_arg_item      : f_norm_arg
                     {
                       $$ = new_arg(p, $1);
                     }
-                | tLPAREN f_margs rparen
+                | tLPAREN
+                    {
+                      $<nd>$ = local_switch(p);
+                    }
+                  f_margs rparen
                     {
-                      $$ = new_masgn(p, $2, 0);
+                      $$ = new_masgn(p, $3, p->locals->car);
+                      local_resume(p, $<nd>2);
+                      local_add_f(p, 0);
                     }
                 ;