cgen: fixes for ... in with index (#11995)

vlib/v/gen/c/cgen.v

@@ -2052,7 +2052,11 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		g.write('${g.typ(node.cond_type)} $t_expr = ')
 		g.expr(node.cond)
 		g.writeln(';')
-		g.writeln('while (1) {')
+		if node.key_var in ['', '_'] {
+			g.writeln('while (1) {')
+		} else  {
+			g.writeln('for (size_t $node.key_var = 0;; ++$node.key_var) {')
+		}
 		t_var := g.new_tmp_var()
 		receiver_typ := next_fn.params[0].typ
 		receiver_styp := g.typ(receiver_typ)

vlib/v/gen/c/testdata/for_in.c.must_have

@@ -0,0 +1,2 @@
+for (size_t ix = 0;; ++ix) {
+while (1) {

vlib/v/gen/c/testdata/for_in.out

@@ -0,0 +1,9 @@
+a.2
+a.4
+a.6
+a.8
+b.0=2
+b.1=4
+b.2=6
+b.3=8
+end

vlib/v/gen/c/testdata/for_in.vv

@@ -0,0 +1,27 @@
+struct CustomIter {
+mut:
+  idx int
+}
+
+fn new_custom_iter() CustomIter {
+	return CustomIter{ idx: 0 }
+}
+
+fn (mut a CustomIter) next() ?int {
+	if a.idx == 4 {
+		return error('')
+	} else {
+		a.idx++
+		return a.idx * 2
+	}
+}
+
+fn main() {
+	for x in new_custom_iter() {
+		println('a.$x')
+	}
+	for ix, val in new_custom_iter() {
+		println('b.$ix=$val')
+	}
+	println('end')
+}