cgen: skip inc generation for ForCStmt, when empty in the v source

vlib/v/ast/ast.v

@@ -434,7 +434,9 @@ pub:
 	init     Stmt // i := 0;
 	has_init bool
 	cond     Expr // i < 10;
+	has_cond bool
 	inc      Expr // i++;
+	has_inc  bool
 	stmts    []Stmt
 	pos      token.Position
 }

vlib/v/gen/cgen.v

@@ -407,10 +407,13 @@ fn (g mut Gen) stmt(node ast.Stmt) {
 			} else {
 				g.stmt(it.init)
 			}
-			g.expr(it.cond)
+			if it.has_cond {
+				g.expr(it.cond)
+			}
 			g.write('; ')
-			// g.stmt(it.inc)
-			g.expr(it.inc)
+			if it.has_inc {
+				g.expr(it.inc)
+			}
 			g.writeln(') {')
 			g.stmts(it.stmts)
 			g.writeln('}')

vlib/v/parser/parser.v

@@ -1059,6 +1059,8 @@ fn (p mut Parser) for_stmt() ast.Stmt {
 		// mut inc := ast.Stmt{}
 		mut inc := ast.Expr{}
 		mut has_init := false
+		mut has_cond := false
+		mut has_inc := false
 		if p.peek_tok.kind in [.assign, .decl_assign] {
 			init = p.assign_stmt()
 			has_init = true
@@ -1070,11 +1072,13 @@ fn (p mut Parser) for_stmt() ast.Stmt {
 		if p.tok.kind != .semicolon {
 			mut typ := table.void_type
 			cond = p.expr(0)
+			has_cond = true
 		}
 		p.check(.semicolon)
 		if p.tok.kind != .lcbr {
 			// inc = p.stmt()
 			inc = p.expr(0)
+			has_inc = true
 		}
 		p.inside_for = false
 		stmts := p.parse_block()
@@ -1082,6 +1086,8 @@ fn (p mut Parser) for_stmt() ast.Stmt {
 		return ast.ForCStmt{
 			stmts: stmts
 			has_init: has_init
+			has_cond: has_cond
+			has_inc: has_inc
 			init: init
 			cond: cond
 			inc: inc