build.hxml

-x Main

FTW.hx

import haxe.macro.Expr;

class FTW
{
	public static function build()
	{
		return haxe.macro.Context.getBuildFields().map(transformField);
	}

	static function transformField(field:Field)
	{
		switch (field.kind)
		{
			case FFun(f): transformExpr(f.expr);
			default:
		}
		return field;
	}

	static function transformExpr(expr:Expr) switch (expr)
	{
		case macro @for($init, $cond, $incr) $block:
			transformExpr(block);
			expr.expr = makeLoop(init, cond, incr, block).expr;
		default:
			haxe.macro.ExprTools.iter(expr, transformExpr);
	}

	static function makeLoop(init:Expr, cond:Expr, incr:Expr, block:Expr) return macro
	{
		$init;
		while ($cond)
		{
			$block;
			$incr;
		}
	}
}

Main.hx

@:build(FTW.build())
class Main
{
	static function main()
	{
		@for(var i = 0, i < 10, i++)
		{
			trace('ftw: $i');
		}
	}
}

A year and a half later, I finally noticed your comment. Sorry for the late response!

Anyway, I figured out that you meant to include a continue statement in your example, because that's what causes duplicate code. (Or maybe you did include one and try.haxe.org didn't save it. Or maybe Haxe improved over the past 1.5 years, and now it optimizes your example better.)

Whichever it was, I've fixed the issue. My code checks for the existence of continue and break statements, and uses the simplest possible loop. Even in the most complicated case, the only code that gets duplicated is "shouldBreak = false," not any anonymous functions or classes.