### It's allowed.

Since, in general, spells in 5e [only do what they say they do][1], and no more or less, we should consult the wording of the spell. The Haste spell reads as follows:

>That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

It does not disallow the use of the Extra Attack feature, merely restricting you to a single attack. That single attack can then be replaced by a Cantrip using the Bladesinger's Extra Attack feature, just like it could be replaced with other "special attacks" like a Shove or a Grapple.


  [1]: https://rpg.stackexchange.com/questions/107637/what-is-the-source-of-the-spells-do-only-what-they-say-they-do-rules-interpret