I think this skips an issue which is that the extra attack granted by Haste is part of a fully-qualified Attack action. So any riders or triggers on the Attack action added by other rules sources all apply - at which point you are trying to decide how the spell's restrictions on the Attack action, and on the attack might interact. I'm not disagreeing with your ruling (I agree, but for other reasons as per my answer), but I somehow feel that the words used are not explaining it. Perhaps I am missing something obvious, but so far I cannot see it in any answer.

@NeilSlater I think the piece that is missing here is a sentence to the effect of "when you make two attacks, one can be a cantrip, but we aren't making two attacks, so we can't substitute a cantrip".

@ThomasMarkov Yes, or something more restrictive in the spell description.

Would you let a bladesinger take the Attack action, cast a cantrip, then decide to not make a second attack?

@Caleth Especially if your target dies, and you have no target for your second attack.

@Caleth Why would you not? That would just be not using your Haste action or Extra attack. You don't have to use all your actions each turn.

@ChristoferWeber this answer is implying that you have to make a weapon attack for the other to be substitutable for a cantrip

@Caleth If you are trying to use the Extra Attack feature, yes. That's a requirement. When you say "take the Attack action, cast a cantrip, then [nothing]", if you mean, just casting a cantrip, then you never took the "attack action" (no need to specify your action) but the "cast spell action". Extra attack doesn't limit your action, it expands your attack action.

@ChristoferWeber I disagree that a bladesinger must do two things when they take the Attack action. They can attack twice, and they can substitute a cantrip for one attack. It's also relevant if they want to cast a bonus-action cantrip as well as a bonus action spell.

@Caleth Oh, they absolutely don't have to do two things when they take an attack action. But then they wouldn't be using their extra attack feature, which is of just fine. But it's not an attack action if they just use a cantrip. As far as the bonus action cantrip goes, normally you can never substitute an action to take a bonus action, since they are different kinds of actions. I see no reason as why you would be able to do it here. Especially since you could then use the feat to just use a bonus action instead of an action, and then just "attack air" to make it count as using the feat.

@ChristoferWeber Bladesinger's Extra Attack does two things. One is the normal Extra Attack thing of making two attacks per Attack, and the other is substituting an attack for casting a cantrip. They absolutely are using the Attack Action to cast a cantrip, they are not using the Cast a Spell Action. And that's why they can have 2 bonus action casting time spells in one turn (so long as one is a cantrip).

@ChristoferWeber "Especially since you could then use the feat to just use a bonus action instead of an action" Which feat is that? AFAICT there is nothing that lets a character trade their action for an additional bonus action.

@Caleth Which feat? Bladesinger's extra attack. It's what we are talking about. You say they can use a bonus action in there, and then you say there is no feat to use a bonus action instead of an action? Sounds like you contradict yourself. I say, you can't use a bonus action in there, because you can't substitute actions for bonus actions.

@ChristoferWeber you can cast a cantrip with a casting time of "one bonus action" as an action, using bladesinger's Extra Attack feature to modify the Attack action

@Caleth Is there anything written that supports that you can use a bonus action cantrip for the extra attack?

@ChristoferWeber The Cast a Spell action tells you that it uses the casting time of the spell, not just an action, whereas the Attack action as modified by the bladesinger's Extra Attack feature doesn't

@Caleth From PHB: "When a character casts any spell, the same basic rules are followed, regardless of the character's class or the spell's effects." "Most spells require a single action to cast, but some spells require a bonus action, a reaction, or much more time to cast." "A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell," You mean to say that this rule is overwritten by the fact that Bladesingers Extra Attack doesn't specifically, redundantly, state that you need to use bonus actions to cast bonus action spells?

@ChristoferWeber the fact that it's replacing part of the Attack action.

A spell with casting time Bonus Action can only be cast as a Bonus Action and not part of an action. But this isn't the place to debate that, it feels like it should be its own question.