The kind of wording in the rules that let you replace a single attack with 2 via extra attack, or a single attack with a grab, are the same wording that let you replace an attack with a cantrip.
Haste, as most people read it, prevent the one-attack-for-two or hasted-grab use of the attack action, so should also block the cantrip casting.
Now, there is a perfectly valid alternative reading. In this reading, Haste prevents just prevents a 2nd weapon attack with that action. Anything you do with that action, so long as it causes no more than 1 weapon attack, is allowed.
Under that reading, a hasted attack action on a barbarian could be a weapon attack and a shove, or a weapon attack and a grapple.
And a bladesinger could make a weapon attack, then substitute her second attack with a non-weapon attack cantrip.
Or, they could replace one attack with a cantrip that involves a weapon attack, and substitute their second attack with a grapple or shove.
Basically, does haste say that you can do any attack action, so long as it is no more than one weapon attack, or does it say that the only thing you can do with the haste action is is an attack action, and in that attack action you can do nothing but a single weapon attack? Under the second wording, arguably you wouldn't even be allowed to smite, as an attack action with a weapon attack and a smite isn't only one weapon attack.
Honestly,. there is an entire spectrum of how strict you can read the haste action attack.
Haste, as most people read it, prevent the one-attack-for-two or hasted-grab use of the attack action, so should also block the cantrip casting.
Now, there is a perfectly valid alternative reading. In this reading, Haste prevents just prevents a 2nd weapon attack with that action. Anything you do with that action, so long as it causes no more than 1 weapon attack, is allowed.
Under that reading, a hasted attack action on a barbarian could be a weapon attack and a shove, or a weapon attack and a grapple.
And a bladesinger could make a weapon attack, then substitute her second attack with a non-weapon attack cantrip.
Or, they could replace one attack with a cantrip that involves a weapon attack, and substitute their second attack with a grapple or shove.
Basically, does haste say that you can do any attack action, so long as it is no more than one weapon attack, or does it say that the only thing you can do with the haste action is is an attack action, and in that attack action you can do nothing but a single weapon attack? Under the second wording, arguably you wouldn't even be allowed to smite, as an attack action with a weapon attack and a smite isn't only one weapon attack.
Honestly,. there is an entire spectrum of how strict you can read the haste action attack.