Haste spell: what does "one weapon attack only" mean?

Question

While making a Bladesinger Wizard, I wondered if I would be able to use Booming Blade twice while hasted.

Booming Blade: As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails.

Haste: Until the spell ends, the target’s speed is doubled, it gains a +2 bonus to AC, it has advantage on Dexterity saving throws, and it gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

What exactly does "one weapon attack only" mean? Is this to prevent characters with the Extra Attack feature from making 2 attacks? Does this prevent me from casting 2 spells (cantrips or higher-level spells) in a single turn?

Answer 1

You are correct in assuming that the additional action is restricting use of the extra attack class feature. The "one attack only" stipulation is preventing the use of the extra attack feature in the additional action, so a character with extra attack could use his regular action to make 2 attacks and use the additional action granted by haste to attack once more. This is to prevent say, a fighter at level 20 who gets 3 extra attacks from having 8 attacks in a single turn on top of a possible bonus action.

As for booming blade, you cannot cast a spell with the additional action because the actions granted by the haste spell do not include the Cast a Spell action. Booming blade in particular describes that as part of the action used to cast the spell you make an attack, this does not change the action to an attack action, you are still casting a spell.

Answer 2

"one weapon attack only" is for characters with multi-attack, so they can only take one attack with their attack action instead of being able to take two, three or four attacks.

While you are hasted, you cannot cast a spell (cantrip or otherwise) with your second action on your turn (only allowed to attack (melee or ranged), dash, disengage, hide or use an object). Thus you definitely can't use Booming Blade twice in a turn, since you can't cast another spell after you've already cast one.