Anyone who says the wording is obvious, I envy your certainty. It certainly doesn't seem obvious to me.
That said:
1. The BS can replace one of his attacks with cantrip;
2. Haste allows 1 weapon attack only as a possible extra action.
I don't think the BS could grapple or shove with this extra action. Grapple and shove are special actions that can replace an attack. But this isn't "an attack" it's "one weapon attack."
This rings the same (at least to me). The BS can replace "an attack" with a cantrip, but this isn't "an attack" it's "one weapon attack" which seems to have a separate, fairly specific, meaning.
The designers could have said "one attack only..." they didn't, they chose the far more specific "one weapon attack only.."