Can you attack with a creature the same turn it becomes a creature from a noncreature permanent?

Summoning sickness (the rule preventing a creature from attacking or using tap abilities) applies to any creature that hasn't been under its controller's control continuously since the beginning of that player's most recent turn. This is governed by CR 302.6 and the underlying rule at CR 508.1c, which checks how long the permanent has been under your control — not how long it has been a creature.

So if you control a land, artifact, or enchantment and it becomes a creature (via a spell, ability, or its own effect) on the same turn you first gained control of it, it cannot attack. However, if that permanent has been under your control since the beginning of your current turn (or longer), it is free from summoning sickness and can attack the moment it becomes a creature.

The key point: the game tracks control duration of the permanent, not the duration of it being a creature. CR 302.6 explicitly states that the summoning sickness rule checks the permanent's time under your control, regardless of its card type history.

Example: You have an Inkmoth Nexus (a land) that you've controlled since your last turn. On your current turn, you activate its ability to turn it into a 1/1 creature. Because the permanent (the Nexus) has been under your control since the beginning of your turn, it does not have summoning sickness and can attack immediately.

Contrast that with an opponent's artifact you gain control of via a spell during your turn — if that artifact then becomes a creature, it cannot attack this turn because the permanent hasn't been under your control since the beginning of your turn (CR 302.6, CR 508.1c).