Rubblebelt Rioters — ruling 1

The value of X is determined only as the triggered ability of Rubblebelt Rioters resolves. Once that happens, the value of X won’t change later in the turn even if the greatest power among creatures you control changes.

Rubblebelt Rioters — ruling 2

If Rubblebelt Rioters is the only creature you control as its triggered ability resolves, X is equal to its own power. Unless another effect or a +1/+1 counter has raised its power above 0, this won’t have any effect.

Rubblebelt Rioters : règles, fonctionnement et interactions (MTG)

Rubblebelt Rioters is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Haste Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among creatures you control.

How it works : Rubblebelt Rioters

Original English text — Wizards of the Coast / Scryfall

#1May 3, 2019· Wizards of the Coast
The value of X is determined only as the triggered ability of Rubblebelt Rioters resolves. Once that happens, the value of X won’t change later in the turn even if the greatest power among creatures you control changes.
No other card is named in this ruling.
#2May 3, 2019· Wizards of the Coast
If Rubblebelt Rioters is the only creature you control as its triggered ability resolves, X is equal to its own power. Unless another effect or a +1/+1 counter has raised its power above 0, this won’t have any effect.
No other card is named in this ruling.