Char-Rumbler

• WOTC Official Reminder Textofficial rules
  Double strike — This creature deals both first strike and regular combat damage.

• WWizards of the Coast2007-05-01
  Official Ruling
  A first strike damage step will be created if Char-Rumbler is in combat, even if its power is 0 or less. The normal combat damage step will follow.
• WWizards of the Coast2007-05-01
  Official Ruling
  If Char-Rumbler's power changes between the assignment of first-strike combat damage and the assignment of normal combat damage, Char-Rumbler will deal a different amount of damage in each combat damage step.
• WWizards of the Coast2007-05-01
  Official Ruling
  Yes, Char-Rumbler's printed power is -1. While its power is -1 or 0, it simply deals no combat damage.
• WWizards of the Coast2021-03-19
  Official Ruling
  Because Char-Rumber's power is normally negative, you'll have to give it at least +2/+0 before it'll deal damage. It won't deal negative damage.

Rulings via Scryfall