Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.

Can’t be used on mana abilities because they don’t use the stack. They’ve already resolved by the time you get priority.

Rust : règles, fonctionnement et interactions (MTG)

Rust 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

Counter target activated ability from an artifact source. (Mana abilities can't be targeted.)

How it works : Rust

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2016· Wizards of the Coast
Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Can’t be used on mana abilities because they don’t use the stack. They’ve already resolved by the time you get priority.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Rust.

Trusty Boomerang

Equipped creature has "{1}, {T}: Tap target creature. Return Trusty Boomerang to its owner's hand." Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Corrosion

Cumulative upkeep {1} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) At the beginning of your upkeep, put a rust counter on each artifact target opponent controls. Then destroy each artifact with mana value less than or equal to the number of rust counters on it. Artifacts destroyed this way can't be regenerated. When this enchantment leaves the battlefield, remove all rust counters from all permanents.

Rusting Golem

Fading 5 (This creature enters with five fade counters on it. At the beginning of your upkeep, remove a fade counter from it. If you can't, sacrifice it.) Rusting Golem's power and toughness are each equal to the number of fade counters on it.