Stable Memory & Canister Upgrades

What This Is

Stable memory is persistent storage on Internet Computer that survives canister upgrades. Whether ordinary variables survive depends on the language:

Rust -- heap data (thread_local! { RefCell<T> }, plain statics) is wiped on every upgrade. Any data you care about must live in stable memory, either through ic-stable-structures or by serializing it yourself in pre_upgrade/post_upgrade.
Motoko -- enhanced orthogonal persistence is on by default, so let and var in a persistent actor already survive upgrades with no stable keyword and no upgrade hooks. Only transient declarations are wiped.

For Motoko: mops with core = "2.0.0" in mops.toml, and moc >= 1.7.0 (dot notation on Map/List needs it)
For Rust: ic-stable-structures = "0.7" in Cargo.toml

No external canister dependencies. Stable memory is a local canister feature.

Using thread_local! { RefCell<T> } for user data (Rust) -- This is heap memory. It is wiped on every canister upgrade. All user data, balances, settings stored this way will vanish after icp deploy. Use StableBTreeMap instead.