The Promises in a Signature

pub fn arm(&mut self) -> Result<ArmedToken, ArmError>

Element	The promise
`pub`	This is API. Callers you can’t see may depend on it.
`&mut self`	Arming changes this object and needs exclusive access.
`Result<..>`	Arming can fail, and the caller must handle that.
`ArmedToken`	Success produces a value that later operations will require.
`ArmError`	The failure modes are enumerated in a type.

Method receivers

Receiver	Contract
`&self`	Observation. No visible state change. Shareable.
`&mut self`	In-place modification with exclusive access.
`self`	Consumption. The value transitions or ends, and its old state can’t be used again.

Generated code rarely uses self by value, but it’s the most useful receiver for design: it makes a state transition irreversible at compile time. The Typestate page builds on this.

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The Promises in a Signature

Method receivers