When should we mark our functions unsafe?

[birkenfeld]

@steveklabnik

Any time that calling the function could introduce memory unsafety. "Memory safety" is the absence of "data races." Data races are: Race Condition vs. Data Race – Embedded in Academia

Or any time calling it can directly crash, e.g. by attempting invalid reads/writes, or violate internal invariants, which can later lead to invalid reads/writes. (For example, str::from_utf8_unchecked.)