> The purpose of the unsafe keyword is to mark code that could cause safety issues (undefined behavior) if its expectations about the surrounding code are not met.

That is one use of the `unsafe` keyword. You see this version in the qualifiers for unsafe functions, where safety depends on the inputs and other context. It means that the marked function cannot be called from safe code. The `unsafe` keyword is also used to mark code blocks where the content of the block must be manually proved to be safe, even for valid inputs—this includes calls to unsafe functions, dereferencing raw pointers, etc. (Unsafe functions use this mode by default.)

The same keyword is also used to mark unsafe traits and their implementations, where an incorrect implementation of the trait—even the members which are not themselves marked as `unsafe` and thus can be called from safe code—can impact the safety of other parts of the program (via unsafe code blocks or unsafe functions). An example of this would be a trait with a member which returns a raw pointer (itself a safe operation) which is then called from within an `unsafe` block and dereferenced.