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Inside an unsafe block you may dereference a raw pointer, call an unsafe function, access a mutable static, implement an unsafe trait, and read a union field. Everything else is checked exactly as before. The block is a promise that you have verified an invariant the compiler cannot, which is why the convention is to write that invariant down as a SAFETY comment.
An unsafe block permits a small set of operations whose safety conditions cannot be fully checked by the compiler. It does not relax ordinary borrowing rules for safe references or make a wrong pointer valid. The lesson's get_unchecked(0) example is safe only because the slice is known to contain index zero at that exact call site. The SAFETY comment should connect the preceding check to the API's precondition. If a future edit changes that control flow, the proof must be reviewed again.
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