Unsafe operations are those that potentially violate the memory-safety guarantees of Rust's static semantics.

The following language level features cannot be used in the safe subset of Rust:

• Dereferencing a raw pointer.
• Reading or writing a mutable static variable.
• Reading a field of a union, or writing to a field of a union that isn't Copy.
• Calling an unsafe function (including an intrinsic or foreign function).
• Implementing an unsafe trait.