What Rust provides is a way to build safe abstractions over unsafe code. Rust's type system (including ownership and borrowing, Sync/Send, etc), along with it'

What Rust provides is a way to build safe abstractions over unsafe code. Rust's type system (including ownership and borrowing, Sync/Send, etc), along with it's privacy features (allowing types to have private fields that can only be accessed by code in the module that defined them) allows you to create fully safe interfaces around code that uses unsafe; there is provably no combination of uses of the interface which lead to undefined behavior. Now, yeah, it's possible to also use unsafe in Rust just for applying a local optimisation. And that has fewer benefits than a fully encapsulated safe interface, though is still easier to audit for potential UB than C. So you're right that it's on a continuum, but the distinction between safe and unsafe code means you can more easily find the specific places where UB could occur, and the encapsulation and type system makes it possible to create safe abstractions over unsafe code.