[jeeyoungk]

It also depends on how "deep" you want to get.

A lot of built-in constructs uses unsafe under the hood. Vector (Rust's dynamic arrays) does memory allocation / resizing under the hood for example, and there's no safe way to do it, unless some safer array allocation primitive is exposed.

Also things like mem::swap(x, y) cannot be implemented at all with safe rust. in order to perform swap, you need a temporary variable. That temporary variable would be uninitialized, which Rust does not allow.

Note that in c++ it invokes copy constructor - http://www.cplusplus.com/reference/algorithm/swap/ - but Rust's mem::swap works for types that does not implement the Copy trait (Rust's equivalent to copy constructor).

Same can be said about slice splitting functions, which is primarily used to work around Rust's borrow checker.