“Pinning, or holding a value’s memory address in a fixed location,is one of the more challenging concepts in Rust.”

“Normally only seen within async code, i.e. poll(self: Pin<&mut Self>), pinning has wider applicability.”

Some data structures that are difficult or impossible to write without the unsafe keyword, including self-referential structs and intrusive data structures.

FFI with C++ is a prominent use case that’s related to this. Rust must assume that any C++ with a reference might be a self-referential data structure.

“To understand this conflict in more detail, we’ll first need to make sure that we have a strong understanding of Rust’s move semantics.”