Memory model - The Rust Reference

# Memory model

A Rust program's memory consists of a static set of items and a heap. Immutable portions of the heap may be safely shared between threads, mutable portions may not be safely shared, but several mechanisms for effectively-safe sharing of mutable values, built on unsafe code but enforcing a safe locking discipline, exist in the standard library.

Allocations in the stack consist of variables, and allocations in the heap consist of boxes.