What's the relationship between Safe Rust and Unsafe Rust? How do they interact?

The separation between Safe Rust and Unsafe Rust is controlled with the unsafe keyword, which acts as an interface from one to the other. This is why we can say Safe Rust is a safe language: all the unsafe parts are kept exclusively behind the unsafe boundary. If you wish, you can even toss #![forbid(unsafe_code)] into your code base to statically guarantee that you're only writing Safe Rust.

The unsafe keyword has two uses: to declare the existence of contracts the compiler can't check, and to declare that a programmer has checked that these contracts have been upheld.

You can use unsafe to indicate the existence of unchecked contracts on functions and trait declarations. On functions, unsafe means that users of the function must check that function's documentation to ensure they are using it in a way that maintains the contracts the function requires. On trait declarations, unsafe means that implementors of the trait must check the trait documentation to ensure their implementation maintains the contracts the trait requires.

You can use unsafe on a block to declare that all unsafe actions performed within are verified to uphold the contracts of those operations. For instance, the index passed to slice::get_unchecked is in-bounds.

You can use unsafe on a trait implementation to declare that the implementation upholds the trait's contract. For instance, that a type implementing Send is really safe to move to another thread.

The standard library has a number of unsafe functions, including:

• slice::get_unchecked, which performs unchecked indexing, allowing memory safety to be freely violated.
• mem::transmute reinterprets some value as having a given type, bypassing type safety in arbitrary ways (see conversions for details).
• Every raw pointer to a sized type has an offset method that invokes Undefined Behavior if the passed offset is not "in bounds".
• All FFI (Foreign Function Interface) functions are unsafe to call because the other language can do arbitrary operations that the Rust compiler can't check.

As of Rust 1.29.2 the standard library defines the following unsafe traits (there are others, but they are not stabilized yet and some of them may never be):

• Send is a marker trait (a trait with no API) that promises implementors are safe to send (move) to another thread.
• Sync is a marker trait that promises threads can safely share implementors through a shared reference.
• GlobalAlloc allows customizing the memory allocator of the whole program.

Much of the Rust standard library also uses Unsafe Rust internally. These implementations have generally been rigorously manually checked, so the Safe Rust interfaces built on top of these implementations can be assumed to be safe.

The need for all of this separation boils down a single fundamental property of Safe Rust:

No matter what, Safe Rust can't cause Undefined Behavior.

The design of the safe/unsafe split means that there is an asymmetric trust relationship between Safe and Unsafe Rust. Safe Rust inherently has to trust that any Unsafe Rust it touches has been written correctly. On the other hand, Unsafe Rust has to be very careful about trusting Safe Rust.

As an example, Rust has the PartialOrd and Ord traits to differentiate between types which can "just" be compared, and those that provide a "total" ordering (which basically means that comparison behaves reasonably).

BTreeMap doesn't really make sense for partially-ordered types, and so it requires that its keys implement Ord. However, BTreeMap has Unsafe Rust code inside of its implementation. Because it would be unacceptable for a sloppy Ord implementation (which is Safe to write) to cause Undefined Behavior, the Unsafe code in BTreeMap must be written to be robust against Ord implementations which aren't actually total — even though that's the whole point of requiring Ord.

The Unsafe Rust code just can't trust the Safe Rust code to be written correctly. That said, BTreeMap will still behave completely erratically if you feed in values that don't have a total ordering. It just won't ever cause Undefined Behavior.

One may wonder, if BTreeMap cannot trust Ord because it's Safe, why can it trust any Safe code? For instance BTreeMap relies on integers and slices to be implemented correctly. Those are safe too, right?

The difference is one of scope. When BTreeMap relies on integers and slices, it's relying on one very specific implementation. This is a measured risk that can be weighed against the benefit. In this case there's basically zero risk; if integers and slices are broken, everyone is broken. Also, they're maintained by the same people who maintain BTreeMap, so it's easy to keep tabs on them.

On the other hand, BTreeMap's key type is generic. Trusting its Ord implementation means trusting every Ord implementation in the past, present, and future. Here the risk is high: someone somewhere is going to make a mistake and mess up their Ord implementation, or even just straight up lie about providing a total ordering because "it seems to work". When that happens, BTreeMap needs to be prepared.

The same logic applies to trusting a closure that's passed to you to behave correctly.

This problem of unbounded generic trust is the problem that unsafe traits exist to resolve. The BTreeMap type could theoretically require that keys implement a new trait called UnsafeOrd, rather than Ord, that might look like this:

# #![allow(unused_variables)]
#fn main() {
use std::cmp::Ordering;

unsafe trait UnsafeOrd {
    fn cmp(&self, other: &Self) -> Ordering;
}
#}

Then, a type would use unsafe to implement UnsafeOrd, indicating that they've ensured their implementation maintains whatever contracts the trait expects. In this situation, the Unsafe Rust in the internals of BTreeMap would be justified in trusting that the key type's UnsafeOrd implementation is correct. If it isn't, it's the fault of the unsafe trait implementation, which is consistent with Rust's safety guarantees.

The decision of whether to mark a trait unsafe is an API design choice. Rust has traditionally avoided doing this because it makes Unsafe Rust pervasive, which isn't desirable. Send and Sync are marked unsafe because thread safety is a fundamental property that unsafe code can't possibly hope to defend against in the way it could defend against a buggy Ord implementation. Similarly, GlobalAllocator is keeping accounts of all the memory in the program and other things like Box or Vec build on top of it. If it does something weird (giving the same chunk of memory to another request when it is still in use), there's no chance to detect that and do anything about it.

The decision of whether to mark your own traits unsafe depends on the same sort of consideration. If unsafe code can't reasonably expect to defend against a broken implementation of the trait, then marking the trait unsafe is a reasonable choice.

As an aside, while Send and Sync are unsafe traits, they are also automatically implemented for types when such derivations are provably safe to do. Send is automatically derived for all types composed only of values whose types also implement Send. Sync is automatically derived for all types composed only of values whose types also implement Sync. This minimizes the pervasive unsafety of making these two traits unsafe. And not many people are going to implement memory allocators (or use them directly, for that matter).

This is the balance between Safe and Unsafe Rust. The separation is designed to make using Safe Rust as ergonomic as possible, but requires extra effort and care when writing Unsafe Rust. The rest of this book is largely a discussion of the sort of care that must be taken, and what contracts Unsafe Rust must uphold.