nomicon/safe-unsafe-meaning.md

% How Safe and Unsafe Interact

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

Rust models the separation between Safe and Unsafe Rust with the unsafe keyword, which can be thought as a sort of foreign function interface (FFI) between Safe and Unsafe Rust. This is the magic behind why we can say Safe Rust is a safe language: all the scary unsafe bits are relegated exclusively to FFI just like every other safe language.

However because one language is a subset of the other, the two can be cleanly intermixed as long as the boundary between Safe and Unsafe Rust is denoted with the unsafe keyword. No need to write headers, initialize runtimes, or any of that other FFI boiler-plate.

There are several places unsafe can appear in Rust today, which can largely be grouped into two categories:

• There are unchecked contracts here. To declare you understand this, I require you to write unsafe elsewhere:

  • On functions, unsafe is declaring the function to be unsafe to call. Users of the function must check the documentation to determine what this means, and then have to write unsafe somewhere to identify that they're aware of the danger.
  • On trait declarations, unsafe is declaring that implementing the trait is an unsafe operation, as it has contracts that other unsafe code is free to trust blindly. (More on this below.)

• I am declaring that I have, to the best of my knowledge, adhered to the unchecked contracts:

  • On trait implementations, unsafe is declaring that the contract of the unsafe trait has been upheld.
  • On blocks, unsafe is declaring any unsafety from an unsafe operation within to be handled, and therefore the parent function is safe.

There is also #[unsafe_no_drop_flag], which is a special case that exists for historical reasons and is in the process of being phased out. See the section on drop flags for details.

Some examples of unsafe functions:

• slice::get_unchecked will perform unchecked indexing, allowing memory safety to be freely violated.
• every raw pointer to sized type has intrinsic offset method that invokes Undefined Behaviour if it is not "in bounds" as defined by LLVM.
• mem::transmute reinterprets some value as having the given type, bypassing type safety in arbitrary ways. (see conversions for details)
• All FFI functions are unsafe because they can do arbitrary things. C being an obvious culprit, but generally any language can do something that Rust isn't happy about.

As of Rust 1.0 there are exactly two unsafe traits:

• Send is a marker trait (it has no actual API) that promises implementors are safe to send (move) to another thread.
• Sync is a marker trait that promises that threads can safely share implementors through a shared reference.

The need for unsafe traits boils down to the fundamental property of safe code:

No matter how completely awful Safe code is, it can't cause Undefined Behavior.

This means that Unsafe Rust, the royal vanguard of Undefined Behavior, has to be super paranoid about generic safe code. To be clear, Unsafe Rust is totally free to trust specific safe code. Anything else would degenerate into infinite spirals of paranoid despair. In particular it's generally regarded as ok to trust the standard library to be correct. std is effectively an extension of the language, and you really just have to trust the language. If std fails to uphold the guarantees it declares, then it's basically a language bug.

That said, it would be best to minimize needlessly relying on properties of concrete safe code. Bugs happen! Of course, I must reinforce that this is only a concern for Unsafe code. Safe code can blindly trust anyone and everyone as far as basic memory-safety is concerned.

On the other hand, safe traits are free to declare arbitrary contracts, but because implementing them is safe, unsafe code can't trust those contracts to actually be upheld. This is different from the concrete case because anyone can randomly implement the interface. There is something fundamentally different about trusting a particular piece of code to be correct, and trusting all the code that will ever be written to be correct.

For instance Rust has PartialOrd and Ord traits to try to differentiate between types which can "just" be compared, and those that actually implement a total ordering. Pretty much every API that wants to work with data that can be compared wants Ord data. For instance, a sorted map like BTreeMap doesn't even make sense for partially ordered types. If you claim to implement Ord for a type, but don't actually provide a proper total ordering, BTreeMap will get really confused and start making a total mess of itself. Data that is inserted may be impossible to find!

But that's okay. BTreeMap is safe, so it guarantees that even if you give it a completely garbage Ord implementation, it will still do something safe. You won't start reading uninitialized or unallocated memory. In fact, BTreeMap manages to not actually lose any of your data. When the map is dropped, all the destructors will be successfully called! Hooray!

However BTreeMap is implemented using a modest spoonful of Unsafe Rust (most collections are). That means that it's not necessarily trivially true that a bad Ord implementation will make BTreeMap behave safely. BTreeMap must be sure not to rely on Ord where safety is at stake. Ord is provided by safe code, and safety is not safe code's responsibility to uphold.

But wouldn't it be grand if there was some way for Unsafe to trust some trait contracts somewhere? This is the problem that unsafe traits tackle: by marking the trait itself as unsafe to implement, unsafe code can trust the implementation to uphold the trait's contract. Although the trait implementation may be incorrect in arbitrary other ways.

For instance, given a hypothetical UnsafeOrd trait, this is technically a valid implementation:

# use std::cmp::Ordering;
# struct MyType;
# unsafe trait UnsafeOrd { fn cmp(&self, other: &Self) -> Ordering; }
unsafe impl UnsafeOrd for MyType {
    fn cmp(&self, other: &Self) -> Ordering {
        Ordering::Equal
    }
}

But it's probably not the implementation you want.

Rust has traditionally avoided making traits unsafe because it makes Unsafe pervasive, which is not desirable. The reason Send and Sync are unsafe is because thread safety is a fundamental property that unsafe code cannot possibly hope to defend against in the same way it would defend against a bad Ord implementation. The only way to possibly defend against thread-unsafety would be to not use threading at all. Making every load and store atomic isn't even sufficient, because it's possible for complex invariants to exist between disjoint locations in memory. For instance, the pointer and capacity of a Vec must be in sync.

Even concurrent paradigms that are traditionally regarded as Totally Safe like message passing implicitly rely on some notion of thread safety -- are you really message-passing if you pass a pointer? Send and Sync therefore require some fundamental level of trust that Safe code can't provide, so they must be unsafe to implement. To help obviate the pervasive unsafety that this would introduce, Send (resp. Sync) is automatically derived for all types composed only of Send (resp. Sync) values. 99% of types are Send and Sync, and 99% of those never actually say it (the remaining 1% is overwhelmingly synchronization primitives).