nomicon/safe-unsafe-meaning.md

% What do Safe and Unsafe really mean?

Rust cares about preventing the following things:

• Dereferencing null or dangling pointers
• Reading uninitialized memory
• Breaking the pointer aliasing rules
• Producing invalid primitive values:
  • dangling/null references
  • a bool that isn't 0 or 1
  • an undefined enum discriminant
  • a char larger than char::MAX (TODO: check if stronger restrictions apply)
  • A non-utf8 str
• Unwinding into another language
• Causing a data race
• Invoking Misc. Undefined Behaviour (in e.g. compiler intrinsics)

That's it. That's all the Undefined Behaviour in Rust. Libraries are free to declare arbitrary requirements if they could transitively cause memory safety issues, but it all boils down to the above actions. Rust is otherwise quite permisive with respect to other dubious operations. Rust considers it "safe" to:

• Deadlock
• Have a Race Condition
• Leak memory
• Fail to call destructors
• Overflow integers
• Delete the production database

However any program that does such a thing is probably incorrect. Rust provides lots of tools to make doing these things rare, but these problems are considered impractical to categorically prevent.

Rust models the seperation between Safe and Unsafe with the unsafe keyword. 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.

• 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 destructors for details.

Some examples of unsafe functions:

• slice::get_unchecked will perform unchecked indexing, allowing memory safety to be freely violated.
• ptr::offset is an intrinsic that invokes Undefined Behaviour if it is not "in bounds" as defined by LLVM (see the lifetimes section for details).
• 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 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 lack of trust that Unsafe has for Safe. All safe traits are free to declare arbitrary contracts, but because implementing them is a job for Safe, Unsafe can't trust those contracts to actually be upheld.

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 really 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 ok. 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 memory 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 (most collections are). That means that it is not necessarily trivially true that a bad Ord implementation will make BTreeMap behave safely. Unsafe must be sure not to rely on Ord where safety is at stake, because Ord is provided by Safe, and memory safety is not Safe's responsibility to uphold. It must be impossible for Safe code to violate memory safety.

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 can trust the implementation to be correct (because Unsafe can trust themself).

Rust has traditionally avoided making traits unsafe because it makes Unsafe pervasive, which is not desirable. Send and Sync are unsafe is because thread safety is a fundamental property that Unsafe 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 operation atomic isn't even sufficient, because it's possible for complex invariants between disjoint locations in memory.

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 send 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).