What Unsafe Rust Can Do

The only things that are different in Unsafe Rust are that you can:

  • Dereference raw pointers
  • Call unsafe functions (including C functions, compiler intrinsics, and the raw allocator)
  • Implement unsafe traits
  • Mutate statics
  • Access fields of unions

That's it. The reason these operations are relegated to Unsafe is that misusing any of these things will cause the ever dreaded Undefined Behavior. Invoking Undefined Behavior gives the compiler full rights to do arbitrarily bad things to your program. You definitely should not invoke Undefined Behavior.

Unlike C, Undefined Behavior is pretty limited in scope in Rust. All the core language cares about is preventing the following things:

  • Dereferencing (using the * operator on) dangling or unaligned pointers (see below)
  • Breaking the pointer aliasing rules
  • Calling a function with the wrong call ABI or unwinding from a function with the wrong unwind ABI.
  • Causing a data race
  • Executing code compiled with target features that the current thread of execution does not support
  • Producing invalid values (either alone or as a field of a compound type such as enum/struct/array/tuple):
    • a bool that isn't 0 or 1
    • an enum with an invalid discriminant
    • a null fn pointer
    • a char outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
    • a ! (all values are invalid for this type)
    • an integer (i*/u*), floating point value (f*), or raw pointer read from uninitialized memory, or uninitialized memory in a str.
    • a reference/Box that is dangling, unaligned, or points to an invalid value.
    • a wide reference, Box, or raw pointer that has invalid metadata:
      • dyn Trait metadata is invalid if it is not a pointer to a vtable for Trait that matches the actual dynamic trait the pointer or reference points to
      • slice metadata is invalid if the length is not a valid usize (i.e., it must not be read from uninitialized memory)
    • a type with custom invalid values that is one of those values, such as a NonNull that is null. (Requesting custom invalid values is an unstable feature, but some stable libstd types, like NonNull, make use of it.)

"Producing" a value happens any time a value is assigned, passed to a function/primitive operation or returned from a function/primitive operation.

A reference/pointer is "dangling" if it is null or not all of the bytes it points to are part of the same allocation (so in particular they all have to be part of some allocation). The span of bytes it points to is determined by the pointer value and the size of the pointee type. As a consequence, if the span is empty, "dangling" is the same as "null". Note that slices and strings point to their entire range, so it's important that the length metadata is never too large (in particular, allocations and therefore slices and strings cannot be bigger than isize::MAX bytes). If for some reason this is too cumbersome, consider using raw pointers.

That's it. That's all the causes of Undefined Behavior baked into Rust. Of course, unsafe functions and traits are free to declare arbitrary other constraints that a program must maintain to avoid Undefined Behavior. For instance, the allocator APIs declare that deallocating unallocated memory is Undefined Behavior.

However, violations of these constraints generally will just transitively lead to one of the above problems. Some additional constraints may also derive from compiler intrinsics that make special assumptions about how code can be optimized. For instance, Vec and Box make use of intrinsics that require their pointers to be non-null at all times.

Rust is otherwise quite permissive with respect to other dubious operations. Rust considers it "safe" to:

  • Deadlock
  • Have a race condition
  • Leak memory
  • Fail to call destructors
  • Overflow integers
  • Abort the program
  • Delete the production database

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