nomicon/src/what-unsafe-does.md

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) null, dangling, or unaligned pointers, or fat pointers with invalid metadata (see below)
• Reading uninitialized memory
• Breaking the pointer aliasing rules
• Producing invalid primitive 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 undefined enum discriminant
  • null fn pointers
  • a char outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
  • a ! (all values are invalid for this type)
  • dangling/null/unaligned references, references that do themselves point to invalid values, or fat references (to a dynamically sized type) with invalid metadata
    • slice metadata is invalid if the slice has a total size larger than isize::MAX bytes in memory
    • dyn Trait metadata is invalid if it is not a pointer to a vtable for Trait that matches the actual dynamic trait the reference points to
  • a non-utf8 str
  • an uninitialized integer (i*/u*) or floating point value (f*)
  • an invalid library type with custom invalid values, such as a NonNull or NonZero* that is 0
• Unwinding into another language
• Causing a data race
• Executing code compiled with platform features that the current platform does not support (see target_feature)

"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 not all of the bytes it points to are part of the same allocation. The span of bytes it points to is determined by the pointer value and the size of the pointee type.

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.