From Unsafe To Safe | Comprehensive Understanding of Unsafe Rust

Introduction#

As is well known, the Rust language consists of two major parts: Safe Rust and Unsafe Rust. Unsafe Rust is a superset of Safe Rust. This means that all code written in Safe Rust can also work normally in Unsafe Rust, but Unsafe Rust provides additional features and operations that cannot be used directly in Safe Rust.

However, one of the most frequently asked questions in the community is: Why is Rust considered a safe language when a large amount of Unsafe Rust is used in the standard library?

Moreover, some people in the community even develop PTSD towards Unsafe Rust, deeming anything involving Unsafe as unsafe. Admittedly, we should minimize the use of Unsafe, but when faced with situations where Unsafe is indispensable, we should know how to use Unsafe safely, know how to review Unsafe code, and thus not be so afraid and resistant when seeing Unsafe.

So, the goal of this article is to guide readers through a systematic understanding of Unsafe Rust, truly grasping the ingenious uses of Unsafe Rust.

Table of Contents:

Why Unsafe Rust is needed

What Unsafe Rust can do

The safety philosophy of Unsafe Rust

Safe abstractions: maintaining safety invariants and validity invariants

Case studies from the standard library

Examples from FFI

Guidelines for Unsafe Rust programming

Hot discussion: Unsafe Rust vs Zig

Introduction to the Zig language

Is Zig really safer than Unsafe Rust?

Summary

Why Unsafe Rust is Needed#

In general, Safe Rust is abstracted from Unsafe Rust. The world itself is Unsafe, so Unsafe comes first, followed by Safe. This is the basic worldview of the Rust language.

This worldview is actually very consistent with the objective world and is very easy for people to understand. The universe is Unsafe, so we need Safe spaceships and spacesuits to explore it; the Earth is Unsafe, so we humans use science and civilization to continuously create Safe homes.

From a language design perspective, the reasons for needing Unsafe Rust are as follows:

Providing low-level support for high-level abstractions: Rust provides some high-level abstractions, such as reference counting, smart pointers, synchronization primitives, etc. These abstractions require Unsafe operations at the lower level, such as raw pointer operations and memory management. Through Unsafe Rust, library developers can implement these low-level operations and encapsulate them in safe interfaces.

High-performance optimization: To ensure memory safety, the Rust compiler automatically inserts some runtime checks, such as array boundary checks. In some performance-critical scenarios, these checks can become performance bottlenecks. Unsafe Rust allows developers to bypass these checks while ensuring safety, to achieve higher performance.

System-level programming: As a systems programming language, Rust needs to handle low-level operations, such as operating systems or embedded systems, or driver development. Unsafe Rust provides this flexibility, making Rust suitable for these tasks.

Interoperability with other languages: In real-world projects, Rust code may need to interact with code written in other languages (such as C/C++). Since these languages may use different memory management and type systems, Rust needs to provide a way to safely communicate with this external code. Unsafe Rust provides such a mechanism, allowing the handling of raw pointers and type conversions in Rust, thus enabling interoperability with other languages.

Language extensibility: Unsafe Rust provides possibilities for future language features and libraries. By allowing low-level operations, Unsafe Rust enables the Rust community to continually explore new ideas and integrate them into the language.

In summary, from the perspective of Rust language design, Unsafe Rust is an intentional design compromise. It meets the needs of low-level operations, performance optimization, and interoperability while ensuring overall safety through strict restrictions and encapsulation.

What Unsafe Rust Can Do#

As mentioned earlier, Unsafe Rust is a superset of Safe Rust. Therefore, Unsafe Rust also includes all compiler safety checks in Safe Rust. However, Unsafe Rust also includes operations not found in Safe Rust, i.e., operations that can only be executed in Unsafe Rust:

Raw pointer operations: You can create, dereference, and manipulate raw pointers (*const T and *mut T). This allows you to directly access memory addresses, perform memory allocation, deallocation, and modification, etc.

Calling Unsafe functions: Unsafe Rust can call functions marked as unsafe. These functions may lead to undefined behavior, so they need to be called within an unsafe code block. These functions are typically used to implement low-level operations, such as memory management, hardware access, etc.

Implementing Unsafe traits: You can implement traits marked as unsafe. These traits may contain potentially risky operations, and they need to be explicitly marked as unsafe when implemented.

Accessing and modifying mutable static variables: In Unsafe Rust, you can access and modify mutable static variables with a global lifetime. These variables remain active throughout the entire program execution and may lead to potential data race issues.

Working with Union types. Since multiple fields share the same memory location, using union carries certain risks. When accessing a union field, the compiler cannot guarantee type safety because it cannot determine which field the currently stored value belongs to. To ensure safe access to union fields, you need to perform operations within an unsafe code block.

Disabling runtime boundary checks: Unsafe Rust allows you to bypass array boundary checks. By using get_unchecked and get_unchecked_mut methods, you can access array and slice elements without performing boundary checks, thereby improving performance.

Inline assembly: In Unsafe Rust, you can use inline assembly (the asm! macro) to write processor instructions directly. This allows you to implement platform-specific optimizations and operations.

Foreign Function Interface (FFI): Unsafe Rust allows you to interact with code written in other programming languages (such as C/C++). This usually involves native pointer operations, type conversions, and calling unsafe functions.

It is important to note that caution is needed when using Unsafe Rust. Whenever possible, prefer using Safe Rust to write code. Although it provides powerful capabilities, it can also lead to undefined behavior and memory safety issues. Therefore, Rust's official standard library source code implementation and the official Unsafe Code Guideline both contain the safety philosophy of Unsafe Rust to maintain its security.

Safety Philosophy of Unsafe Rust#

The safety philosophy of Unsafe Rust is allowing developers to perform low-level operations and performance optimizations under restricted conditions while ensuring that the overall code remains safe.

Safety Abstraction: Maintaining Safety Invariants and Validity Invariants#

What does it mean for the overall code to remain safe? Unsafe Rust has a term called Safety Invariant specifically for defining this.

Safe Rust has compiler safety checks to ensure memory safety and concurrency safety, but for those specialized operation scenarios of Unsafe Rust, the Rust compiler cannot help, so developers themselves need to ensure the memory safety and concurrency safety of the code. Unsafe Rust is the developer's safety commitment to the Rust compiler: "Leave safety to me to guard!"

In order to abide by this safety commitment when writing Unsafe Rust code, developers must always maintain safety invariants. Safety invariants refer to the conditions that must be maintained throughout the entire program execution process to ensure memory safety. These conditions usually include pointer validity, data structure integrity, data access synchronization, etc. Safety invariants primarily focus on the correct execution of the program and the avoidance of undefined behavior. When using Unsafe Rust, developers are responsible for ensuring that these safety invariants are met to avoid memory safety issues.

In the process of maintaining safety invariants, another concept to understand is the Validity Invariant. Validity invariants refer to the conditions that certain data types and structures must meet during their lifetime. Validity invariants mainly focus on the correctness of data types and structures. For example, for reference types, validity invariants include non-null pointers and valid memory being pointed to. When writing and using Unsafe Rust code, developers need to ensure that these validity invariants are maintained.

There is a certain degree of association between safety invariants and validity invariants because they both focus on the correctness and safety of the code. Maintaining validity invariants often helps ensure that safety invariants are met. For example, ensuring the validity of reference type pointers (validity invariant) can prevent null pointer dereferencing (safety invariant). Although they are related, the areas of focus for safety invariants and validity invariants are different. Safety invariants mainly focus on the memory safety of the entire program and the avoidance of undefined behavior, while validity invariants mainly focus on the correctness of specific data types and structures.

The relationship between them can be summarized in the following aspects:

Purpose: Safety invariants mainly focus on memory safety and data integrity to prevent undefined behavior. Validity invariants focus on the conditions that type instances must meet during their lifetime for correct use.

Scope: Safety invariants typically involve the memory safety of the entire program or module, while validity invariants are specific to type constraints. To some extent, safety invariants can be regarded as global restrictions, while validity invariants are local restrictions.

Hierarchy: Safety invariants and validity invariants can interact at different levels. Generally, maintaining validity invariants is the foundation for implementing safety invariants. In other words, the validity invariants of a type are often required for implementing higher-level safety invariants.

Dependency: Safety invariants depend on validity invariants. When the validity invariants of a type are satisfied, it helps ensure the conditions required for implementing safety invariants. For example, in Rust, safe reference access depends on the underlying type's validity invariants being satisfied.

Therefore, when the Unsafe Rust code violates safety invariants or validity invariants, we say the code is unsound. Unsound code may lead to undefined behavior, memory leaks, data races, and other issues. Rust attempts to avoid unsound situations by ensuring memory safety for most code through the compiler and type system, especially avoiding undefined behavior.

Read More: Two Kinds of Invariants: Safety and Validity

Undefined behavior refers to situations where the program's execution result is unpredictable. This may be due to out-of-bounds memory access, null pointer dereferencing, data races, and other errors. When encountering undefined behavior, the program may crash, produce incorrect results, or exhibit other unexpected behavior.

However, some people argue that using the term "undefined behavior" in newer languages like Zig and Rust may not be precise.

In Rust, pay special attention to situations that may lead to undefined behavior:

Data races: Data races occur when multiple threads access the same memory location simultaneously, and at least one thread is performing a write operation. Rust's ownership system and borrow checker prevent most data races at compile time, but in Unsafe Rust, developers need to be extra careful to avoid data races.

Invalid pointer dereferencing: Dereferencing an invalid pointer (e.g., a null pointer, a pointer to freed memory) leads to undefined behavior.

Integer overflow: In some cases, integer overflow (e.g., integer addition, subtraction, multiplication, etc.) may cause unexpected program situations. Rust enables integer overflow checks by default in debug mode, but in Release mode, integer overflow behavior is defined, see RFC 0560.

Accessing uninitialized memory: Accessing uninitialized memory leads to undefined behavior. This includes reading or writing uninitialized memory or passing uninitialized memory to external functions.

Incorrect type casting: Forcing a pointer of one type to be converted to a pointer of another type and then dereferencing it may lead to undefined behavior. This usually occurs in Unsafe Rust code and requires special attention to ensure type conversion is safe.

In summary, when using Unsafe Rust, developers need to pay special attention to maintaining safety invariants and validity invariants to avoid Unsound and undefined behavior. We call Unsafe Rust code that strictly follows these principles Unsafe safety abstractions. They can be considered Safe, allowing for low-level operations and performance optimization while maintaining overall safety.

Examples from the standard library#

Vec<T>#

The push method in the Vec<T> type in the standard library is a typical example of an Unsafe safety abstraction. The following code is a simplified implementation of this method:

Copy
pub struct Vec<T> {
ptr: *mut T,
len: usize,
cap: usize,
}

impl<T> Vec<T> {
pub fn push(&mut self, value: T) {
if self.len == self.cap {
// Reallocate memory (detailed implementation omitted here)
self.reallocate();
}

// Ensure safety invariants and validity invariants are satisfied here
unsafe {
let end = self.ptr.add(self.len); // end is a valid pointer because len < cap
std::ptr::write(end, value); // Write value to memory pointed to by end, assuming end is valid
self.len += 1; // Update the length
}
}
}

In this simplified version of the Vec<T> implementation, the push method uses Unsafe Rust. Now let's analyze how safety invariants and validity invariants are satisfied:

Safety invariants:

Memory allocation and deallocation: The push method calls the reallocate method (detailed implementation omitted here) when the length equals the capacity, ensuring sufficient memory is allocated. This ensures that there will be no out-of-bounds memory access.

Data access synchronization: In this example, the push method is a mutable reference (&mut self), ensuring that there are no other mutable or immutable references when called. This ensures that there is no data race.

Validity invariants:

Pointer validity: The end pointer is calculated using self.ptr.add(self.len). Since self.len < self.cap (after reallocate), we can ensure that the memory address pointed to by end is valid.

Data type correctness: std::ptr::write(end, value) writes value to the memory pointed to by end. Since we have already ensured the validity of end, this operation is safe and ensures data type correctness.

By maintaining safety invariants and validity invariants, the Vec<T> type in the Rust standard library can provide high-performance operations while ensuring memory safety. In this simplified version of the push method, Unsafe Rust is used to perform low-level memory operations, but throughout the process, developers ensure that safety invariants and validity invariants are satisfied.

In conclusion, the Rust standard library uses Unsafe Rust for low-level operations and optimizations while ensuring safety invariants and validity invariants are satisfied, achieving safe and high-performance abstractions. This approach is also applied in many other standard library types and methods, such as String, HashMap, etc. This allows developers to take advantage of low-level operations and performance optimizations while writing safe, high-level code.

String#

In the standard library's String type, there is a pair of similar methods: from_utf8 and from_utf8_unchecked. The difference is that the former is a Safe function, while the latter is an Unsafe function.

Copy
pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
// This is not the source code of the standard library but a demonstration
let vec = match run_utf8_validation(v) {
Ok(_) => {
// SAFETY: validation succeeded.
Ok(unsafe { from_utf8_unchecked(v) })
}
Err(err) => Err(err),
};
Ok(String { vec })
}

In the from_utf8 code, the input byte sequence is checked for UTF8 encoding to ensure the data validity of the vec argument. Therefore, the entire function will not exhibit Unsound behavior when handling arbitrary byte sequences, especially without undefined behavior. At this point, we can consider it to maintain safety invariants. Then the function is safe.

from_utf8_unchecked source code example:

Copy
/// # Safety
///
/// This function is unsafe because it does not check that the bytes passed
/// to it are valid UTF-8. If this constraint is violated, it may cause
/// memory unsafety issues with future users of the `String`, as the rest of
/// the standard library assumes that `String`s are valid UTF-8.
pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
String { vec: bytes }
}

In this function, the input byte sequence is not checked for UTF8 encoding but is directly constructed as a String, which is risky. Since the data validity of the bytes argument cannot be guaranteed, the entire function is unsafe, so it needs to be marked as unsafe fn. A Safety documentation comment should also be added to inform developers using this function under what circumstances it is safe to use, that is, to let the caller maintain the safety invariants.

Some readers might wonder, why do we need a from_utf8_unchecked function when there is already a from_utf8 function?

This is a convention in Unsafe Rust practice, providing an unsafe function with an _unchecked name suffix as a performance outlet. For example, in some environments, the bytes argument of the from_utf8_unchecked function is validated for UTF8 encoding externally (e.g., in C interfaces), so there is no need to validate it twice here. For performance reasons, the _unchecked method can be used.

Example from FFI#

One common scenario for Unsafe Rust is to interact with various other languages through C-ABI, i.e., FFI (Foreign Function Interface) scenarios. In this situation, we need to consider many safety factors to achieve Unsafe safety abstraction. Let's demonstrate how to achieve safety abstraction in FFI with an example.

Suppose we have a C language library (my_c_lib.c):

Copy
#include <stdint.h>

int32_t add(int32_t a, int32_t b) {
return a + b;
}

We need to write a Rust program to call this add function. First, we need to create an FFI binding in Rust:

Copy
// my_c_lib.rs
extern "C" {
fn add(a: i32, b: i32) -> i32;
}

Now, we can create a safe Rust function that wraps this Unsafe FFI binding:

Copy
// my_c_lib.rs
pub fn safe_add(a: i32, b: i32) -> i32 {
// The developer is responsible for maintaining safety invariants and validity invariants
// If a or b are not valid i32, return 0 (depending on the specific business choice)
if a >= i32::MAX || b >= i32::MAX || (i32::MAX - a - b) < 0 {return 0}
unsafe {
add(a, b);
}
}

In this example, we wrapped the Unsafe FFI binding add function in a safe safe_add function. This way, when other Rust code calls safe_add, they don't have to worry about potential safety issues (e.g., an integer overflow). Unsafe Rust code is limited to the internals of the safe_add function, and the developer is responsible for ensuring that safety invariants and validity invariants are satisfied.

Now we can use the safe_add function safely in Rust:

Copy
fn main() {
let result = my_c_lib::safe_add(5, 7);
println!("5 + 7 = {}", result);
}

In this FFI scenario, Unsafe Rust is used to call the add function implemented in C. By encapsulating Unsafe code in a safe API, we ensure the safety of Rust code when calling this API. In practice, developers need to pay attention to potential safety invariants and validity invariants issues and ensure they are satisfied when encapsulating Unsafe Rust.

Unsafe Rust Programming Guidelines#

Following these guidelines in Unsafe Rust practices can effectively achieve Unsafe safety abstraction:

Minimize unsafe code: The use of Unsafe Rust should be limited and used only when necessary. Most features should be implemented using Safe Rust to ensure memory safety and avoid undefined behavior. When possible, Unsafe Rust code should be encapsulated in a safe API to provide users with a safe interface.

Explicitly unsafe: Unsafe Rust must be explicitly marked as unsafe so that developers can clearly identify potential safety risks. When encountering the unsafe keyword, developers should pay special attention and carefully review the code to ensure proper handling of potential memory safety issues and undefined behavior. Developers need to carefully consider whether a function or trait should be marked unsafe, and should not intentionally remove or not use the unsafe mark where it should be marked as unsafe.

Review and testing: Unsafe Rust code requires stricter review and testing to ensure its correctness. This includes testing low-level operations, memory allocation and release, and concurrent behavior. Ensuring the correctness of Unsafe Rust code is crucial, as errors can lead to serious memory safety issues and undefined behavior.

Documentation and comments: Unsafe Rust code should be well documented and commented on so that other developers can understand its purpose, operation, and potential risks. This helps maintainers and other contributors follow correct safety practices when modifying the code.

Reduce complexity: Unsafe Rust code should be as simple and straightforward as possible to facilitate understanding and maintenance. Complex Unsafe Rust code may lead to hard-to-find errors, increasing potential safety risks.

By following these principles, Unsafe Rust's safety philosophy aims to balance the need for low-level operations and performance optimization while ensuring that the overall code remains safe. Although the use of Unsafe Rust may lead to potential safety issues, the Rust language attempts to minimize these risks through explicit marking, limiting the scope of use, strict review, and thorough documentation.

Hotspot Discussion: Unsafe Rust vs Zig#

Finally, I'd like to discuss which language is safer when writing Unsafe code: Unsafe Rust or Zig.

The origin of this topic is a recent hotly debated post on the Reddit rust channel: When Zig is safer and faster than Rust. Although the title says Rust, it's actually comparing with Unsafe Rust.

Zig Language Introduction#

Zig is a modern, high-performance systems programming language aimed at simplifying some of the complexities of C while providing greater safety and ease of use. Zig was initiated by Andrew Kelley in 2016 and has received support from an active open-source community. Zig is suitable for various scenarios, such as operating system development, embedded systems, game development, high-performance computing, etc. Although the Zig language is relatively new, it has attracted the attention of many developers and has been applied in actual projects. With the development of the community and ecosystem, Zig is expected to become an important choice in the field of systems programming.

Feature | Zig | Rust
Design Goals | Simplicity, high-performance, ease of use, C compatibility | Safety, concurrency, high-performance, memory safety
Syntax | Closer to C language, simpler | Closer to ML series languages, expressive type system
Memory Safety | Compile-time checks, no borrow checker and ownership system | Borrow checker and ownership system, compile-time guaranteed memory safety
Performance | High performance, close to C language | High performance, comparable to C++
Line-by-Line Compilation | Supports line-by-line compilation, allows compile-time execution of code | Supports const generics, limited compile-time execution capabilities
Error Handling | Error return values and error union types, no exceptions | Result and Option types, no exceptions
FFI | Excellent C language compatibility, easy to interoperate with existing C code | Good FFI support, requires additional bindings creation
Package Management | Built-in package manager | Cargo package manager
Runtime | No runtime overhead | Minimal runtime, can run without std
Community and Ecosystem | Relatively new, community in development | Mature community and rich ecosystem

Please note that while this table summarizes the main differences, each language has its unique features in practice, so you may need to delve deeper into the characteristics of each language before making a choice.

Zig language is designed with a focus on memory safety, but unlike Rust, it does not have a ownership system and borrow checker. Nevertheless, Zig enhances memory safety through some compile-time checks and language features. Here are some ways Zig language implements memory safety:

Compile-time checks: The Zig compiler performs many checks during compilation to catch potential memory errors, such as array out-of-bounds access, null pointer dereference, etc. When the Zig compiler detects these errors, it stops the compilation and reports the error.

Error handling: Zig improves code robustness through explicit error handling. Zig does not have exceptions; instead, it uses error return values and error union types to handle errors. This forces developers to explicitly handle potential errors, helping to reduce memory safety issues caused by unhandled errors.

Optional types: Zig provides optional types (Optionals) to represent values that may be null. By using optional types, null value cases can be explicitly handled, reducing the risk of null pointer dereferences.

Defined behavior: Zig designs defined behavior for many memory-related operations to avoid security risks posed by undefined behavior. For example, when dereferencing a null pointer, Zig ensures that a clearly defined error occurs, rather than producing undefined behavior.

Memory management: Zig offers flexible memory management options, including manual memory management, built-in allocators, and the use of user-defined allocators. Through explicit memory management, developers can better control memory usage, reducing the risks of memory leaks and memory errors.

In summary, the Zig language, like C, entrusts memory management to humans, placing full trust in human development. Zig then provides some memory safety checks to ensure memory safety. However, it still lacks the strict compile-time guarantees of Rust's ownership system and borrow checker. Therefore, when writing Zig code, developers need to pay more attention to potential memory safety issues and ensure that errors and exceptional situations are handled correctly.

Is Zig really safer than Unsafe Rust?#

Compared to Safe Rust, the Zig language gives developers more freedom but is less safe than Safe Rust. However, is Zig safer than Unsafe Rust?

Let's go back to the Reddit article When Zig is safer and faster than Rust.

1. The author says that using Unsafe Rust is difficult and relies entirely on Miri's checks. This statement seems correct, but not entirely correct.

Miri is a MIR interpreter with many functions. One of them is UB checking in Unsafe Rust.

First, Unsafe Rust is indeed challenging. After understanding the content about Unsafe Rust safety abstractions mentioned earlier, this difficulty should be reduced by half for Rust developers. At least, the usage of Unsafe Rust is not as confusing, and there is a correct direction.

UB issues also exist in the Zig language, and Zig will also face the problems of Unsafe code. When writing Unsafe code in Zig, memory safety guarantees mainly depend on the developer's experience and coding practices. Although the Zig compiler provides some compile-time checks, in Unsafe code, these checks may not be enough to capture all potential memory errors. To ensure memory safety when writing Unsafe code, developers can follow these practices:

Reduce the use of Unsafe code: Try to minimize the use of Unsafe code without compromising performance and functionality. Limit Unsafe code to the smallest possible scope, making it easier to review and maintain.

Use the type system: Make the most of Zig's type system to represent different types of data and constraints. The type system can help developers capture potential errors at compile-time, reducing the risk of memory errors.

Explicit error handling: Ensure that potential errors are explicitly handled in Unsafe code, using error return values and error union types to represent possible error situations. This helps improve code robustness and reduce memory safety issues caused by unhandled errors.

Proper encapsulation and abstraction: For parts that require the use of Unsafe code, consider encapsulating them into safe abstractions, isolating Unsafe code. This ensures that other parts of the code do not touch potential memory errors when called.

Code review: Conduct a detailed code review for parts involving Unsafe code, ensuring that developers understand potential memory risks and take appropriate measures to prevent errors.

Testing: Write test cases for Unsafe code, ensuring that it works correctly under different scenarios. Testing can help discover potential memory errors and verify the effectiveness of fixes.

When writing Unsafe code in Zig, it is also necessary to perform safety abstractions like Unsafe Rust and pay attention to maintaining safety invariants and validity invariants.

2. The author provides two examples to illustrate the difficulty of using Unsafe Rust

Firstly, using *mut T and *const T already loses the compiler's safety guarantees, and even in C, using pointers requires developers to ensure safety themselves. In Zig, the usage of pointers is similar to that of C, so there is no particular safety advantage for Zig in this regard.

The author also states that in Unsafe Rust, using pointers leads to scattered code, such as (*ptr).field, since pointers cannot call methods. However, there is a better and safer solution in Unsafe Rust:

Copy
impl Foo {
/// # Safety
/// When calling this method, you have to ensure that _either_ the pointer is null _or_ all of the following is true:
/// - The pointer must be properly aligned.
/// - The pointer must point to an initialized instance of `T`.
unsafe fn as_ref<'a>(ptr: *const Foo) -> &'a Foo {
unsafe {
if let Some(foo) = ptr.as_ref() {
println!("We got back the value: {foo}!");
foo
}
}
}
/// # Safety
/// When calling this method, you have to ensure that _either_ the pointer is null _or_ all of the following is true:
/// - The pointer must be properly aligned.
/// - The pointer must point to an initialized instance of `T`.
unsafe fn as_mut<'a>(ptr: *mut Foo) -> &'a mut Foo {
unsafe {
if let Some(foo) = ptr.as_mut() {
println!("We got back the value: {foo}!");
foo
}
}
}
}

The standard library provides as_ref and as_mut methods for raw pointers to convert them into immutable and mutable references. By implementing these two methods for Foo, safety conditions can be fully considered, and it becomes more convenient to call methods on instances of Foo without the need for (*ptr).field-like code.

Regarding the author's point about arrays, I have provided an improved version of their example:

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#[derive(Debug)]
struct Value(i32);

unsafe fn do_stuff_with_array(values: *mut Value, len: usize) {
let values: &mut [Value] = std::slice::from_raw_parts_mut(values, len);
// I can use the ergonomics of iterators!
for val in values.iter_mut() {
// ...
// Perform actions on each `val` of type &mut Value
val.0 += 1;
}
}

fn main() {
// Example usage of `do_stuff_with_array`
let mut values = vec![Value(1), Value(2), Value(3)];
unsafe { do_stuff_with_array(values.as_mut_ptr(), values.len()) };
println!("{values:?}");
}

I don't see any problem with this code. Perhaps I missed the author's point. At least I don't agree with their suggestion of avoiding the use of references as a solution to this issue.

The author refers to these issues as "dark arts" in Rust, but I have reason to believe that they may not fully understand Rust's safety philosophy regarding Unsafe Rust.

3. The Author Favors Zig's Built-in Safety Strategy

The built-in safety strategies in Zig that the author favors include:

An explicit allocation policy and a special allocator that detects memory errors.

Pointers are non-null by default, but their nullability can be expressed with ?*Value.

The dot operator is used for pointer dereferencing and distinguishing between single and array value pointers.

From this perspective, Zig's safety measures for pointers are similar to Rust's references. Zig may be more flexible to use, like C, without having to consider as many safety factors. It appears that Zig may be safer than Unsafe Rust.

Although Unsafe Rust's pointers are no different from those in C, their incorrect usage can lead to safety issues just like with C. However, Unsafe Rust's safety philosophy allows developers to fully consider the safety issues related to raw pointers. The standard library also provides some methods to assist developers in using pointers more safely. Developers can completely transform pointers into references or use NoNull<T> to make pointers non-null. Additionally, there are safety checking tools like Miri or kani that perform safety checks on pointer-related issues.

Unsafe Rust has higher requirements for developers, and its safety may be better because the unsafe keyword in Unsafe Rust can propagate. For developers and reviewers, abstracting safety from Unsafe Rust requires more effort. However, these efforts are worthwhile.

Zig's safety strategies are not 100% safe and still require developers to consider safety factors. For example, explicit memory management allows developers to explicitly allocate and release memory, thus more clearly controlling the memory's lifecycle, but also means that developers have to assume more responsibility to ensure that memory errors do not occur.

Therefore, there is no such thing as who is safer than whom. However, Zig's performance is better than Unsafe Rust, at least according to the author's benchmark tests.

Conclusion#

This article attempts to help readers understand the safety philosophy of Unsafe Rust and provides comments on the comparison between Zig and Unsafe Rust on who is safer, which has recently been discussed on Reddit. However, one article is not enough to cover all the various details of Unsafe Rust's safety abstractions. Readers should also pay attention to subsequent articles in the Unsafe Rust series.

Finally, it is worth noting that this article's creation was also aided by GPT4.

Thank you for reading.