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## Unsafe Rust

All the code we’ve discussed so far has had Rust’s memory safety guarantees enforced at compile time. However, Rust has a second language hidden inside it that doesn’t enforce these memory safety guarantees: it’s called unsafe Rust and works just like regular Rust, but gives us extra superpowers.

Unsafe Rust exists because, by nature, static analysis is conservative. When the compiler tries to determine whether or not code upholds the guarantees, it’s better for it to reject some valid programs than to accept some invalid programs. Although the code might be okay, if the Rust compiler doesn’t have enough information to be confident, it will reject the code. In these cases, you can use unsafe code to tell the compiler, “Trust me, I know what I’m doing.” Be warned, however, that you use unsafe Rust at your own risk: if you use unsafe code incorrectly, problems can occur due to memory unsafety, such as null pointer dereferencing.

Another reason Rust has an unsafe alter ego is that the underlying computer hardware is inherently unsafe. If Rust didn’t let you do unsafe operations, you couldn’t do certain tasks. Rust needs to allow you to do low-level systems programming, such as directly interacting with the operating system or even writing your own operating system. Working with low-level systems programming is one of the goals of the language. Let’s explore what we can do with unsafe Rust and how to do it.

### Unsafe Superpowers

To switch to unsafe Rust, use the `unsafe` keyword and then start a new block that holds the unsafe code. You can take five actions in unsafe Rust that you can’t in safe Rust, which we call unsafe superpowers. Those superpowers include the ability to:

1. Dereference a raw pointer
2. Call an unsafe function or method
3. Access or modify a mutable static variable
4. Implement an unsafe trait
5. Access fields of `union`s

It’s important to understand that `unsafe` doesn’t turn off the borrow checker or disable any of Rust’s other safety checks: if you use a reference in unsafe code, it will still be checked. The `unsafe` keyword only gives you access to these five features that are then not checked by the compiler for memory safety. You’ll still get some degree of safety inside an unsafe block.

In addition, `unsafe` does not mean the code inside the block is necessarily dangerous or that it will definitely have memory safety problems: the intent is that as the programmer, you’ll ensure the code inside an `unsafe` block will access memory in a valid way.

People are fallible and mistakes will happen, but by requiring these five unsafe operations to be inside blocks annotated with `unsafe`, you’ll know that any errors related to memory safety must be within an `unsafe` block. Keep `unsafe` blocks small; you’ll be thankful later when you investigate memory bugs.

To isolate unsafe code as much as possible, it’s best to enclose such code within a safe abstraction and provide a safe API, which we’ll discuss later in the chapter when we examine unsafe functions and methods. Parts of the standard library are implemented as safe abstractions over unsafe code that has been audited. Wrapping unsafe code in a safe abstraction prevents uses of `unsafe` from leaking out into all the places that you or your users might want to use the functionality implemented with `unsafe` code, because using a safe abstraction is safe.

Let’s look at each of the five unsafe superpowers in turn. We’ll also look at some abstractions that provide a safe interface to unsafe code.

### Dereferencing a Raw Pointer

In Chapter 4, in “Dangling References”, we mentioned that the compiler ensures references are always valid. Unsafe Rust has two new types called raw pointers that are similar to references. As with references, raw pointers can be immutable or mutable and are written as `*const T` and `*mut T`, respectively. The asterisk isn’t the dereference operator; it’s part of the type name. In the context of raw pointers, immutable means that the pointer can’t be directly assigned to after being dereferenced.

Different from references and smart pointers, raw pointers:

- Are allowed to ignore the borrowing rules by having both immutable and mutable pointers or multiple mutable pointers to the same location
- Aren’t guaranteed to point to valid memory
- Are allowed to be null
- Don’t implement any automatic cleanup

By opting out of having Rust enforce these guarantees, you can give up guaranteed safety in exchange for greater performance or the ability to interface with another language or hardware where Rust’s guarantees don’t apply.

Listing 20-1 shows how to create an immutable and a mutable raw pointer.

    fn main() {
        let mut num = 5;

        let r1 = &raw const num;
        let r2 = &raw mut num;
    }

Listing 20-1: Creating raw pointers with the raw borrow operators

Notice that we don’t include the `unsafe` keyword in this code. We can create raw pointers in safe code; we just can’t dereference raw pointers outside an unsafe block, as you’ll see in a bit.

We’ve created raw pointers by using the raw borrow operators: `&raw const num` creates a `*const i32` immutable raw pointer, and `&raw mut num` creates a `*mut i32` mutable raw pointer. Because we created them directly from a local variable, we know these particular raw pointers are valid, but we can’t make that assumption about just any raw pointer.

To demonstrate this, next we’ll create a raw pointer whose validity we can’t be so certain of, using the keyword `as` to cast a value instead of using the raw borrow operator. Listing 20-2 shows how to create a raw pointer to an arbitrary location in memory. Trying to use arbitrary memory is undefined: there might be data at that address or there might not, the compiler might optimize the code so there is no memory access, or the program might terminate with a segmentation fault. Usually, there is no good reason to write code like this, especially in cases where you can use a raw borrow operator instead, but it is possible.

    fn main() {
        let address = 0x012345usize;
        let r = address as *const i32;
    }

Listing 20-2: Creating a raw pointer to an arbitrary memory address

Recall that we can create raw pointers in safe code, but we can’t dereference raw pointers and read the data being pointed to. In Listing 20-3, we use the dereference operator `*` on a raw pointer that requires an `unsafe` block.

    fn main() {
        let mut num = 5;

        let r1 = &raw const num;
        let r2 = &raw mut num;

        unsafe {
            println!("r1 is: {}", *r1);
            println!("r2 is: {}", *r2);
        }
    }

Listing 20-3: Dereferencing raw pointers within an `unsafe` block

Creating a pointer does no harm; it’s only when we try to access the value that it points at that we might end up dealing with an invalid value.

Note also that in Listings 20-1 and 20-3, we created `*const i32` and `*mut i32` raw pointers that both pointed to the same memory location, where `num` is stored. If we instead tried to create an immutable and a mutable reference to `num`, the code would not have compiled because Rust’s ownership rules don’t allow a mutable reference at the same time as any immutable references. With raw pointers, we can create a mutable pointer and an immutable pointer to the same location and change data through the mutable pointer, potentially creating a data race. Be careful!

With all of these dangers, why would you ever use raw pointers? One major use case is when interfacing with C code, as you’ll see in the next section. Another case is when building up safe abstractions that the borrow checker doesn’t understand. We’ll introduce unsafe functions and then look at an example of a safe abstraction that uses unsafe code.

### Calling an Unsafe Function or Method

The second type of operation you can perform in an unsafe block is calling unsafe functions. Unsafe functions and methods look exactly like regular functions and methods, but they have an extra `unsafe` before the rest of the definition. The `unsafe` keyword in this context indicates the function has requirements we need to uphold when we call this function, because Rust can’t guarantee we’ve met these requirements. By calling an unsafe function within an `unsafe` block, we’re saying that we’ve read this function’s documentation and we take responsibility for upholding the function’s contracts.

Here is an unsafe function named `dangerous` that doesn’t do anything in its body:

    fn main() {
        unsafe fn dangerous() {}

        unsafe {
            dangerous();
        }
    }

We must call the `dangerous` function within a separate `unsafe` block. If we try to call `dangerous` without the `unsafe` block, we’ll get an error:

    $ cargo run
       Compiling unsafe-example v0.1.0 (file:///projects/unsafe-example)
    error[E0133]: call to unsafe function `dangerous` is unsafe and requires unsafe block
     --> src/main.rs:4:5
      |
    4 |     dangerous();
      |     ^^^^^^^^^^^ call to unsafe function
      |
      = note: consult the function's documentation for information on how to avoid undefined behavior

    For more information about this error, try `rustc --explain E0133`.
    error: could not compile `unsafe-example` (bin "unsafe-example") due to 1 previous error

With the `unsafe` block, we’re asserting to Rust that we’ve read the function’s documentation, we understand how to use it properly, and we’ve verified that we’re fulfilling the contract of the function.

To perform unsafe operations in the body of an `unsafe` function, you still need to use an `unsafe` block, just as within a regular function, and the compiler will warn you if you forget. This helps us keep `unsafe` blocks as small as possible, as unsafe operations may not be needed across the whole function body.

#### Creating a Safe Abstraction over Unsafe Code

Just because a function contains unsafe code doesn’t mean we need to mark the entire function as unsafe. In fact, wrapping unsafe code in a safe function is a common abstraction. As an example, let’s study the `split_at_mut` function from the standard library, which requires some unsafe code. We’ll explore how we might implement it. This safe method is defined on mutable slices: it takes one slice and makes it two by splitting the slice at the index given as an argument. Listing 20-4 shows how to use `split_at_mut`.

    fn main() {
        let mut v = vec![1, 2, 3, 4, 5, 6];

        let r = &mut v[..];

        let (a, b) = r.split_at_mut(3);

        assert_eq!(a, &mut [1, 2, 3]);
        assert_eq!(b, &mut [4, 5, 6]);
    }

Listing 20-4: Using the safe `split_at_mut` function

We can’t implement this function using only safe Rust. An attempt might look something like Listing 20-5, which won’t compile. For simplicity, we’ll implement `split_at_mut` as a function rather than a method and only for slices of `i32` values rather than for a generic type `T`.

    fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
        let len = values.len();

        assert!(mid <= len);

        (&mut values[..mid], &mut values[mid..])
    }

    fn main() {
        let mut vector = vec![1, 2, 3, 4, 5, 6];
        let (left, right) = split_at_mut(&mut vector, 3);
    }

Listing 20-5: An attempted implementation of `split_at_mut` using only safe Rust

This function first gets the total length of the slice. Then it asserts that the index given as a parameter is within the slice by checking whether it’s less than or equal to the length. The assertion means that if we pass an index that is greater than the length to split the slice at, the function will panic before it attempts to use that index.

Then we return two mutable slices in a tuple: one from the start of the original slice to the `mid` index and another from `mid` to the end of the slice.

When we try to compile the code in Listing 20-5, we’ll get an error:

    $ cargo run
       Compiling unsafe-example v0.1.0 (file:///projects/unsafe-example)
    error[E0499]: cannot borrow `*values` as mutable more than once at a time
     --> src/main.rs:6:31
      |
    1 | fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
      |                         - let's call the lifetime of this reference `'1`
    ...
    6 |     (&mut values[..mid], &mut values[mid..])
      |     --------------------------^^^^^^--------
      |     |     |                   |
      |     |     |                   second mutable borrow occurs here
      |     |     first mutable borrow occurs here
      |     returning this value requires that `*values` is borrowed for `'1`
      |
      = help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

    For more information about this error, try `rustc --explain E0499`.
    error: could not compile `unsafe-example` (bin "unsafe-example") due to 1 previous error

Rust’s borrow checker can’t understand that we’re borrowing different parts of the slice; it only knows that we’re borrowing from the same slice twice. Borrowing different parts of a slice is fundamentally okay because the two slices aren’t overlapping, but Rust isn’t smart enough to know this. When we know code is okay, but Rust doesn’t, it’s time to reach for unsafe code.

Listing 20-6 shows how to use an `unsafe` block, a raw pointer, and some calls to unsafe functions to make the implementation of `split_at_mut` work.

    use std::slice;

    fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
        let len = values.len();
        let ptr = values.as_mut_ptr();

        assert!(mid <= len);

        unsafe {
            (
                slice::from_raw_parts_mut(ptr, mid),
                slice::from_raw_parts_mut(ptr.add(mid), len - mid),
            )
        }
    }

    fn main() {
        let mut vector = vec![1, 2, 3, 4, 5, 6];
        let (left, right) = split_at_mut(&mut vector, 3);
    }

Listing 20-6: Using unsafe code in the implementation of the `split_at_mut` function

Recall from “The Slice Type” in Chapter 4 that a slice is a pointer to some data and the length of the slice. We use the `len` method to get the length of a slice and the `as_mut_ptr` method to access the raw pointer of a slice. In this case, because we have a mutable slice to `i32` values, `as_mut_ptr` returns a raw pointer with the type `*mut i32`, which we’ve stored in the variable `ptr`.

We keep the assertion that the `mid` index is within the slice. Then we get to the unsafe code: the `slice::from_raw_parts_mut` function takes a raw pointer and a length, and it creates a slice. We use this function to create a slice that starts from `ptr` and is `mid` items long. Then we call the `add` method on `ptr` with `mid` as an argument to get a raw pointer that starts at `mid`, and we create a slice using that pointer and the remaining number of items after `mid` as the length.

The function `slice::from_raw_parts_mut` is unsafe because it takes a raw pointer and must trust that this pointer is valid. The `add` method on raw pointers is also unsafe because it must trust that the offset location is also a valid pointer. Therefore, we had to put an `unsafe` block around our calls to `slice::from_raw_parts_mut` and `add` so we could call them. By looking at the code and by adding the assertion that `mid` must be less than or equal to `len`, we can tell that all the raw pointers used within the `unsafe` block will be valid pointers to data within the slice. This is an acceptable and appropriate use of `unsafe`.

Note that we don’t need to mark the resultant `split_at_mut` function as `unsafe`, and we can call this function from safe Rust. We’ve created a safe abstraction to the unsafe code with an implementation of the function that uses `unsafe` code in a safe way, because it creates only valid pointers from the data this function has access to.

In contrast, the use of `slice::from_raw_parts_mut` in Listing 20-7 would likely crash when the slice is used. This code takes an arbitrary memory location and creates a slice 10,000 items long.

    fn main() {
        use std::slice;

        let address = 0x01234usize;
        let r = address as *mut i32;

        let values: &[i32] = unsafe { slice::from_raw_parts_mut(r, 10000) };
    }

Listing 20-7: Creating a slice from an arbitrary memory location

We don’t own the memory at this arbitrary location, and there is no guarantee that the slice this code creates contains valid `i32` values. Attempting to use `values` as though it’s a valid slice results in undefined behavior.

#### Using `extern` Functions to Call External Code

Sometimes your Rust code might need to interact with code written in another language. For this, Rust has the keyword `extern` that facilitates the creation and use of a Foreign Function Interface (FFI), which is a way for a programming language to define functions and enable a different (foreign) programming language to call those functions.

Listing 20-8 demonstrates how to set up an integration with the `abs` function from the C standard library. Functions declared within `extern` blocks are generally unsafe to call from Rust code, so `extern` blocks must also be marked `unsafe`. The reason is that other languages don’t enforce Rust’s rules and guarantees, and Rust can’t check them, so responsibility falls on the programmer to ensure safety.

Filename: src/main.rs

    unsafe extern "C" {
        fn abs(input: i32) -> i32;
    }

    fn main() {
        unsafe {
            println!("Absolute value of -3 according to C: {}", abs(-3));
        }
    }

Listing 20-8: Declaring and calling an `extern` function defined in another language

Within the `unsafe extern "C"` block, we list the names and signatures of external functions from another language we want to call. The `"C"` part defines which application binary interface (ABI) the external function uses: the ABI defines how to call the function at the assembly level. The `"C"` ABI is the most common and follows the C programming language’s ABI. Information about all the ABIs Rust supports is available in the Rust Reference.

Every item declared within an `unsafe extern` block is implicitly unsafe. However, some FFI functions are safe to call. For example, the `abs` function from C’s standard library does not have any memory safety considerations and we know it can be called with any `i32`. In cases like this, we can use the `safe` keyword to say that this specific function is safe to call even though it is in an `unsafe extern` block. Once we make that change, calling it no longer requires an `unsafe` block, as shown in Listing 20-9.

Filename: src/main.rs

    unsafe extern "C" {
        safe fn abs(input: i32) -> i32;
    }

    fn main() {
        println!("Absolute value of -3 according to C: {}", abs(-3));
    }

Listing 20-9: Explicitly marking a function as `safe` within an `unsafe extern` block and calling it safely

Marking a function as `safe` does not inherently make it safe! Instead, it is like a promise you are making to Rust that it is safe. It is still your responsibility to make sure that promise is kept!

#### Calling Rust Functions from Other Languages

We can also use `extern` to create an interface that allows other languages to call Rust functions. Instead of creating a whole `extern` block, we add the `extern` keyword and specify the ABI to use just before the `fn` keyword for the relevant function. We also need to add an `#[unsafe(no_mangle)]` annotation to tell the Rust compiler not to mangle the name of this function. Mangling is when a compiler changes the name we’ve given a function to a different name that contains more information for other parts of the compilation process to consume but is less human readable. Every programming language compiler mangles names slightly differently, so for a Rust function to be nameable by other languages, we must disable the Rust compiler’s name mangling. This is unsafe because there might be name collisions across libraries without the built-in mangling, so it is our responsibility to make sure the name we choose is safe to export without mangling.

In the following example, we make the `call_from_c` function accessible from C code, after it’s compiled to a shared library and linked from C:

    #[unsafe(no_mangle)]
    pub extern "C" fn call_from_c() {
        println!("Just called a Rust function from C!");
    }

This usage of `extern` requires `unsafe` only in the attribute, not on the `extern` block.

### Accessing or Modifying a Mutable Static Variable

In this book, we’ve not yet talked about global variables, which Rust does support but can be problematic with Rust’s ownership rules. If two threads are accessing the same mutable global variable, it can cause a data race.

In Rust, global variables are called static variables. Listing 20-10 shows an example declaration and use of a static variable with a string slice as a value.

Filename: src/main.rs

    static HELLO_WORLD: &str = "Hello, world!";

    fn main() {
        println!("value is: {HELLO_WORLD}");
    }

Listing 20-10: Defining and using an immutable static variable

Static variables are similar to constants, which we discussed in “Constants” in Chapter 3. The names of static variables are in `SCREAMING_SNAKE_CASE` by convention. Static variables can only store references with the `'static` lifetime, which means the Rust compiler can figure out the lifetime and we aren’t required to annotate it explicitly. Accessing an immutable static variable is safe.

A subtle difference between constants and immutable static variables is that values in a static variable have a fixed address in memory. Using the value will always access the same data. Constants, on the other hand, are allowed to duplicate their data whenever they’re used. Another difference is that static variables can be mutable. Accessing and modifying mutable static variables is unsafe. Listing 20-11 shows how to declare, access, and modify a mutable static variable named `COUNTER`.

Filename: src/main.rs

    static mut COUNTER: u32 = 0;

    /// SAFETY: Calling this from more than a single thread at a time is undefined
    /// behavior, so you *must* guarantee you only call it from a single thread at
    /// a time.
    unsafe fn add_to_count(inc: u32) {
        unsafe {
            COUNTER += inc;
        }
    }

    fn main() {
        unsafe {
            // SAFETY: This is only called from a single thread in `main`.
            add_to_count(3);
            println!("COUNTER: {}", *(&raw const COUNTER));
        }
    }

Listing 20-11: Reading from or writing to a mutable static variable is unsafe.

As with regular variables, we specify mutability using the `mut` keyword. Any code that reads or writes from `COUNTER` must be within an `unsafe` block. The code in Listing 20-11 compiles and prints `COUNTER: 3` as we would expect because it’s single threaded. Having multiple threads access `COUNTER` would likely result in data races, so it is undefined behavior. Therefore, we need to mark the entire function as `unsafe` and document the safety limitation, so anyone calling the function knows what they are and are not allowed to do safely.

Whenever we write an unsafe function, it is idiomatic to write a comment starting with `SAFETY` and explaining what the caller needs to do to call the function safely. Likewise, whenever we perform an unsafe operation, it is idiomatic to write a comment starting with `SAFETY` to explain how the safety rules are upheld.

Additionally, the compiler will deny by default any attempt to create references to a mutable static variable through a compiler lint. You must either explicitly opt-out of that lint’s protections by adding an `#[allow(static_mut_refs)]` annotation or access the mutable static variable via a raw pointer created with one of the raw borrow operators. That includes cases where the reference is created invisibly, as when it is used in the `println!` in this code listing. Requiring references to static mutable variables to be created via raw pointers helps make the safety requirements for using them more obvious.

With mutable data that is globally accessible, it’s difficult to ensure there are no data races, which is why Rust considers mutable static variables to be unsafe. Where possible, it’s preferable to use the concurrency techniques and thread-safe smart pointers we discussed in Chapter 16 so the compiler checks that data access from different threads is done safely.

### Implementing an Unsafe Trait

We can use `unsafe` to implement an unsafe trait. A trait is unsafe when at least one of its methods has some invariant that the compiler can’t verify. We declare that a trait is `unsafe` by adding the `unsafe` keyword before `trait` and marking the implementation of the trait as `unsafe` too, as shown in Listing 20-12.

    unsafe trait Foo {
        // methods go here
    }

    unsafe impl Foo for i32 {
        // method implementations go here
    }

    fn main() {}

Listing 20-12: Defining and implementing an unsafe trait

By using `unsafe impl`, we’re promising that we’ll uphold the invariants that the compiler can’t verify.

As an example, recall the `Send` and `Sync` marker traits we discussed in “Extensible Concurrency with the `Send` and `Sync` Traits” in Chapter 16: the compiler implements these traits automatically if our types are composed entirely of other types that implement `Send` and `Sync`. If we implement a type that contains a type that does not implement `Send` or `Sync`, such as raw pointers, and we want to mark that type as `Send` or `Sync`, we must use `unsafe`. Rust can’t verify that our type upholds the guarantees that it can be safely sent across threads or accessed from multiple threads; therefore, we need to do those checks manually and indicate as such with `unsafe`.

### Accessing Fields of a Union

The final action that works only with `unsafe` is accessing fields of a union. A union is similar to a `struct`, but only one declared field is used in a particular instance at one time. Unions are primarily used to interface with unions in C code. Accessing union fields is unsafe because Rust can’t guarantee the type of the data currently being stored in the union instance. You can learn more about unions in the Rust Reference.

### Using Miri to Check Unsafe Code

When writing unsafe code, you might want to check that what you have written actually is safe and correct. One of the best ways to do that is to use Miri, an official Rust tool for detecting undefined behavior. Whereas the borrow checker is a static tool which works at compile time, Miri is a dynamic tool which works at runtime. It checks your code by running your program, or its test suite, and detecting when you violate the rules it understands about how Rust should work.

Using Miri requires a nightly build of Rust (which we talk about more in Appendix G: How Rust is Made and “Nightly Rust”). You can install both a nightly version of Rust and the Miri tool by typing `rustup +nightly component add miri`. This does not change what version of Rust your project uses; it only adds the tool to your system so you can use it when you want to. You can run Miri on a project by typing `cargo +nightly miri run` or `cargo +nightly miri test`.

For an example of how helpful this can be, consider what happens when we run it against Listing 20-7.

    $ cargo +nightly miri run
       Compiling unsafe-example v0.1.0 (file:///projects/unsafe-example)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
         Running `file:///home/.rustup/toolchains/nightly/bin/cargo-miri runner target/miri/debug/unsafe-example`
    warning: integer-to-pointer cast
     --> src/main.rs:5:13
      |
    5 |     let r = address as *mut i32;
      |             ^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
      |
      = help: this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program
      = help: see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation
      = help: to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
      = help: you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics
      = help: alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning
      = note: BACKTRACE:
      = note: inside `main` at src/main.rs:5:13: 5:32

    error: Undefined Behavior: pointer not dereferenceable: pointer must be dereferenceable for 40000 bytes, but got 0x1234[noalloc] which is a dangling pointer (it has no provenance)
     --> src/main.rs:7:35
      |
    7 |     let values: &[i32] = unsafe { slice::from_raw_parts_mut(r, 10000) };
      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
      |
      = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
      = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
      = note: BACKTRACE:
      = note: inside `main` at src/main.rs:7:35: 7:70

    note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

    error: aborting due to 1 previous error; 1 warning emitted

Miri correctly warns us that we’re casting an integer to a pointer, which might be a problem but Miri can’t detect if there is because it doesn’t know how the pointer originated. Then, Miri returns an error where Listing 20-7 has undefined behavior because we have a dangling pointer. Thanks to Miri, we now know there is a risk of undefined behavior, and we can think about how to make the code safe. In some cases, Miri can even make recommendations about how to fix errors.

Miri doesn’t catch everything you might get wrong when writing unsafe code. Miri is a dynamic analysis tool, so it only catches problems with code that actually gets run. That means you will need to use it in conjunction with good testing techniques to increase your confidence about the unsafe code you have written. Miri also does not cover every possible way your code can be unsound.

Put another way: If Miri does catch a problem, you know there’s a bug, but just because Miri doesn’t catch a bug doesn’t mean there isn’t a problem. It can catch a lot, though. Try running it on the other examples of unsafe code in this chapter and see what it says!

You can learn more about Miri at its GitHub repository.

### When to Use Unsafe Code

Using `unsafe` to use one of the five superpowers just discussed isn’t wrong or even frowned upon, but it is trickier to get `unsafe` code correct because the compiler can’t help uphold memory safety. When you have a reason to use `unsafe` code, you can do so, and having the explicit `unsafe` annotation makes it easier to track down the source of problems when they occur. Whenever you write unsafe code, you can use Miri to help you be more confident that the code you have written upholds Rust’s rules.

For a much deeper exploration of how to work effectively with unsafe Rust, read Rust’s official guide to the subject, the Rustonomicon.