diff options
Diffstat (limited to '')
| -rw-r--r-- | src/doc/book/nostarch/chapter19.md | 684 |
1 files changed, 336 insertions, 348 deletions
diff --git a/src/doc/book/nostarch/chapter19.md b/src/doc/book/nostarch/chapter19.md index 5ef3fd7a2..410e7eb62 100644 --- a/src/doc/book/nostarch/chapter19.md +++ b/src/doc/book/nostarch/chapter19.md @@ -9,7 +9,7 @@ directory, so all fixes need to be made in `/src/`. # Advanced Features By now, you’ve learned the most commonly used parts of the Rust programming -language. Before we do one more project in Chapter 20, we’ll look at a few +language. Before we do one more project, in Chapter 20, we’ll look at a few aspects of the language you might run into every once in a while, but may not use every day. You can use this chapter as a reference for when you encounter any unknowns. The features covered here are useful in very specific situations. @@ -19,11 +19,11 @@ grasp of all the features Rust has to offer. In this chapter, we’ll cover: * Unsafe Rust: how to opt out of some of Rust’s guarantees and take - responsibility for manually upholding those guarantees +responsibility for manually upholding those guarantees * Advanced traits: associated types, default type parameters, fully qualified - syntax, supertraits, and the newtype pattern in relation to traits +syntax, supertraits, and the newtype pattern in relation to traits * Advanced types: more about the newtype pattern, type aliases, the never type, - and dynamically sized types +and dynamically sized types * Advanced functions and closures: function pointers and returning closures * Macros: ways to define code that defines more code at compile time @@ -61,30 +61,30 @@ 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: -* Dereference a raw pointer -* Call an unsafe function or method -* Access or modify a mutable static variable -* Implement an unsafe trait -* Access fields of `union`s +1. Dereference a raw pointer +1. Call an unsafe function or method +1. Access or modify a mutable static variable +1. Implement an unsafe trait +1. Access fields of `union`s It’s important to understand that `unsafe` doesn’t turn off the borrow checker -or disable any other of Rust’s safety checks: if you use a reference in unsafe +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 of an unsafe block. +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 +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 unsafe code +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 @@ -98,18 +98,18 @@ some abstractions that provide a safe interface to unsafe code. ### Dereferencing a Raw Pointer -In Chapter 4, in the “Dangling References” section, 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 +In “Dangling References” on page XX, 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 +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 @@ -144,9 +144,9 @@ To demonstrate this, next we’ll create a raw pointer whose validity we can’t so certain of. Listing 19-2 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 error with a segmentation -fault. Usually, there is no good reason to write code like this, but it is -possible. +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, +but it is possible. ``` let address = 0x012345usize; @@ -176,8 +176,8 @@ Listing 19-3: 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 Listing 19-1 and 19-3, we created `*const i32` and `*mut i32` -raw pointers that both pointed to the same memory location, where `num` is +Note also that in Listings 19-1 and 19-3, 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 @@ -186,8 +186,8 @@ 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, -“Calling an Unsafe Function or Method.” Another case is when building up safe +case is when interfacing with C code, as you’ll see in “Calling an Unsafe +Function or Method” on page XX. 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. @@ -201,7 +201,7 @@ 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 -take responsibility for upholding the function’s contracts. +we take responsibility for upholding the function’s contracts. Here is an unsafe function named `dangerous` that doesn’t do anything in its body: @@ -218,13 +218,15 @@ 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: ``` -error[E0133]: call to unsafe function is unsafe and requires unsafe function or block +error[E0133]: call to unsafe function is unsafe and requires +unsafe function or 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 + = note: consult the function's documentation for information on +how to avoid undefined behavior ``` With the `unsafe` block, we’re asserting to Rust that we’ve read the function’s @@ -264,7 +266,10 @@ 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]) { +fn split_at_mut( + values: &mut [i32], + mid: usize, +) -> (&mut [i32], &mut [i32]) { let len = values.len(); assert!(mid <= len); @@ -289,12 +294,12 @@ When we try to compile the code in Listing 19-5, we’ll get an error: ``` error[E0499]: cannot borrow `*values` as mutable more than once at a time - --> src/main.rs:6:31 + --> src/main.rs:9:31 | -1 | fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) { - | - let's call the lifetime of this reference `'1` +2 | values: &mut [i32], + | - let's call the lifetime of this reference `'1` ... -6 | (&mut values[..mid], &mut values[mid..]) +9 | (&mut values[..mid], &mut values[mid..]) | --------------------------^^^^^^-------- | | | | | | | second mutable borrow occurs here @@ -314,16 +319,19 @@ 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]) { - [1] let len = values.len(); - [2] let ptr = values.as_mut_ptr(); +fn split_at_mut( + values: &mut [i32], + mid: usize, +) -> (&mut [i32], &mut [i32]) { + 1 let len = values.len(); + 2 let ptr = values.as_mut_ptr(); - [3] assert!(mid <= len); + 3 assert!(mid <= len); - [4] unsafe { + 4 unsafe { ( - [5] slice::from_raw_parts_mut(ptr, mid), - [6] slice::from_raw_parts_mut(ptr.add(mid), len - mid), + 5 slice::from_raw_parts_mut(ptr, mid), + 6 slice::from_raw_parts_mut(ptr.add(mid), len - mid), ) } } @@ -332,13 +340,12 @@ fn split_at_mut(values: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) { Listing 19-6: Using unsafe code in the implementation of the `split_at_mut` function - -Recall from “The Slice Type” section 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 [1] and the `as_mut_ptr` method to access the raw pointer of -a slice [2]. 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`. +Recall from “The Slice Type” on page XX 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 [1] and the `as_mut_ptr` method to access the raw pointer of a slice [2]. +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 [3]. Then we get to the unsafe code [4]: the `slice::from_raw_parts_mut` function takes a raw @@ -350,15 +357,15 @@ we create a slice using that pointer and the remaining number of items after 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 +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 +`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 resulting `split_at_mut` function as +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 @@ -374,7 +381,9 @@ use std::slice; let address = 0x01234usize; let r = address as *mut i32; -let values: &[i32] = unsafe { slice::from_raw_parts_mut(r, 10000) }; +let values: &[i32] = unsafe { + slice::from_raw_parts_mut(r, 10000) +}; ``` Listing 19-7: Creating a slice from an arbitrary memory location @@ -383,11 +392,11 @@ 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 +#### Using extern Functions to Call External Code -Sometimes, your Rust code might need to interact with code written in another +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)*. An FFI is a way for a +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. @@ -406,7 +415,10 @@ extern "C" { fn main() { unsafe { - println!("Absolute value of -3 according to C: {}", abs(-3)); + println!( + "Absolute value of -3 according to C: {}", + abs(-3) + ); } } ``` @@ -416,33 +428,25 @@ language Within the `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 +*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. -<!-- Totally optional - but do we want to mention the other external types -that Rust supports here? Also, do we want to mention there are helper -crates for connecting to other languages, include C++? -/JT --> -<!-- I don't really want to get into the other external types or other -languages; there are other resources that cover these topics better than I -could here. /Carol --> - -> #### Calling Rust Functions from Other Languages +> ### 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 an 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 a `#[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. +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 a `#[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. > > 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: +C code, after it’s compiled to a shared library and linked from C: > > ``` > #[no_mangle] @@ -455,13 +459,12 @@ could here. /Carol --> ### Accessing or Modifying a Mutable Static Variable -In this book, we’ve not yet talked about *global variables*, which Rust does +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 19-9 shows an -example declaration and use of a static variable with a string slice as a -value. +example declaration and use of a static variable with a string slice as a value. Filename: src/main.rs @@ -469,18 +472,18 @@ Filename: src/main.rs static HELLO_WORLD: &str = "Hello, world!"; fn main() { - println!("name is: {}", HELLO_WORLD); + println!("value is: {HELLO_WORLD}"); } ``` Listing 19-9: Defining and using an immutable static variable -Static variables are similar to constants, which we discussed in the -“Differences Between Variables and Constants” section 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. +Static variables are similar to constants, which we discussed in “Constants” on +page XX. 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 @@ -505,12 +508,12 @@ fn main() { add_to_count(3); unsafe { - println!("COUNTER: {}", COUNTER); + println!("COUNTER: {COUNTER}"); } } ``` -Listing 19-10: Reading from or writing to a mutable static variable is unsafe +Listing 19-10: 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. This @@ -522,7 +525,7 @@ 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 accessed from different threads is done safely. +that data access from different threads is done safely. ### Implementing an Unsafe Trait @@ -540,8 +543,6 @@ unsafe trait Foo { unsafe impl Foo for i32 { // method implementations go here } - -fn main() {} ``` Listing 19-11: Defining and implementing an unsafe trait @@ -549,25 +550,25 @@ Listing 19-11: 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 `Sync` and `Send` marker traits we discussed in the -“Extensible Concurrency with the `Sync` and `Send` Traits” section in Chapter -16: the compiler implements these traits automatically if our types are -composed entirely of `Send` and `Sync` types. If we implement a type that -contains a type that is not `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`. +As an example, recall the `Send` and `Sync` marker traits we discussed in +“Extensible Concurrency with the Send and Sync Traits” on page XX: the compiler +implements these traits automatically if our types are composed entirely of +`Send` and `Sync` types. If we implement a type that contains a type that is +not `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 at -*https://doc.rust-lang.org/reference/items/unions.html*. +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 at +*https://doc.rust-lang.org/reference/items/unions.html**.* ### When to Use Unsafe Code @@ -579,11 +580,11 @@ it easier to track down the source of problems when they occur. ## Advanced Traits -We first covered traits in the “Traits: Defining Shared Behavior” section of -Chapter 10, but we didn’t discuss the more advanced details. Now that you know -more about Rust, we can get into the nitty-gritty. +We first covered traits in “Traits: Defining Shared Behavior” on page XX, but +we didn’t discuss the more advanced details. Now that you know more about Rust, +we can get into the nitty-gritty. -### Specifying Placeholder Types in Trait Definitions with Associated Types +### Associated Types *Associated types* connect a type placeholder with a trait such that the trait method definitions can use these placeholder types in their signatures. The @@ -632,7 +633,7 @@ impl Iterator for Counter { type Item = u32; fn next(&mut self) -> Option<Self::Item> { - // --snip-- + --snip-- ``` This syntax seems comparable to that of generics. So why not just define the @@ -648,7 +649,7 @@ Listing 19-13: A hypothetical definition of the `Iterator` trait using generics The difference is that when using generics, as in Listing 19-13, we must annotate the types in each implementation; because we can also implement -`Iterator<String> for Counter` or any other type, we could have multiple +`Iterator<``String``> for Counter` or any other type, we could have multiple implementations of `Iterator` for `Counter`. In other words, when a trait has a generic parameter, it can be implemented for a type multiple times, changing the concrete types of the generic type parameters each time. When we use the @@ -657,29 +658,23 @@ indicate which implementation of `Iterator` we want to use. With associated types, we don’t need to annotate types because we can’t implement a trait on a type multiple times. In Listing 19-12 with the -definition that uses associated types, we can only choose what the type of -`Item` will be once, because there can only be one `impl Iterator for Counter`. -We don’t have to specify that we want an iterator of `u32` values everywhere -that we call `next` on `Counter`. +definition that uses associated types, we can choose what the type of `Item` +will be only once because there can be only one `impl Iterator for Counter`. We +don’t have to specify that we want an iterator of `u32` values everywhere we +call `next` on `Counter`. Associated types also become part of the trait’s contract: implementors of the trait must provide a type to stand in for the associated type placeholder. Associated types often have a name that describes how the type will be used, -and documenting the associated type in the API documentation is good practice. - -<!-- It also makes the type a part of the trait's contract. Not sure if -too subtle of a point, but the associated type of a trait is part of the -require things that the implementor must provide. They often also have a name -that may clue you in as to how that required type will be used. -/JT --> -<!-- Great points, I've added a small paragraph here! /Carol --> +and documenting the associated type in the API documentation is a good practice. ### Default Generic Type Parameters and Operator Overloading When we use generic type parameters, we can specify a default concrete type for the generic type. This eliminates the need for implementors of the trait to specify a concrete type if the default type works. You specify a default type -when declaring a generic type with the `<PlaceholderType=ConcreteType>` syntax. +when declaring a generic type with the `<`PlaceholderType`=`ConcreteType`>` +syntax. A great example of a situation where this technique is useful is with *operator overloading*, in which you customize the behavior of an operator (such as `+`) @@ -690,7 +685,7 @@ operators. But you can overload the operations and corresponding traits listed in `std::ops` by implementing the traits associated with the operator. For example, in Listing 19-14 we overload the `+` operator to add two `Point` instances together. We do this by implementing the `Add` trait on a `Point` -struct: +struct. Filename: src/main.rs @@ -743,7 +738,7 @@ trait Add<Rhs=Self> { This code should look generally familiar: a trait with one method and an associated type. The new part is `Rhs=Self`: this syntax is called *default -type parameters*. The `Rhs` generic type parameter (short for “right hand +type parameters*. The `Rhs` generic type parameter (short for “right-hand side”) defines the type of the `rhs` parameter in the `add` method. If we don’t specify a concrete type for `Rhs` when we implement the `Add` trait, the type of `Rhs` will default to `Self`, which will be the type we’re implementing @@ -756,11 +751,11 @@ default. We have two structs, `Millimeters` and `Meters`, holding values in different units. This thin wrapping of an existing type in another struct is known as the -*newtype pattern*, which we describe in more detail in the “Using the Newtype -Pattern to Implement External Traits on External Types” section. We want to add -values in millimeters to values in meters and have the implementation of `Add` -do the conversion correctly. We can implement `Add` for `Millimeters` with -`Meters` as the `Rhs`, as shown in Listing 19-15. +*newtype pattern*, which we describe in more detail in “Using the Newtype +Pattern to Implement External Traits on External Types” on page XX. We want to +add values in millimeters to values in meters and have the implementation of +`Add` do the conversion correctly. We can implement `Add` for `Millimeters` +with `Meters` as the `Rhs`, as shown in Listing 19-15. Filename: src/lib.rs @@ -780,15 +775,15 @@ impl Add<Meters> for Millimeters { ``` Listing 19-15: Implementing the `Add` trait on `Millimeters` to add -`Millimeters` to `Meters` +`Millimeters` and `Meters` To add `Millimeters` and `Meters`, we specify `impl Add<Meters>` to set the value of the `Rhs` type parameter instead of using the default of `Self`. You’ll use default type parameters in two main ways: -* To extend a type without breaking existing code -* To allow customization in specific cases most users won’t need +1. To extend a type without breaking existing code +1. To allow customization in specific cases most users won’t need The standard library’s `Add` trait is an example of the second purpose: usually, you’ll add two like types, but the `Add` trait provides the ability to @@ -802,7 +797,7 @@ type parameter to an existing trait, you can give it a default to allow extension of the functionality of the trait without breaking the existing implementation code. -### Fully Qualified Syntax for Disambiguation: Calling Methods with the Same Name +### Disambiguating Between Methods with the Same Name Nothing in Rust prevents a trait from having a method with the same name as another trait’s method, nor does Rust prevent you from implementing both traits @@ -849,7 +844,7 @@ impl Human { Listing 19-16: Two traits are defined to have a `fly` method and are implemented on the `Human` type, and a `fly` method is implemented on `Human` -directly +directly. When we call `fly` on an instance of `Human`, the compiler defaults to calling the method that is directly implemented on the type, as shown in Listing 19-17. @@ -894,10 +889,6 @@ disambiguate. Running this code prints the following: ``` -$ cargo run - Compiling traits-example v0.1.0 (file:///projects/traits-example) - Finished dev [unoptimized + debuginfo] target(s) in 0.46s - Running `target/debug/traits-example` This is your captain speaking. Up! *waving arms furiously* @@ -909,12 +900,12 @@ trait to use based on the type of `self`. However, associated functions that are not methods don’t have a `self` parameter. When there are multiple types or traits that define non-method -functions with the same function name, Rust doesn't always know which type you -mean unless you use *fully qualified syntax*. For example, in Listing 19-19 we -create a trait for an animal shelter that wants to name all baby dogs *Spot*. -We make an `Animal` trait with an associated non-method function `baby_name`. -The `Animal` trait is implemented for the struct `Dog`, on which we also -provide an associated non-method function `baby_name` directly. +functions with the same function name, Rust doesn’t always know which type you +mean unless you use fully qualified syntax. For example, in Listing 19-19 we +create a trait for an animal shelter that wants to name all baby dogs Spot. We +make an `Animal` trait with an associated non-method function `baby_name`. The +`Animal` trait is implemented for the struct `Dog`, on which we also provide an +associated non-method function `baby_name` directly. Filename: src/main.rs @@ -984,7 +975,8 @@ error[E0283]: type annotations needed --> src/main.rs:20:43 | 20 | println!("A baby dog is called a {}", Animal::baby_name()); - | ^^^^^^^^^^^^^^^^^ cannot infer type + | ^^^^^^^^^^^^^^^^^ cannot infer +type | = note: cannot satisfy `_: Animal` ``` @@ -998,7 +990,10 @@ Filename: src/main.rs ``` fn main() { - println!("A baby dog is called a {}", <Dog as Animal>::baby_name()); + println!( + "A baby dog is called a {}", + <Dog as Animal>::baby_name() + ); } ``` @@ -1028,20 +1023,20 @@ in the program. You only need to use this more verbose syntax in cases where there are multiple implementations that use the same name and Rust needs help to identify which implementation you want to call. -### Using Supertraits to Require One Trait’s Functionality Within Another Trait +### Using Supertraits -Sometimes, you might write a trait definition that depends on another trait: -for a type to implement the first trait, you want to require that type to also +Sometimes you might write a trait definition that depends on another trait: for +a type to implement the first trait, you want to require that type to also implement the second trait. You would do this so that your trait definition can make use of the associated items of the second trait. The trait your trait definition is relying on is called a *supertrait* of your trait. For example, let’s say we want to make an `OutlinePrint` trait with an -`outline_print` method that will print a given value formatted so that it's +`outline_print` method that will print a given value formatted so that it’s framed in asterisks. That is, given a `Point` struct that implements the -standard library trait `Display` to result in `(x, y)`, when we -call `outline_print` on a `Point` instance that has `1` for `x` and `3` for -`y`, it should print the following: +standard library trait `Display` to result in `(x, y)`, when we call +`outline_print` on a `Point` instance that has `1` for `x` and `3` for `y`, it +should print the following: ``` ********** @@ -1111,7 +1106,8 @@ error[E0277]: `Point` doesn't implement `std::fmt::Display` | ^^^^^^^^^^^^ `Point` cannot be formatted with the default formatter | = help: the trait `std::fmt::Display` is not implemented for `Point` - = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead + = note: in format strings you may be able to use `{:?}` (or {:#?} for +pretty-print) instead note: required by a bound in `OutlinePrint` --> src/main.rs:3:21 | @@ -1134,24 +1130,23 @@ impl fmt::Display for Point { } ``` -Then implementing the `OutlinePrint` trait on `Point` will compile +Then, implementing the `OutlinePrint` trait on `Point` will compile successfully, and we can call `outline_print` on a `Point` instance to display it within an outline of asterisks. -### Using the Newtype Pattern to Implement External Traits on External Types - -In Chapter 10 in the “Implementing a Trait on a Type” section, we mentioned the -orphan rule that states we’re only allowed to implement a trait on a type if -either the trait or the type are local to our crate. -It’s possible to get -around this restriction using the *newtype pattern*, which involves creating a -new type in a tuple struct. (We covered tuple structs in the “Using Tuple -Structs without Named Fields to Create Different Types” section of Chapter 5.) -The tuple struct will have one field and be a thin wrapper around the type we -want to implement a trait for. Then the wrapper type is local to our crate, and -we can implement the trait on the wrapper. *Newtype* is a term that originates -from the Haskell programming language. There is no runtime performance penalty -for using this pattern, and the wrapper type is elided at compile time. +### Using the Newtype Pattern to Implement External Traits + +In “Implementing a Trait on a Type” on page XX, we mentioned the orphan rule +that states we’re only allowed to implement a trait on a type if either the +trait or the type, or both, are local to our crate. It’s possible to get around +this restriction using the *newtype pattern*, which involves creating a new +type in a tuple struct. (We covered tuple structs in “Using Tuple Structs +Without Named Fields to Create Different Types” on page XX.) The tuple struct +will have one field and be a thin wrapper around the type for which we want to +implement a trait. Then the wrapper type is local to our crate, and we can +implement the trait on the wrapper. *Newtype* is a term that originates from +the Haskell programming language. There is no runtime performance penalty for +using this pattern, and the wrapper type is elided at compile time. As an example, let’s say we want to implement `Display` on `Vec<T>`, which the orphan rule prevents us from doing directly because the `Display` trait and the @@ -1173,15 +1168,18 @@ impl fmt::Display for Wrapper { } fn main() { - let w = Wrapper(vec![String::from("hello"), String::from("world")]); - println!("w = {}", w); + let w = Wrapper(vec![ + String::from("hello"), + String::from("world"), + ]); + println!("w = {w}"); } ``` Listing 19-23: Creating a `Wrapper` type around `Vec<String>` to implement `Display` -The implementation of `Display` uses `self.0` to access the inner `Vec<T>`, +The implementation of `Display` uses `self.0` to access the inner `Vec<T>` because `Wrapper` is a tuple struct and `Vec<T>` is the item at index 0 in the tuple. Then we can use the functionality of the `Display` type on `Wrapper`. @@ -1190,9 +1188,9 @@ doesn’t have the methods of the value it’s holding. We would have to impleme all the methods of `Vec<T>` directly on `Wrapper` such that the methods delegate to `self.0`, which would allow us to treat `Wrapper` exactly like a `Vec<T>`. If we wanted the new type to have every method the inner type has, -implementing the `Deref` trait (discussed in Chapter 15 in the “Treating Smart -Pointers Like Regular References with the `Deref` Trait” section) on the -`Wrapper` to return the inner type would be a solution. If we don’t want the +implementing the `Deref` trait on the `Wrapper` to return the inner type would +be a solution (we discussed implementing the `Deref` trait in “Treating Smart +Pointers Like Regular References with Deref” on page XX). If we didn’t want the `Wrapper` type to have all the methods of the inner type—for example, to restrict the `Wrapper` type’s behavior—we would have to implement just the methods we do want manually. @@ -1210,16 +1208,15 @@ the `!` type and dynamically sized types. ### Using the Newtype Pattern for Type Safety and Abstraction -> Note: This section assumes you’ve read the earlier section “Using the -> Newtype Pattern to Implement External Traits on External -> Types.” +> Note: This section assumes you’ve read the earlier section “Using the Newtype +Pattern to Implement External Traits” on page XX. The newtype pattern is also useful for tasks beyond those we’ve discussed so far, including statically enforcing that values are never confused and indicating the units of a value. You saw an example of using newtypes to indicate units in Listing 19-15: recall that the `Millimeters` and `Meters` structs wrapped `u32` values in a newtype. If we wrote a function with a -parameter of type `Millimeters`, we couldn’t compile a program that +parameter of type `Millimeters`, we wouldn’t be able to compile a program that accidentally tried to call that function with a value of type `Meters` or a plain `u32`. @@ -1233,8 +1230,8 @@ associated with their name. Code using `People` would only interact with the public API we provide, such as a method to add a name string to the `People` collection; that code wouldn’t need to know that we assign an `i32` ID to names internally. The newtype pattern is a lightweight way to achieve encapsulation -to hide implementation details, which we discussed in the “Encapsulation that -Hides Implementation Details” section of Chapter 17. +to hide implementation details, which we discussed in “Encapsulation That Hides +Implementation Details” on page XX. ### Creating Type Synonyms with Type Aliases @@ -1246,7 +1243,7 @@ the alias `Kilometers` to `i32` like so: type Kilometers = i32; ``` -Now, the alias `Kilometers` is a *synonym* for `i32`; unlike the `Millimeters` +Now the alias `Kilometers` is a *synonym* for `i32`; unlike the `Millimeters` and `Meters` types we created in Listing 19-15, `Kilometers` is not a separate, new type. Values that have the type `Kilometers` will be treated the same as values of type `i32`: @@ -1262,20 +1259,11 @@ println!("x + y = {}", x + y); Because `Kilometers` and `i32` are the same type, we can add values of both types and we can pass `Kilometers` values to functions that take `i32` -parameters. However, using this method, we don’t get the type checking benefits +parameters. However, using this method, we don’t get the type-checking benefits that we get from the newtype pattern discussed earlier. In other words, if we mix up `Kilometers` and `i32` values somewhere, the compiler will not give us an error. -<!-- Having a few battle wounds trying to debug using this pattern, it's -definitely good to warn people that if they use type aliases to the same base -type in their program (like multiple aliases to `usize`), they're asking for -trouble as the typechecker will not help them if they mix up their types. -/JT --> -<!-- I'm not sure if JT was saying this paragraph was good or it could use more -emphasis? I've added a sentence to the end of the paragraph above in case it -was the latter /Carol --> - The main use case for type synonyms is to reduce repetition. For example, we might have a lengthy type like this: @@ -1288,14 +1276,16 @@ over the code can be tiresome and error prone. Imagine having a project full of code like that in Listing 19-24. ``` -let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi")); +let f: Box<dyn Fn() + Send + 'static> = Box::new(|| { + println!("hi"); +}); fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) { - // --snip-- + --snip-- } fn returns_long_type() -> Box<dyn Fn() + Send + 'static> { - // --snip-- + --snip-- } ``` @@ -1311,11 +1301,11 @@ type Thunk = Box<dyn Fn() + Send + 'static>; let f: Thunk = Box::new(|| println!("hi")); fn takes_long_type(f: Thunk) { - // --snip-- + --snip-- } fn returns_long_type() -> Thunk { - // --snip-- + --snip-- } ``` @@ -1343,7 +1333,10 @@ pub trait Write { fn flush(&mut self) -> Result<(), Error>; fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>; - fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<(), Error>; + fn write_fmt( + &mut self, + fmt: fmt::Arguments, + ) -> Result<(), Error>; } ``` @@ -1374,7 +1367,7 @@ us a consistent interface across all of `std::io`. Because it’s an alias, it just another `Result<T, E>`, which means we can use any methods that work on `Result<T, E>` with it, as well as special syntax like the `?` operator. -### The Never Type that Never Returns +### The Never Type That Never Returns Rust has a special type named `!` that’s known in type theory lingo as the *empty type* because it has no values. We prefer to call it the *never type* @@ -1383,16 +1376,16 @@ return. Here is an example: ``` fn bar() -> ! { - // --snip-- + --snip-- } ``` This code is read as “the function `bar` returns never.” Functions that return -never are called *diverging functions*. We can’t create values of the type `!` +never are called *diverging functions*. We can’t create values of the type `!`, so `bar` can never possibly return. But what use is a type you can never create values for? Recall the code from -Listing 2-5, part of the number guessing game; we’ve reproduced a bit of it +Listing 2-5, part of the number-guessing game; we’ve reproduced a bit of it here in Listing 19-26. ``` @@ -1404,9 +1397,9 @@ let guess: u32 = match guess.trim().parse() { Listing 19-26: A `match` with an arm that ends in `continue` -At the time, we skipped over some details in this code. In Chapter 6 in “The -`match` Control Flow Operator” section, we discussed that `match` arms must all -return the same type. So, for example, the following code doesn’t work: +At the time, we skipped over some details in this code. In “The match Control +Flow Construct” on page XX, we discussed that `match` arms must all return the +same type. So, for example, the following code doesn’t work: ``` let guess = match guess.trim().parse() { @@ -1440,7 +1433,9 @@ impl<T> Option<T> { pub fn unwrap(self) -> T { match self { Some(val) => val, - None => panic!("called `Option::unwrap()` on a `None` value"), + None => panic!( + "called `Option::unwrap()` on a `None` value" + ), } } } @@ -1466,7 +1461,7 @@ Here, the loop never ends, so `!` is the value of the expression. However, this wouldn’t be true if we included a `break`, because the loop would terminate when it got to the `break`. -### Dynamically Sized Types and the `Sized` Trait +### Dynamically Sized Types and the Sized Trait Rust needs to know certain details about its types, such as how much space to allocate for a value of a particular type. This leaves one corner of its type @@ -1493,25 +1488,25 @@ storage and `s2` needs 15. This is why it’s not possible to create a variable holding a dynamically sized type. So what do we do? In this case, you already know the answer: we make the types -of `s1` and `s2` a `&str` rather than a `str`. Recall from the “String Slices” -section of Chapter 4 that the slice data structure just stores the starting -position and the length of the slice. So although a `&T` is a single value that -stores the memory address of where the `T` is located, a `&str` is *two* -values: the address of the `str` and its length. As such, we can know the size -of a `&str` value at compile time: it’s twice the length of a `usize`. That is, -we always know the size of a `&str`, no matter how long the string it refers to -is. In general, this is the way in which dynamically sized types are used in -Rust: they have an extra bit of metadata that stores the size of the dynamic +of `s1` and `s2` a `&str` rather than a `str`. Recall from “String Slices” on +page XX that the slice data structure just stores the starting position and the +length of the slice. So, although a `&T` is a single value that stores the +memory address of where the `T` is located, a `&str` is *two* values: the +address of the `str` and its length. As such, we can know the size of a `&str` +value at compile time: it’s twice the length of a `usize`. That is, we always +know the size of a `&str`, no matter how long the string it refers to is. In +general, this is the way in which dynamically sized types are used in Rust: +they have an extra bit of metadata that stores the size of the dynamic information. The golden rule of dynamically sized types is that we must always put values of dynamically sized types behind a pointer of some kind. We can combine `str` with all kinds of pointers: for example, `Box<str>` or `Rc<str>`. In fact, you’ve seen this before but with a different dynamically sized type: traits. Every trait is a dynamically sized type we can refer to by -using the name of the trait. In Chapter 17 in the “Using Trait Objects That -Allow for Values of Different Types” section, we mentioned that to use traits -as trait objects, we must put them behind a pointer, such as `&dyn Trait` or -`Box<dyn Trait>` (`Rc<dyn Trait>` would work too). +using the name of the trait. In “Using Trait Objects That Allow for Values of +Different Types” on page XX, we mentioned that to use traits as trait objects, +we must put them behind a pointer, such as `&dyn Trait` or `Box<dyn Trait>` +(`Rc<dyn Trait>` would work too). To work with DSTs, Rust provides the `Sized` trait to determine whether or not a type’s size is known at compile time. This trait is automatically implemented @@ -1521,7 +1516,7 @@ generic function definition like this: ``` fn generic<T>(t: T) { - // --snip-- + --snip-- } ``` @@ -1529,7 +1524,7 @@ is actually treated as though we had written this: ``` fn generic<T: Sized>(t: T) { - // --snip-- + --snip-- } ``` @@ -1539,7 +1534,7 @@ restriction: ``` fn generic<T: ?Sized>(t: &T) { - // --snip-- + --snip-- } ``` @@ -1564,14 +1559,14 @@ including function pointers and returning closures. We’ve talked about how to pass closures to functions; you can also pass regular functions to functions! This technique is useful when you want to pass a function you’ve already defined rather than defining a new closure. Functions -coerce to the type `fn` (with a lowercase f), not to be confused with the `Fn` -closure trait. The `fn` type is called a *function pointer*. Passing functions -with function pointers will allow you to use functions as arguments to other -functions. +coerce to the type `fn` (with a lowercase *f*), not to be confused with the +`Fn` closure trait. The `fn` type is called a *function pointer*. Passing +functions with function pointers will allow you to use functions as arguments +to other functions. The syntax for specifying that a parameter is a function pointer is similar to that of closures, as shown in Listing 19-27, where we’ve defined a function -`add_one` that adds one to its parameter. The function `do_twice` takes two +`add_one` that adds 1 to its parameter. The function `do_twice` takes two parameters: a function pointer to any function that takes an `i32` parameter and returns an `i32`, and one `i32 value`. The `do_twice` function calls the function `f` twice, passing it the `arg` value, then adds the two function call @@ -1592,7 +1587,7 @@ fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 { fn main() { let answer = do_twice(add_one, 5); - println!("The answer is: {}", answer); + println!("The answer is: {answer}"); } ``` @@ -1619,13 +1614,15 @@ functions can accept functions as arguments, but C doesn’t have closures. As an example of where you could use either a closure defined inline or a named function, let’s look at a use of the `map` method provided by the `Iterator` -trait in the standard library. To use the `map` function to turn a -vector of numbers into a vector of strings, we could use a closure, like this: +trait in the standard library. To use the `map` function to turn a vector of +numbers into a vector of strings, we could use a closure, like this: ``` let list_of_numbers = vec![1, 2, 3]; -let list_of_strings: Vec<String> = - list_of_numbers.iter().map(|i| i.to_string()).collect(); +let list_of_strings: Vec<String> = list_of_numbers + .iter() + .map(|i| i.to_string()) + .collect(); ``` Or we could name a function as the argument to `map` instead of the closure, @@ -1633,23 +1630,25 @@ like this: ``` let list_of_numbers = vec![1, 2, 3]; -let list_of_strings: Vec<String> = - list_of_numbers.iter().map(ToString::to_string).collect(); +let list_of_strings: Vec<String> = list_of_numbers + .iter() + .map(ToString::to_string) + .collect(); ``` -Note that we must use the fully qualified syntax that we talked about earlier -in the “Advanced Traits” section because there are multiple functions available +Note that we must use the fully qualified syntax that we talked about in +“Advanced Traits” on page XX because there are multiple functions available named `to_string`. -Here, we’re using the `to_string` function defined in the -`ToString` trait, which the standard library has implemented for any type that -implements `Display`. +Here, we’re using the `to_string` function defined in the `ToString` trait, +which the standard library has implemented for any type that implements +`Display`. -Recall from the “Enum values” section of Chapter 6 that the name of each enum -variant that we define also becomes an initializer function. We can use these -initializer functions as function pointers that implement the closure traits, -which means we can specify the initializer functions as arguments for methods -that take closures, like so: +Recall from “Enum Values” on page XX that the name of each enum variant that we +define also becomes an initializer function. We can use these initializer +functions as function pointers that implement the closure traits, which means +we can specify the initializer functions as arguments for methods that take +closures, like so: ``` enum Status { @@ -1657,12 +1656,14 @@ enum Status { Stop, } -let list_of_statuses: Vec<Status> = (0u32..20).map(Status::Value).collect(); +let list_of_statuses: Vec<Status> = (0u32..20) + .map(Status::Value) + .collect(); ``` -Here we create `Status::Value` instances using each `u32` value in the range +Here, we create `Status::Value` instances using each `u32` value in the range that `map` is called on by using the initializer function of `Status::Value`. -Some people prefer this style, and some people prefer to use closures. They +Some people prefer this style and some people prefer to use closures. They compile to the same code, so use whichever style is clearer to you. ### Returning Closures @@ -1689,10 +1690,14 @@ error[E0746]: return type cannot have an unboxed trait object --> src/lib.rs:1:25 | 1 | fn returns_closure() -> dyn Fn(i32) -> i32 { - | ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time + | ^^^^^^^^^^^^^^^^^^ doesn't have a size known at +compile-time | - = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits> -help: use `impl Fn(i32) -> i32` as the return type, as all return paths are of type `[closure@src/lib.rs:2:5: 2:14]`, which implements `Fn(i32) -> i32` + = note: for information on `impl Trait`, see +<https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that- +implement-traits> +help: use `impl Fn(i32) -> i32` as the return type, as all return paths are of +type `[closure@src/lib.rs:2:5: 2:14]`, which implements `Fn(i32) -> i32` | 1 | fn returns_closure() -> impl Fn(i32) -> i32 { | ~~~~~~~~~~~~~~~~~~~ @@ -1708,9 +1713,8 @@ fn returns_closure() -> Box<dyn Fn(i32) -> i32> { } ``` -This code will compile just fine. For more about trait objects, refer to the -section “Using Trait Objects That Allow for Values of Different Types” in -Chapter 17. +This code will compile just fine. For more about trait objects, refer to “Using +Trait Objects That Allow for Values of Different Types” on page XX. Next, let’s look at macros! @@ -1722,10 +1726,10 @@ of features in Rust: *declarative* macros with `macro_rules!` and three kinds of *procedural* macros: * Custom `#[derive]` macros that specify code added with the `derive` attribute - used on structs and enums +used on structs and enums * Attribute-like macros that define custom attributes usable on any item * Function-like macros that look like function calls but operate on the tokens - specified as their argument +specified as their argument We’ll talk about each of these in turn, but first, let’s look at why we even need macros when we already have functions. @@ -1740,7 +1744,7 @@ macros *expand* to produce more code than the code you’ve written manually. Metaprogramming is useful for reducing the amount of code you have to write and maintain, which is also one of the roles of functions. However, macros have -some additional powers that functions don’t. +some additional powers that functions don’t have. A function signature must declare the number and type of parameters the function has. Macros, on the other hand, can take a variable number of @@ -1760,14 +1764,14 @@ Another important difference between macros and functions is that you must define macros or bring them into scope *before* you call them in a file, as opposed to functions you can define anywhere and call anywhere. -### Declarative Macros with `macro_rules!` for General Metaprogramming +### Declarative Macros with macro_rules! for General Metaprogramming The most widely used form of macros in Rust is the *declarative macro*. These are also sometimes referred to as “macros by example,” “`macro_rules!` macros,” or just plain “macros.” At their core, declarative macros allow you to write something similar to a Rust `match` expression. As discussed in Chapter 6, `match` expressions are control structures that take an expression, compare the -resulting value of the expression to patterns, and then run the code associated +resultant value of the expression to patterns, and then run the code associated with the matching pattern. Macros also compare a value to patterns that are associated with particular code: in this situation, the value is the literal Rust source code passed to the macro; the patterns are compared with the @@ -1794,15 +1798,15 @@ Listing 19-28 shows a slightly simplified definition of the `vec!` macro. Filename: src/lib.rs ``` -[1] #[macro_export] -[2] macro_rules! vec { - [3] ( $( $x:expr ),* ) => { +1 #[macro_export] +2 macro_rules! vec { + 3 ( $( $x:expr ),* ) => { { let mut temp_vec = Vec::new(); - [4] $( - [5] temp_vec.push($x [6]); + 4 $( + 5 temp_vec.push(6 $x); )* - [7] temp_vec + 7 temp_vec } }; } @@ -1811,8 +1815,8 @@ Filename: src/lib.rs Listing 19-28: A simplified version of the `vec!` macro definition > Note: The actual definition of the `vec!` macro in the standard library -> includes code to preallocate the correct amount of memory up front. That code -> is an optimization that we don’t include here to make the example simpler. +includes code to pre-allocate the correct amount of memory up front. That code +is an optimization that we don’t include here, to make the example simpler. The `#[macro_export]` annotation [1] indicates that this macro should be made available whenever the crate in which the macro is defined is brought into @@ -1830,20 +1834,19 @@ is the only pattern in this macro, there is only one valid way to match; any other pattern will result in an error. More complex macros will have more than one arm. -Valid pattern syntax in macro definitions is different than the pattern syntax +Valid pattern syntax in macro definitions is different from the pattern syntax covered in Chapter 18 because macro patterns are matched against Rust code structure rather than values. Let’s walk through what the pattern pieces in Listing 19-28 mean; for the full macro pattern syntax, see the Rust Reference at *https://doc.rust-lang.org/reference/macros-by-example.html*. -First, we use a set of parentheses to encompass the whole pattern. We use a +First we use a set of parentheses to encompass the whole pattern. We use a dollar sign (`$`) to declare a variable in the macro system that will contain the Rust code matching the pattern. The dollar sign makes it clear this is a -macro variable as opposed to a regular Rust variable. -Next comes a set of parentheses that captures values that match the -pattern within the parentheses for use in the replacement code. Within `$()` is -`$x:expr`, which matches any Rust expression and gives the expression the name -`$x`. +macro variable as opposed to a regular Rust variable. Next comes a set of +parentheses that captures values that match the pattern within the parentheses +for use in the replacement code. Within `$()` is `$x:expr`, which matches any +Rust expression and gives the expression the name `$x`. The comma following `$()` indicates that a literal comma separator character could optionally appear after the code that matches the code in `$()`. The `*` @@ -1853,11 +1856,11 @@ When we call this macro with `vec![1, 2, 3];`, the `$x` pattern matches three times with the three expressions `1`, `2`, and `3`. Now let’s look at the pattern in the body of the code associated with this arm: -`temp_vec.push()` [5] within `$()*` [4][7] is generated for each part that -matches `$()` in the pattern zero or more times depending on how many times the -pattern matches. The `$x` [6] is replaced with each expression matched. When we -call this macro with `vec![1, 2, 3];`, the code generated that replaces this -macro call will be the following: +`temp_vec.push()` [5] within `$()* at [4] and [7] is generated for each part +that matches `$()` in the pattern zero or more times depending on how many +times the pattern matches. The `$x` [6] is replaced with each expression +matched. When we call this macro with `vec![1, 2, 3];`, the code generated that +replaces this macro call will be the following: ``` { @@ -1874,24 +1877,16 @@ generate code to create a vector containing the specified elements. To learn more about how to write macros, consult the online documentation or other resources, such as “The Little Book of Rust Macros” at -*https://veykril.github.io/tlborm/* started by Daniel Keep and continued by +*https://veykril.github.io/tlborm* started by Daniel Keep and continued by Lukas Wirth. -<!-- Not sure what "In the future, Rust will have a second kind of declarative -macro" means here. I suspect we're "stuck" with the two kinds of macros we -already have today, at least I don't see much energy in pushing to add a third -just yet. -/JT --> -<!-- Yeah, great catch, I think that part was back when we had more dreams that -have now been postponed/abandoned. I've removed. /Carol --> - ### Procedural Macros for Generating Code from Attributes -The second form of macros is the *procedural macro*, which acts more like a -function (and is a type of procedure). Procedural macros accept some code as an -input, operate on that code, and produce some code as an output rather than +The second form of macros is the procedural macro, which acts more like a +function (and is a type of procedure). *Procedural macros* accept some code as +an input, operate on that code, and produce some code as an output rather than matching against patterns and replacing the code with other code as declarative -macros do. The three kinds of procedural macros are custom derive, +macros do. The three kinds of procedural macros are custom `derive`, attribute-like, and function-like, and all work in a similar fashion. When creating procedural macros, the definitions must reside in their own crate @@ -1903,7 +1898,7 @@ macro variety. Filename: src/lib.rs ``` -use proc_macro; +use proc_macro::TokenStream; #[some_attribute] pub fn some_name(input: TokenStream) -> TokenStream { @@ -1922,20 +1917,20 @@ that specifies which kind of procedural macro we’re creating. We can have multiple kinds of procedural macros in the same crate. Let’s look at the different kinds of procedural macros. We’ll start with a -custom derive macro and then explain the small dissimilarities that make the +custom `derive` macro and then explain the small dissimilarities that make the other forms different. -### How to Write a Custom `derive` Macro +### How to Write a Custom derive Macro Let’s create a crate named `hello_macro` that defines a trait named `HelloMacro` with one associated function named `hello_macro`. Rather than making our users implement the `HelloMacro` trait for each of their types, we’ll provide a procedural macro so users can annotate their type with `#[derive(HelloMacro)]` to get a default implementation of the `hello_macro` -function. The default implementation will print `Hello, Macro! My name is -TypeName!` where `TypeName` is the name of the type on which this trait has -been defined. In other words, we’ll write a crate that enables another -programmer to write code like Listing 19-30 using our crate. +function. The default implementation will print `Hello, Macro! My name is` +TypeName`!` where TypeName is the name of the type on which this trait has been +defined. In other words, we’ll write a crate that enables another programmer to +write code like Listing 19-30 using our crate. Filename: src/main.rs @@ -2002,8 +1997,8 @@ name at runtime. We need a macro to generate code at compile time. The next step is to define the procedural macro. At the time of this writing, procedural macros need to be in their own crate. Eventually, this restriction might be lifted. The convention for structuring crates and macro crates is as -follows: for a crate named `foo`, a custom derive procedural macro crate is -called `foo_derive`. Let’s start a new crate called `hello_macro_derive` inside +follows: for a crate named foo, a custom `derive` procedural macro crate is +called foo`_derive`. Let’s start a new crate called `hello_macro_derive` inside our `hello_macro` project: ``` @@ -2095,11 +2090,11 @@ The `hello_macro_derive` function first converts the `input` from a operations on. This is where `syn` comes into play. The `parse` function in `syn` takes a `TokenStream` and returns a `DeriveInput` struct representing the parsed Rust code. Listing 19-32 shows the relevant parts of the `DeriveInput` -struct we get from parsing the `struct Pancakes;` string: +struct we get from parsing the `struct Pancakes;` string. ``` DeriveInput { - // --snip-- + --snip-- ident: Ident { ident: "Pancakes", @@ -2121,14 +2116,14 @@ Listing 19-32: The `DeriveInput` instance we get when parsing the code that has the macro’s attribute in Listing 19-30 The fields of this struct show that the Rust code we’ve parsed is a unit struct -with the `ident` (identifier, meaning the name) of `Pancakes`. There are more +with the `ident` (*identifier*, meaning the name) of `Pancakes`. There are more fields on this struct for describing all sorts of Rust code; check the `syn` documentation for `DeriveInput` at *https://docs.rs/syn/1.0/syn/struct.DeriveInput.html* for more information. Soon we’ll define the `impl_hello_macro` function, which is where we’ll build the new Rust code we want to include. But before we do, note that the output -for our derive macro is also a `TokenStream`. The returned `TokenStream` is +for our `derive` macro is also a `TokenStream`. The returned `TokenStream` is added to the code that our crate users write, so when they compile their crate, they’ll get the extra functionality that we provide in the modified `TokenStream`. @@ -2153,7 +2148,10 @@ fn impl_hello_macro(ast: &syn::DeriveInput) -> TokenStream { let gen = quote! { impl HelloMacro for #name { fn hello_macro() { - println!("Hello, Macro! My name is {}!", stringify!(#name)); + println!( + "Hello, Macro! My name is {}!", + stringify!(#name) + ); } } }; @@ -2166,7 +2164,7 @@ Listing 19-33: Implementing the `HelloMacro` trait using the parsed Rust code We get an `Ident` struct instance containing the name (identifier) of the annotated type using `ast.ident`. The struct in Listing 19-32 shows that when we run the `impl_hello_macro` function on the code in Listing 19-30, the -`ident` we get will have the `ident` field with a value of `"Pancakes"`. Thus, +`ident` we get will have the `ident` field with a value of `"Pancakes"`. Thus the `name` variable in Listing 19-33 will contain an `Ident` struct instance that, when printed, will be the string `"Pancakes"`, the name of the struct in Listing 19-30. @@ -2185,13 +2183,13 @@ introduction. We want our procedural macro to generate an implementation of our `HelloMacro` trait for the type the user annotated, which we can get by using `#name`. The -trait implementation has the one function `hello_macro`, whose body contains the -functionality we want to provide: printing `Hello, Macro! My name is` and then -the name of the annotated type. +trait implementation has the one function `hello_macro`, whose body contains +the functionality we want to provide: printing `Hello, Macro! My name is` and +then the name of the annotated type. The `stringify!` macro used here is built into Rust. It takes a Rust expression, such as `1 + 2`, and at compile time turns the expression into a -string literal, such as `"1 + 2"`. This is different than `format!` or +string literal, such as `"1 + 2"`. This is different from `format!` or `println!`, macros which evaluate the expression and then turn the result into a `String`. There is a possibility that the `#name` input might be an expression to print literally, so we use `stringify!`. Using `stringify!` also @@ -2203,7 +2201,7 @@ and `hello_macro_derive`. Let’s hook up these crates to the code in Listing your *projects* directory using `cargo new pancakes`. We need to add `hello_macro` and `hello_macro_derive` as dependencies in the `pancakes` crate’s *Cargo.toml*. If you’re publishing your versions of `hello_macro` and -`hello_macro_derive` to *https://crates.io/*, they would be regular +`hello_macro_derive` to *https://crates.io*, they would be regular dependencies; if not, you can specify them as `path` dependencies as follows: ``` @@ -2219,15 +2217,15 @@ should print `Hello, Macro! My name is Pancakes!` The implementation of the trait implementation. Next, let’s explore how the other kinds of procedural macros differ from custom -derive macros. +`derive` macros. -### Attribute-like macros +### Attribute-like Macros -Attribute-like macros are similar to custom derive macros, but instead of +Attribute-like macros are similar to custom `derive` macros, but instead of generating code for the `derive` attribute, they allow you to create new attributes. They’re also more flexible: `derive` only works for structs and enums; attributes can be applied to other items as well, such as functions. -Here’s an example of using an attribute-like macro: say you have an attribute +Here’s an example of using an attribute-like macro. Say you have an attribute named `route` that annotates functions when using a web application framework: ``` @@ -2240,7 +2238,10 @@ macro. The signature of the macro definition function would look like this: ``` #[proc_macro_attribute] -pub fn route(attr: TokenStream, item: TokenStream) -> TokenStream { +pub fn route( + attr: TokenStream, + item: TokenStream +) -> TokenStream { ``` Here, we have two parameters of type `TokenStream`. The first is for the @@ -2248,21 +2249,21 @@ contents of the attribute: the `GET, "/"` part. The second is the body of the item the attribute is attached to: in this case, `fn index() {}` and the rest of the function’s body. -Other than that, attribute-like macros work the same way as custom derive +Other than that, attribute-like macros work the same way as custom `derive` macros: you create a crate with the `proc-macro` crate type and implement a function that generates the code you want! -### Function-like macros +### Function-like Macros Function-like macros define macros that look like function calls. Similarly to `macro_rules!` macros, they’re more flexible than functions; for example, they -can take an unknown number of arguments. However, `macro_rules!` macros can be -defined only using the match-like syntax we discussed in the section -“Declarative Macros with `macro_rules!` for General Metaprogramming” earlier. -Function-like macros take a `TokenStream` parameter and their definition -manipulates that `TokenStream` using Rust code as the other two types of -procedural macros do. An example of a function-like macro is an `sql!` macro -that might be called like so: +can take an unknown number of arguments. However, `macro_rules!` macros can +only be defined using the match-like syntax we discussed in “Declarative Macros +with macro_rules! for General Metaprogramming” on page XX. Function-like macros +take a `TokenStream` parameter, and their definition manipulates that +`TokenStream` using Rust code as the other two types of procedural macros do. +An example of a function-like macro is an `sql!` macro that might be called +like so: ``` let sql = sql!(SELECT * FROM posts WHERE id=1); @@ -2277,32 +2278,19 @@ syntactically correct, which is much more complex processing than a pub fn sql(input: TokenStream) -> TokenStream { ``` -This definition is similar to the custom derive macro’s signature: we receive +This definition is similar to the custom `derive` macro’s signature: we receive the tokens that are inside the parentheses and return the code we wanted to generate. -<!-- I may get a few looks for this, but I wonder if we should trim the -procedural macros section above a bit. There's a lot of information in there, -but it feels like something we could intro and then point people off to other -materials for. Reason being (and I know I may be in the minority here), -procedural macros are something we should use only rarely in our Rust projects. -They are a burden on the compiler, have the potential to hurt readability and -maintainability, and... you know the saying with great power comes great -responsibilty and all that. /JT --> -<!-- I think we felt obligated to have this section when procedural macros were -introduced because there wasn't any documentation for them. I feel like the -custom derive is the most common kind people want to make... While I'd love to -not have to maintain this section, I asked around and people seemed generally -in favor of keeping it, so I think I will, for now. /Carol --> - ## Summary Whew! Now you have some Rust features in your toolbox that you likely won’t use often, but you’ll know they’re available in very particular circumstances. We’ve introduced several complex topics so that when you encounter them in -error message suggestions or in other peoples’ code, you’ll be able to +error message suggestions or in other people’s code, you’ll be able to recognize these concepts and syntax. Use this chapter as a reference to guide you to solutions. Next, we’ll put everything we’ve discussed throughout the book into practice and do one more project! + |