## Macros

We’ve used macros like `println!` throughout this book, but we haven’t fully
explored what a macro is and how it works. The term *macro* refers to a family
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
* 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

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.

### The Difference Between Macros and Functions

Fundamentally, macros are a way of writing code that writes other code, which
is known as *metaprogramming*. In Appendix C, we discuss the `derive`
attribute, which generates an implementation of various traits for you. We’ve
also used the `println!` and `vec!` macros throughout the book. All of these
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.

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
parameters: we can call `println!("hello")` with one argument or
`println!("hello {}", name)` with two arguments. Also, macros are expanded
before the compiler interprets the meaning of the code, so a macro can, for
example, implement a trait on a given type. A function can’t, because it gets
called at runtime and a trait needs to be implemented at compile time.

The downside to implementing a macro instead of a function is that macro
definitions are more complex than function definitions because you’re writing
Rust code that writes Rust code. Due to this indirection, macro definitions are
generally more difficult to read, understand, and maintain than function
definitions.

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

The most widely used form of macros in Rust is *declarative macros*. 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
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
structure of that source code; and the code associated with each pattern, when
matched, replaces the code passed to the macro. This all happens during
compilation.

To define a macro, you use the `macro_rules!` construct. Let’s explore how to
use `macro_rules!` by looking at how the `vec!` macro is defined. Chapter 8
covered how we can use the `vec!` macro to create a new vector with particular
values. For example, the following macro creates a new vector containing three
integers:

```rust
let v: Vec<u32> = vec![1, 2, 3];
```

We could also use the `vec!` macro to make a vector of two integers or a vector
of five string slices. We wouldn’t be able to use a function to do the same
because we wouldn’t know the number or type of values up front.

Listing 19-28 shows a slightly simplified definition of the `vec!` macro.

Filename: src/lib.rs

```rust
#[macro_export]
macro_rules! vec {
    ( $( $x:expr ),* ) => {
        {
            let mut temp_vec = Vec::new();
            $(
                temp_vec.push($x);
            )*
            temp_vec
        }
    };
}
```

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.

The `#[macro_export]` annotation indicates that this macro should be made
available whenever the crate in which the macro is defined is brought into
scope. Without this annotation, the macro can’t be brought into scope.

We then start the macro definition with `macro_rules!` and the name of the
macro we’re defining *without* the exclamation mark. The name, in this case
`vec`, is followed by curly brackets denoting the body of the macro definition.

The structure in the `vec!` body is similar to the structure of a `match`
expression. Here we have one arm with the pattern `( $( $x:expr ),* )`,
followed by `=>` and the block of code associated with this pattern. If the
pattern matches, the associated block of code will be emitted. Given that this
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
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 reference].

[the reference]: ../reference/macros.html

First, a set of parentheses encompasses the whole pattern. A dollar sign (`$`)
is next, followed by a set of parentheses that captures values that match the
pattern within the parentheses for use in the replacement code. Within `$()` 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 `*`
specifies that the pattern matches zero or more of whatever precedes the `*`.

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()` within `$()*` is generated for each part that matches `$()`
in the pattern zero or more times depending on how many times the pattern
matches. The `$x` 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:

```rust,ignore
let mut temp_vec = Vec::new();
temp_vec.push(1);
temp_vec.push(2);
temp_vec.push(3);
temp_vec
```

We’ve defined a macro that can take any number of arguments of any type and can
generate code to create a vector containing the specified elements.

There are some strange edge cases with `macro_rules!`. In the future, Rust will
have a second kind of declarative macro that will work in a similar fashion but
fix some of these edge cases. After that update, `macro_rules!` will be
effectively deprecated. With this in mind, as well as the fact that most Rust
programmers will *use* macros more than *write* macros, we won’t discuss
`macro_rules!` any further. To learn more about how to write macros, consult
the online documentation or other resources, such as [“The Little Book of Rust
Macros”][tlborm].

[tlborm]: https://danielkeep.github.io/tlborm/book/index.html

### Procedural Macros for Generating Code from Attributes

The second form of macros is *procedural macros*, which act more like functions
(and are 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 (custom derive, attribute-like, and
function-like) all work in a similar fashion.

When creating procedural macros, the definitions must reside in their own crate
with a special crate type. This is for complex technical reasons that we hope
to eliminate in the future. Using procedural macros looks like the code in
Listing 19-29, where `some_attribute` is a placeholder for using a specific
macro.

Filename: src/lib.rs

```rust,ignore
use proc_macro;

#[some_attribute]
pub fn some_name(input: TokenStream) -> TokenStream {
}
```

Listing 19-29: An example of using a procedural
macro

The function that defines a procedural macro takes a `TokenStream` as an input
and produces a `TokenStream` as an output. The `TokenStream` type is defined by
the `proc_macro` crate that is included with Rust and represents a sequence of
tokens. This is the core of the macro: the source code that the macro is
operating on makes up the input `TokenStream`, and the code the macro produces
is the output `TokenStream`. The function also has an attribute attached to it
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
other forms different.

### 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 crate 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.

Filename: src/main.rs

```rust,ignore
use hello_macro::HelloMacro;
use hello_macro_derive::HelloMacro;

#[derive(HelloMacro)]
struct Pancakes;

fn main() {
    Pancakes::hello_macro();
}
```

Listing 19-30: The code a user of our crate will be able
to write when using our procedural macro

This code will print `Hello, Macro! My name is Pancakes!` when we’re done. The
first step is to make a new library crate, like this:

```text
$ cargo new hello_macro --lib
```

Next, we’ll define the `HelloMacro` trait and its associated function:

Filename: src/lib.rs

```rust
pub trait HelloMacro {
    fn hello_macro();
}
```

We have a trait and its function. At this point, our crate user could implement
the trait to achieve the desired functionality, like so:

```rust,ignore
use hello_macro::HelloMacro;

struct Pancakes;

impl HelloMacro for Pancakes {
    fn hello_macro() {
        println!("Hello, Macro! My name is Pancakes!");
    }
}

fn main() {
    Pancakes::hello_macro();
}
```

However, they would need to write the implementation block for each type they
wanted to use with `hello_macro`; we want to spare them from having to do this
work.

Additionally, we can’t yet provide the `hello_macro` function with default
implementation that will print the name of the type the trait is implemented
on: Rust doesn’t have reflection capabilities, so it can’t look up the type’s
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
our `hello_macro` project:

```text
$ cargo new hello_macro_derive --lib
```

Our two crates are tightly related, so we create the procedural macro crate
within the directory of our `hello_macro` crate. If we change the trait
definition in `hello_macro`, we’ll have to change the implementation of the
procedural macro in `hello_macro_derive` as well. The two crates will need to
be published separately, and programmers using these crates will need to add
both as dependencies and bring them both into scope. We could instead have the
`hello_macro` crate use `hello_macro_derive` as a dependency and reexport the
procedural macro code. However, the way we’ve structured the project makes it
possible for programmers to use `hello_macro` even if they don’t want the
`derive` functionality.

We need to declare the `hello_macro_derive` crate as a procedural macro crate.
We’ll also need functionality from the `syn` and `quote` crates, as you’ll see
in a moment, so we need to add them as dependencies. Add the following to the
*Cargo.toml* file for `hello_macro_derive`:

Filename: hello_macro_derive/Cargo.toml

```toml
[lib]
proc-macro = true

[dependencies]
syn = "0.14.4"
quote = "0.6.3"
```

To start defining the procedural macro, place the code in Listing 19-31 into
your *src/lib.rs* file for the `hello_macro_derive` crate. Note that this code
won’t compile until we add a definition for the `impl_hello_macro` function.

Filename: hello_macro_derive/src/lib.rs

```rust,ignore
extern crate proc_macro;

use crate::proc_macro::TokenStream;
use quote::quote;
use syn;

#[proc_macro_derive(HelloMacro)]
pub fn hello_macro_derive(input: TokenStream) -> TokenStream {
    // Construct a representation of Rust code as a syntax tree
    // that we can manipulate
    let ast = syn::parse(input).unwrap();

    // Build the trait implementation
    impl_hello_macro(&ast)
}
```

Listing 19-31: Code that most procedural macro crates
will require in order to process Rust code

Notice that we’ve split the code into the `hello_macro_derive` function, which
is responsible for parsing the `TokenStream`, and the `impl_hello_macro`
function, which is responsible for transforming the syntax tree: this makes
writing a procedural macro more convenient. The code in the outer function
(`hello_macro_derive` in this case) will be the same for almost every
procedural macro crate you see or create. The code you specify in the body of
the inner function (`impl_hello_macro` in this case) will be different
depending on your procedural macro’s purpose.

We’ve introduced three new crates: `proc_macro`, [`syn`], and [`quote`]. The
`proc_macro` crate comes with Rust, so we didn’t need to add that to the
dependencies in *Cargo.toml*. The `proc_macro` crate is the compiler’s API that
allows us to read and manipulate Rust code from our code.

[`syn`]: https://crates.io/crates/syn
[`quote`]: https://crates.io/crates/quote

The `syn` crate parses Rust code from a string into a data structure that we
can perform operations on. The `quote` crate turns `syn` data structures back
into Rust code. These crates make it much simpler to parse any sort of Rust
code we might want to handle: writing a full parser for Rust code is no simple
task.

The `hello_macro_derive` function will be called when a user of our library
specifies `#[derive(HelloMacro)]` on a type. This is possible because we’ve
annotated the `hello_macro_derive` function here with `proc_macro_derive` and
specified the name, `HelloMacro`, which matches our trait name; this is the
convention most procedural macros follow.

The `hello_macro_derive` function first converts the `input` from a
`TokenStream` to a data structure that we can then interpret and perform
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:

```rust,ignore
DeriveInput {
    // --snip--

    ident: Ident {
        ident: "Pancakes",
        span: #0 bytes(95..103)
    },
    data: Struct(
        DataStruct {
            struct_token: Struct,
            fields: Unit,
            semi_token: Some(
                Semi
            )
        }
    )
}
```

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
fields on this struct for describing all sorts of Rust code; check the [`syn`
documentation for `DeriveInput`][syn-docs] for more information.

[syn-docs]: https://docs.rs/syn/0.14.4/syn/struct.DeriveInput.html

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
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`.

You might have noticed that we’re calling `unwrap` to cause the
`hello_macro_derive` function to panic if the call to the `syn::parse` function
fails here. It’s necessary for our procedural macro to panic on errors because
`proc_macro_derive` functions must return `TokenStream` rather than `Result` to
conform to the procedural macro API. We’ve simplified this example by using
`unwrap`; in production code, you should provide more specific error messages
about what went wrong by using `panic!` or `expect`.

Now that we have the code to turn the annotated Rust code from a `TokenStream`
into a `DeriveInput` instance, let’s generate the code that implements the
`HelloMacro` trait on the annotated type, as shown in Listing 19-33.

Filename: hello_macro_derive/src/lib.rs

```rust,ignore
fn impl_hello_macro(ast: &syn::DeriveInput) -> TokenStream {
    let name = &ast.ident;
    let gen = quote! {
        impl HelloMacro for #name {
            fn hello_macro() {
                println!("Hello, Macro! My name is {}", stringify!(#name));
            }
        }
    };
    gen.into()
}
```

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,
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.

The `quote!` macro lets us define the Rust code that we want to return. The
compiler expects something different to the direct result of the `quote!`
macro’s execution, so we need to convert it to a `TokenStream`. We do this by
calling the `into` method, which consumes this intermediate representation and
returns a value of the required `TokenStream` type.

The `quote!` macro also provides some very cool templating mechanics: we can
enter `#name`, and `quote!` will replace it with the value in the variable
`name`. You can even do some repetition similar to the way regular macros work.
Check out [the `quote` crate’s docs][quote-docs] for a thorough introduction.

[quote-docs]: https://docs.rs/quote

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 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
`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
saves an allocation by converting `#name` to a string literal at compile time.

At this point, `cargo build` should complete successfully in both `hello_macro`
and `hello_macro_derive`. Let’s hook up these crates to the code in Listing
19-30 to see the procedural macro in action! Create a new binary project in
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 [crates.io](https://crates.io/), they would be regular
dependencies; if not, you can specify them as `path` dependencies as follows:

```toml
[dependencies]
hello_macro = { path = "../hello_macro" }
hello_macro_derive = { path = "../hello_macro/hello_macro_derive" }
```

Put the code in Listing 19-30 into *src/main.rs*, and run `cargo run`: it
should print `Hello, Macro! My name is Pancakes!` The implementation of the
`HelloMacro` trait from the procedural macro was included without the
`pancakes` crate needing to implement it; the `#[derive(HelloMacro)]` added the
trait implementation.

Next, let’s explore how the other kinds of procedural macros differ from custom
derive macros.

### Attribute-like macros

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
named `route` that annotates functions when using a web application framework:

```rust,ignore
#[route(GET, "/")]
fn index() {
```

This `#[route]` attribute would be defined by the framework as a procedural
macro. The signature of the macro definition function would look like this:

```rust,ignore
#[proc_macro_attribute]
pub fn route(attr: TokenStream, item: TokenStream) -> TokenStream {
```

Here, we have two parameters of type `TokenStream`. The first is for the
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
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 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”][decl]
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:

[decl]: #declarative-macros-with-macro_rules-for-general-metaprogramming

```rust,ignore
let sql = sql!(SELECT * FROM posts WHERE id=1);
```

This macro would parse the SQL statement inside it and check that it’s
syntactically correct, which is much more complex processing than a
`macro_rules!` macro can do. The `sql!` macro would be defined like this:

```rust,ignore
#[proc_macro]
pub fn sql(input: TokenStream) -> TokenStream {
```

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.

## Summary

Whew! Now you have some Rust features in your toolbox that you 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 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!
