## Procedural Macros

*Procedural macros* allow creating syntax extensions as execution of a function.
Procedural macros come in one of three flavors:

* [Function-like macros] - `custom!(...)`
* [Derive mode macros] - `#[derive(CustomMode)]`
* [Attribute macros] - `#[CustomAttribute]`

Procedural macros allow you to run code at compile time that operates over Rust
syntax, both consuming and producing Rust syntax. You can sort of think of
procedural macros as functions from an AST to another AST.

Procedural macros must be defined in a crate with the [crate type] of
`proc-macro`.

> **Note**: When using Cargo, Procedural macro crates are defined with the
> `proc-macro` key in your manfiest:
>
> ```toml
> [lib]
> proc-macro = true
> ```

As functions, they must either return syntax, panic, or loop endlessly. Returned
syntax either replaces or adds the syntax depending on the kind of procedural
macro. Panics are caught by the compiler and are turned into a compiler error.
Endless loops are not caught by the compiler which hangs the compiler.

Procedural macros run during compilation, and thus have the same resources that
the compiler has. For example, standard input, error, and output are the same
that the compiler has access to. Similarly, file access is the same. Because
of this, procedural macros have the same security concerns that [Cargo's
build scripts] have.

Procedural macros have two ways of reporting errors. The first is to panic. The
second is to emit a [`compile_error`] macro invocation.

### The `proc_macro` crate

Procedural macro crates almost always will link to the compiler-provided 
[`proc_macro` crate]. The `proc_macro` crate provides types required for
writing procedural macros and facilities to make it easier.

This crate primarily contains a [`TokenStream`] type. Procedural macros operate
over *token streams* instead of AST nodes, which is a far more stable interface
over time for both the compiler and for procedural macros to target. A
*token stream* is roughly equivalent to `Vec<TokenTree>` where a `TokenTree`
can roughly be thought of as lexical token. For example `foo` is an `Ident`
token, `.` is a `Punct` token, and `1.2` is a `Literal` token. The `TokenStream`
type, unlike `Vec<TokenTree>`, is cheap to clone.

All tokens have an associated `Span`. A `Span` is an opaque value that cannot
be modified but can be manufactured. `Span`s represent an extent of source
code within a program and are primarily used for error reporting. You can modify
the `Span` of any token.

### Procedural macro hygiene

Procedural macros are *unhygienic*. This means they behave as if the output
token stream was simply written inline to the code it's next to. This means that
it's affected by external items and also affects external imports.

Macro authors need to be careful to ensure their macros work in as many contexts
as possible given this limitation. This often includes using absolute paths to
items in libraries (for example, `::std::option::Option` instead of `Option`) or
by ensuring that generated functions have names that are unlikely to clash with
other functions (like `__internal_foo` instead of `foo`).

### Function-like procedural macros

*Function-like procedural macros* are procedural macros that are invoked using
the macro invocation operator (`!`).

These macros are defined by a [public] [function] with the `proc_macro`
[attribute] and a signature of `(TokenStream) -> TokenStream`. The input
[`TokenStream`] is what is inside the delimiters of the macro invocation and the
output [`TokenStream`] replaces the entire macro invocation. It may contain an
arbitrary number of [items]. These macros cannot expand to syntax that defines
new `macro_rule` style macros.

For example, the following macro definition ignores its input and outputs a
function `answer` into its scope.

```rust,ignore
extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro]
pub fn make_answer(_item: TokenStream) -> TokenStream {
    "fn answer() -> u32 { 42 }".parse().unwrap()
}
```

And then we use it a binary crate to print "42" to standard output.

```rust,ignore
extern crate proc_macro_examples;
use proc_macro_examples::make_answer;

make_answer!();

fn main() {
    println!("{}", answer());
}
```

These macros are only invokable in [modules]. They cannot even be invoked to
create [item declaration statements]. Furthermore, they must either be invoked
with curly braces and no semicolon or a different delimiter followed by a
semicolon. For example, `make_answer` from the previous example can be invoked
as `make_answer!{}`, `make_answer!();` or `make_answer![];`.

### Derive mode macros

*Derive mode macros* define new modes for the `derive` [attribute]. These macros
define new [items] given the token stream of a [struct], [enum], or [union]. 
They also define [derive mode helper attributes].

Custom deriver modes are defined by a [public] [function] with the
`proc_macro_derive` attribute and a signature of `(TokenStream) -> TokenStream`.

The input [`TokenStream`] is the token stream of the item that has the `derive`
attribute on it. The output [`TokenStream`] must be a set of items that are
then appended to the [module] or [block] that the item from the input
[`TokenStream`] is in.

The following is an example of a derive mode macro. Instead of doing anything
useful with its input, it just appends a function `answer`.

```rust,ignore
extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_derive(AnswerFn)]
pub fn derive_answer_fn(_item: TokenStream) -> TokenStream {
    "fn answer() -> u32 { 42 }".parse().unwrap()
}
```

And then using said derive mode:

```rust,ignore
extern crate proc_macro_examples;
use proc_macro_examples::AnswerFn;

#[derive(AnswerFn)]
struct Struct;

fn main() {
    assert_eq!(42, answer());
}
```

#### Derive mode helper attributes

Derive mode macros can add additional [attributes] into the scope of the [item]
they are on. Said attributes are called *derive mode helper attributes*. These
attributes are [inert], and their only purpose is to be fed into the derive
mode macro that defined them. That said, they can be seen by all macros.

The way to define helper attributes is to put an `attributes` key in the
`proc_macro_derive` macro with a comma separated list of identifiers that are
the names of the helper attributes.

For example, the following derive mode macro defines a helper attribute
`helper`, but ultimately doesn't do anything with it.

```rust,ignore
# #[crate_type="proc-macro"]
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_derive(HelperAttr, attributes(helper))]
pub fn derive_helper_attr(_item: TokenStream) -> TokenStream {
    TokenStream::new();
}
```

And then usage on the derive mode on a struct:

```rust,ignore
# #![crate_type="proc-macro"]
# extern crate proc_macro_examples;
# use proc_macro_examples::HelperAttr;

#[derive(HelperAttr)]
struct Struct {
    #[helper] field: ()
}
```

### Attribute macros

*Attribute macros* define new [attributes] which can be attached to [items].

Attribute macros are defined by a [public] [function] with the
`proc_macro_attribute` [attribute] that a signature of 
`(TokenStream, TokenStream) -> TokenStream`. The first [`TokenStream`] is the
attribute's metaitems, not including the delimiters. If the attribute is written
without a metaitem, the attribute [`TokenStream`] is empty. The second
[`TokenStream`] is of the rest of the [item] including other [attributes] on the
[item]. The returned [`TokenStream`] replaces the [item] with an arbitrary
number of [items]. These macros cannot expand to syntax that defines new
`macro_rule` style macros.

For example, this attribute macro takes the input stream and returns it as is,
effectively being the no-op of attributes.

```rust,ignore
# #![crate_type = "proc-macro"]
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn return_as_is(_attr: TokenStream, item: TokenStream) -> TokenStream {
    item
}
```

This following example shows the stringified [`TokenStream`s] that the attribute
macros see. The output will show in the output of the compiler. The output is
shown in the comments after the function prefixed with "out:".

```rust,ignore
// my-macro/src/lib.rs
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn show_streams(attr: TokenStream, input: TokenStream) -> TokenStream {
    println!("attr: \"{}\"", attr.to_string());
    println!("item: \"{}\"', input.to_string());
    item
}
```

```rust,ignore
// src/lib.rs
extern crate my_macro;

use my_macro::show_streams;

// Example: Basic function
#[show_streams]
fn invoke1() {}
// out: attr: ""
// out: item: "fn invoke1() { }"

// Example: Attribute has a metaitem
#[show_streams(bar)]
fn invoke2() {}
// out: attr: "bar"
// out: item: "fn invoke2() {}"

// Example: Multiple words in metaitem
#[show_streams(multiple words)]
fn invoke3() {}
// out: attr: "multiple words"
// out: item: "fn invoke3() {}"

// Example: 
#[show_streams { delimiters }]
fn invoke4() {}
// out: "delimiters"
// out: "fn invoke4() {}"
```

[`TokenStream`]: ../proc_macro/struct.TokenStream.html
[`TokenStream`s]: ../proc_macro/struct.TokenStream.html
[`compile_error`]: ../std/macro.compile_error.html
[`derive`]: attributes.html#derive
[`proc_macro` crate]: ../proc_macro/index.html
[Cargo's build scripts]: ../cargo/reference/build-scripts.html
[Derive mode macros]: #derive-mode-macros
[Attribute macros]: #attribute-macros
[Function-like macros]: #function-like-procedural-macros
[attribute]: attributes.html
[attributes]: attributes.html
[custom attributes]: attributes.html
[crate type]: linkage.html
[derive mode helper attributes]: #derive-mode-helper-attributes
[enum]: items/enumerations.html
[inert]: attributes.html#active-and-inert-attributes
[item]: items.html
[item declaration statements]: statements.html#item-declarations
[items]: items.html
[function]: items/functions.html
[macro]: macros.html
[module]: items/modules.html
[modules]: items/modules.html
[procedural macro tutorial]: ../book/2018-edition/appendix-04-macros.html#procedural-macros-for-custom-derive
[public]: visibility-and-privacy.html
[struct]: items/structs.html
[unions]: items/unions.html
