## 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 macros - `#[derive(CustomDerive)]`
- 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 manifest:

>     [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.

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

    #![crate_type = "proc-macro"]
    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.

    extern crate proc_macro_examples;
    use proc_macro_examples::make_answer;

    make_answer!();

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

Function-like procedural macros may expand to a type or any number of items, including `macro_rules` definitions. They may be invoked in a type expression, item position (except as a statement), including items in `extern` blocks, inherent and trait implementations, and trait definitions. They cannot be used in a statement, expression, or pattern.

### Derive macros

Derive macros define new inputs for the `derive` attribute. These macros can create new items given the token stream of a struct, enum, or union. They can also define derive macro helper attributes.

Custom derive macros 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 macro. Instead of doing anything useful with its input, it just appends a function `answer`.

    #![crate_type = "proc-macro"]
    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 macro:

    extern crate proc_macro_examples;
    use proc_macro_examples::AnswerFn;

    #[derive(AnswerFn)]
    struct Struct;

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

#### Derive macro helper attributes

Derive macros can add additional attributes into the scope of the item they are on. Said attributes are called derive macro helper attributes. These attributes are inert, and their only purpose is to be fed into the derive 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 macro defines a helper attribute `helper`, but ultimately doesn't do anything with it.

    #![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 macro on a struct:

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

### Attribute macros

Attribute macros define new outer attributes which can be attached to items, including items in `extern` blocks, inherent and trait implementations, and trait definitions.

Attribute macros are defined by a public function with the `proc_macro_attribute` attribute that has a signature of `(TokenStream, TokenStream) -> TokenStream`. The first `TokenStream` is the delimited token tree following the attribute's name, not including the outer delimiters. If the attribute is written as a bare attribute name, the attribute `TokenStream` is empty. The second `TokenStream` is the rest of the item including other attributes on the item. The returned `TokenStream` replaces the item with an arbitrary number of items.

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

    #![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:".

    // my-macro/src/lib.rs
    extern crate proc_macro;
    use proc_macro::TokenStream;

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

    // 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 with input
    #[show_streams(bar)]
    fn invoke2() {}
    // out: attr: "bar"
    // out: item: "fn invoke2() {}"

    // Example: Multiple tokens in the input
    #[show_streams(multiple => tokens)]
    fn invoke3() {}
    // out: attr: "multiple => tokens"
    // out: item: "fn invoke3() {}"

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