Meta programming in Rust

Meta programming is a technique used in computer programming to write programs that generate other programs or manipulate existing programs at runtime. This technique can be used to make programs more flexible, easier to maintain, and more efficient. In Rust, meta programming is supported through a feature called macros.

Macros in Rust are a way of writing code that generates other code at compile time.

We divide them into two main categories:

1. Declarative macros

These are defined using the macro_rules! macro, which allows you to specify a pattern that will match against the input code, and a template that will be used to generate the output code.

Declarative macros can be defined only using match-like syntax:

At their core, declarative macros allow you to write something similar to a Rust match expression.[1]

For example, here's a simple declarative macro that generates a function that adds two numbers together:

macro_rules! add {
    ($x:expr, $y:expr) => {
        fn add() -> i32 {
            $x + $y
        }
    };
}

add!(3, 4);
let result = add();
assert_eq!(result, 7);

In this example, the add! macro takes two arguments, $x and $y, which are expressions that evaluate to integers. The macro generates a function that adds these two numbers together and returns the result.

To use the macro, we call it with the values 3 and 4, which are substituted for $x and $y, respectively. We then call the resulting function and verify that it returns the expected value of 7.

2. Procedural macros

Procedural macros have three kinds:

  • Custom #[derive] macros
  • Attribute-like macros
  • Function-like macros

The definition of a procedural macro takes a TokenStream as an input and produces a TokenStream as an output.

#[proc_macro]
pub fn my_macro(input: TokenStream) -> TokenStream {
    // do something with input and return
}

Macros can also be more complex and generate larger amounts of code. For example, the popular serde library for Rust uses macros to generate serialization and deserialization code for data structures.

#[derive(Serialize, Deserialize)]
struct Point {
    x: i32,
    y: i32,
}

In this example, the serde library provides two macros, Serialize and Deserialize, which are used to generate code that can serialize and deserialize the Point struct.

Meta programming in Rust can be a powerful tool for reducing code duplication and increasing the flexibility of your programs. However, it's important to use macros judiciously, as they can make your code harder to read and understand. When using macros, it's a good idea to write clear and well-documented code that explains how the macro works and what code it generates.

Syn & Quote crates

When it comes to creating macros in Rust, the syn and quote crates are indispensable tools that can help simplify the process.

The syn crate provides a parsing library that can be used to parse Rust code and represent it as a syntax tree, which can then be manipulated using Rust code. This can be incredibly useful when working with macros that need to analyze or modify Rust code.

For example, consider a macro that generates getters and setters for a struct's fields. With the syn crate, we can easily parse the struct's fields and generate code for the getters and setters:

use syn::{parse_macro_input, Data, DeriveInput};

#[proc_macro_derive(GetSet)]
pub fn get_set_derive(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let fields = match input.data {
        Data::Struct(ref data) => data.fields.iter(),
        _ => panic!("Can only derive GetSet on a struct"),
    };

    let getters = fields.map(|field| {
        let field_name = &field.ident;
        quote! {
            pub fn #field_name(&self) -> & #field.ty {
                &self.#field_name
            }
        }
    });

    let setters = fields.map(|field| {
        let field_name = &field.ident;
        quote! {
            pub fn set_#field_name(&mut self, val: #field.ty) {
                self.#field_name = val;
            }
        }
    });

    let output = quote! {
        impl GetSet for #input {
            #(#getters)*
            #(#setters)*
        }
    };

    output.into()
}

In this example, the parse_macro_input macro from the syn crate is used to parse the input DeriveInput struct. Then, we iterate over the fields of the struct and generate code for both the getters and setters using the quote crate. Finally, the generated code is assembled into an impl block and returned as a TokenStream.

The quote crate provides a convenient way to generate Rust code programmatically. It allows you to write Rust code as a template with placeholders that can be filled in with values at runtime. This makes it easy to generate code that closely matches the structure and syntax of Rust code.

Conclusion

In conclusion, Rust's support for meta programming through macros is a powerful feature that can help you write more flexible and efficient programs. By learning how to use macros effectively, you can reduce code duplication and increase the readability and maintainability of your code.

The syn and quote crates are essential tools for working with macros in Rust. They can help simplify the process of generating and manipulating Rust code, making it easier to write powerful and flexible macros. By using these tools effectively, you can create macros that are easy to read, maintain, and understand.

References