Crate paste

  

The nightly-only concat_idents! macro in the Rust standard library is notoriously underpowered in that its concatenated identifiers can only refer to existing items, they can never be used to define something new.

This crate provides a flexible way to paste together identifiers in a macro, including using pasted identifiers to define new items.

This approach works with any Rust compiler 1.31+.

Pasting identifiers

Within the paste! macro, identifiers inside [<…>] are pasted together to form a single identifier.

use paste::paste;

paste! {
    // Defines a const called `QRST`.
    const [<Q R S T>]: &str = "success!";
}

fn main() {
    assert_eq!(
        paste! { [<Q R S T>].len() },
        8,
    );
}

More elaborate example

The next example shows a macro that generates accessor methods for some struct fields. It demonstrates how you might find it useful to bundle a paste invocation inside of a macro_rules macro.

use paste::paste;

macro_rules! make_a_struct_and_getters {
    ($name:ident { $($field:ident),* }) => {
        // Define a struct. This expands to:
        //
        //     pub struct S {
        //         a: String,
        //         b: String,
        //         c: String,
        //     }
        pub struct $name {
            $(
                $field: String,
            )*
        }

        // Build an impl block with getters. This expands to:
        //
        //     impl S {
        //         pub fn get_a(&self) -> &str { &self.a }
        //         pub fn get_b(&self) -> &str { &self.b }
        //         pub fn get_c(&self) -> &str { &self.c }
        //     }
        paste! {
            impl $name {
                $(
                    pub fn [<get_ $field>](&self) -> &str {
                        &self.$field
                    }
                )*
            }
        }
    }
}

make_a_struct_and_getters!(S { a, b, c });

fn call_some_getters(s: &S) -> bool {
    s.get_a() == s.get_b() && s.get_c().is_empty()
}

Case conversion

Use $var:lower or $var:upper in the segment list to convert an interpolated segment to lower- or uppercase as part of the paste. For example, [<ld_ $reg:lower _expr>] would paste to ld_bc_expr if invoked with $reg=Bc.

Use $var:snake to convert CamelCase input to snake_case. Use $var:camel to convert snake_case to CamelCase. These compose, so for example $var:snake:upper would give you SCREAMING_CASE.

The precise Unicode conversions are as defined by str::to_lowercase and str::to_uppercase.

Pasting documentation strings

Within the paste! macro, arguments to a #[doc …] attribute are implicitly concatenated together to form a coherent documentation string.

use paste::paste;

macro_rules! method_new {
    ($ret:ident) => {
        paste! {
            #[doc = "Create a new `" $ret "` object."]
            pub fn new() -> $ret { todo!() }
        }
    };
}

pub struct Paste {}

method_new!(Paste);  // expands to #[doc = "Create a new `Paste` object"]

Macros

paste