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 stable or nightly Rust compiler 1.30+.

Pasting identifiers

There are two entry points, paste::expr! for macros in expression position and paste::item! for macros in item position.

Within either one, identifiers inside [<…>] are pasted together to form a single identifier.

// Macro in item position: at module scope or inside of an impl block.
paste::item! {
    // Defines a const called `QRST`.
    const [<Q R S T>]: &str = "success!";
}

fn main() {
    // Macro in expression position: inside a function body.
    assert_eq!(
        paste::expr! { [<Q R S T>].len() },
        8,
    );
}

More elaborate examples

This program demonstrates how you may want to bundle a paste invocation inside of a more convenient user-facing macro of your own. Here the routes!(A, B) macro expands to a vector containing ROUTE_A and ROUTE_B.

const ROUTE_A: &str = "/a";
const ROUTE_B: &str = "/b";

macro_rules! routes {
    ($($route:ident),*) => {{
        paste::expr! {
            vec![$( [<ROUTE_ $route>] ),*]
        }
    }}
}

fn main() {
    let routes = routes!(A, B);
    assert_eq!(routes, vec!["/a", "/b"]);
}

The next example shows a macro that generates accessor methods for some struct fields.

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::item! {
            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.

Macros

expr	Paste identifiers within a macro invocation that expands to an expression.
item	Paste identifiers within a macro invocation that expands to one or more items.
item_with_macros	Paste identifiers within a macro invocation that expands to one or more macro_rules macros or items containing macros.