# paste - Rust
github crates-io docs-rs
Keywords: rust, rustlang, rust-lang, paste

# Crate paste [−] [src]

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.