The nightly-onlyconcat_idents! macro in the Rust standard library isnotoriously underpowered in that its concatenated identifiers can only refer toexisting 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 thepaste! macro, identifiers inside[<…>] are pastedtogether to form a single identifier.

usepaste::paste;paste! {// Defines a const called `QRST`.const[<Q R S T>]:&str ="success!";}fnmain() {assert_eq!(paste! { [<Q R S T>].len() },8,    );}

§More elaborate example

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

usepaste::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 });fncall_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 aninterpolated segment to lower- or uppercase as part of the paste. Forexample,[<ld_ $reg:lower _expr>] would paste told_bc_expr if invokedwith $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 bystr::to_lowercase andstr::to_uppercase.

§Pasting documentation strings

Within thepaste! macro, arguments to a #[doc …] attribute areimplicitly concatenated together to form a coherent documentation string.

usepaste::paste;macro_rules! method_new {    ($ret:ident) => {paste! {#[doc ="Create a new `"$ret"` object."]pub fnnew() ->$ret{todo!() }        }    };}pub structPaste {}method_new!(Paste);// expands to #[doc = "Create a new `Paste` object"]