Rust Quasi-Quoting ================== [![Build Status](https://api.travis-ci.org/dtolnay/quote.svg?branch=master)](https://travis-ci.org/dtolnay/quote) [![Latest Version](https://img.shields.io/crates/v/quote.svg)](https://crates.io/crates/quote) [![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/quote/) This crate provides the [`quote!`] macro for turning Rust syntax tree data structures into tokens of source code. [`quote!`]: https://docs.rs/quote/1.0/quote/macro.quote.html Procedural macros in Rust receive a stream of tokens as input, execute arbitrary Rust code to determine how to manipulate those tokens, and produce a stream of tokens to hand back to the compiler to compile into the caller's crate. Quasi-quoting is a solution to one piece of that — producing tokens to return to the compiler. The idea of quasi-quoting is that we write *code* that we treat as *data*. Within the `quote!` macro, we can write what looks like code to our text editor or IDE. We get all the benefits of the editor's brace matching, syntax highlighting, indentation, and maybe autocompletion. But rather than compiling that as code into the current crate, we can treat it as data, pass it around, mutate it, and eventually hand it back to the compiler as tokens to compile into the macro caller's crate. This crate is motivated by the procedural macro use case, but is a general-purpose Rust quasi-quoting library and is not specific to procedural macros. *Version requirement: Quote supports any compiler version back to Rust's very first support for procedural macros in Rust 1.15.0.* [*Release notes*](https://github.com/dtolnay/quote/releases) ```toml [dependencies] quote = "1.0" ``` ## Syntax The quote crate provides a [`quote!`] macro within which you can write Rust code that gets packaged into a [`TokenStream`] and can be treated as data. You should think of `TokenStream` as representing a fragment of Rust source code. [`TokenStream`]: https://docs.rs/proc-macro2/1.0/proc_macro2/struct.TokenStream.html Within the `quote!` macro, interpolation is done with `#var`. Any type implementing the [`quote::ToTokens`] trait can be interpolated. This includes most Rust primitive types as well as most of the syntax tree types from [`syn`]. [`quote::ToTokens`]: https://docs.rs/quote/1.0/quote/trait.ToTokens.html [`syn`]: https://github.com/dtolnay/syn ```rust let tokens = quote! { struct SerializeWith #generics #where_clause { value: &'a #field_ty, phantom: core::marker::PhantomData<#item_ty>, } impl #generics serde::Serialize for SerializeWith #generics #where_clause { fn serialize(&self, serializer: S) -> Result where S: serde::Serializer, { #path(self.value, serializer) } } SerializeWith { value: #value, phantom: core::marker::PhantomData::<#item_ty>, } }; ``` ## Repetition Repetition is done using `#(...)*` or `#(...),*` similar to `macro_rules!`. This iterates through the elements of any variable interpolated within the repetition and inserts a copy of the repetition body for each one. The variables in an interpolation may be anything that implements `IntoIterator`, including `Vec` or a pre-existing iterator. - `#(#var)*` — no separators - `#(#var),*` — the character before the asterisk is used as a separator - `#( struct #var; )*` — the repetition can contain other things - `#( #k => println!("{}", #v), )*` — even multiple interpolations Note that there is a difference between `#(#var ,)*` and `#(#var),*`—the latter does not produce a trailing comma. This matches the behavior of delimiters in `macro_rules!`. ## Returning tokens to the compiler The `quote!` macro evaluates to an expression of type `proc_macro2::TokenStream`. Meanwhile Rust procedural macros are expected to return the type `proc_macro::TokenStream`. The difference between the two types is that `proc_macro` types are entirely specific to procedural macros and cannot ever exist in code outside of a procedural macro, while `proc_macro2` types may exist anywhere including tests and non-macro code like main.rs and build.rs. This is why even the procedural macro ecosystem is largely built around `proc_macro2`, because that ensures the libraries are unit testable and accessible in non-macro contexts. There is a [`From`]-conversion in both directions so returning the output of `quote!` from a procedural macro usually looks like `tokens.into()` or `proc_macro::TokenStream::from(tokens)`. [`From`]: https://doc.rust-lang.org/std/convert/trait.From.html ## Examples ### Combining quoted fragments Usually you don't end up constructing an entire final `TokenStream` in one piece. Different parts may come from different helper functions. The tokens produced by `quote!` themselves implement `ToTokens` and so can be interpolated into later `quote!` invocations to build up a final result. ```rust let type_definition = quote! {...}; let methods = quote! {...}; let tokens = quote! { #type_definition #methods }; ``` ### Constructing identifiers Suppose we have an identifier `ident` which came from somewhere in a macro input and we need to modify it in some way for the macro output. Let's consider prepending the identifier with an underscore. Simply interpolating the identifier next to an underscore will not have the behavior of concatenating them. The underscore and the identifier will continue to be two separate tokens as if you had written `_ x`. ```rust // incorrect quote! { let mut _#ident = 0; } ``` The solution is to build a new identifier token with the correct value. As this is such a common case, the `format_ident!` macro provides a convenient utility for doing so correctly. ```rust let varname = format_ident!("_{}", ident); quote! { let mut #varname = 0; } ``` Alternatively, the APIs provided by Syn and proc-macro2 can be used to directly build the identifier. This is roughly equivalent to the above, but will not handle `ident` being a raw identifier. ```rust let concatenated = format!("_{}", ident); let varname = syn::Ident::new(&concatenated, ident.span()); quote! { let mut #varname = 0; } ``` ### Making method calls Let's say our macro requires some type specified in the macro input to have a constructor called `new`. We have the type in a variable called `field_type` of type `syn::Type` and want to invoke the constructor. ```rust // incorrect quote! { let value = #field_type::new(); } ``` This works only sometimes. If `field_type` is `String`, the expanded code contains `String::new()` which is fine. But if `field_type` is something like `Vec` then the expanded code is `Vec::new()` which is invalid syntax. Ordinarily in handwritten Rust we would write `Vec::::new()` but for macros often the following is more convenient. ```rust quote! { let value = <#field_type>::new(); } ``` This expands to `>::new()` which behaves correctly. A similar pattern is appropriate for trait methods. ```rust quote! { let value = <#field_type as core::default::Default>::default(); } ``` ## Hygiene Any interpolated tokens preserve the `Span` information provided by their `ToTokens` implementation. Tokens that originate within a `quote!` invocation are spanned with [`Span::call_site()`]. [`Span::call_site()`]: https://docs.rs/proc-macro2/1.0/proc_macro2/struct.Span.html#method.call_site A different span can be provided explicitly through the [`quote_spanned!`] macro. [`quote_spanned!`]: https://docs.rs/quote/1.0/quote/macro.quote_spanned.html
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