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diff --git a/quote/README.md b/quote/README.md new file mode 100644 index 0000000..7c7f743 --- /dev/null +++ b/quote/README.md @@ -0,0 +1,237 @@ +Rust Quasi-Quoting +================== + +[](https://travis-ci.org/dtolnay/quote) +[](https://crates.io/crates/quote) +[](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<S>(&self, serializer: S) -> Result<S::Ok, S::Error> + 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<i32>` then the expanded code is `Vec<i32>::new()` which is invalid syntax. +Ordinarily in handwritten Rust we would write `Vec::<i32>::new()` but for macros +often the following is more convenient. + +```rust +quote! { + let value = <#field_type>::new(); +} +``` + +This expands to `<Vec<i32>>::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 + +<br> + +#### License + +<sup> +Licensed under either of <a href="LICENSE-APACHE">Apache License, Version +2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option. +</sup> + +<br> + +<sub> +Unless you explicitly state otherwise, any contribution intentionally submitted +for inclusion in this crate by you, as defined in the Apache-2.0 license, shall +be dual licensed as above, without any additional terms or conditions. +</sub> |