commit	9d15f7843f28d7ddcaaca8558b3c5680e2920cc2	[log] [tgz]
author	David Tolnay <dtolnay@gmail.com>	Sat Jan 19 14:09:23 2019 -0800
committer	David Tolnay <dtolnay@gmail.com>	Sat Jan 19 18:50:06 2019 -0800
tree	1ff93c83631b1707ac590d1463c3e205c6d7a414
parent	d13a84a83fe94b15dd46dc52748e0144a77348f3 [diff]

tree: 1ff93c83631b1707ac590d1463c3e205c6d7a414

README.md

Rust Quasi-Quoting

This crate provides the quote! macro for turning Rust syntax tree data structures into tokens of source code.

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

[dependencies]
quote = "0.6"

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.

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.

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).

Examples

Quoting other quotes

Often you don't want to write your whole TokenStream in one piece. The TokenStream produced by quote!{...} (syn::export::TokenStream2 not (!) proc_macro::TokenStream) implements ToTokens. Therefore it can be directly quoted.

let quote1 = quote! {...};
let quote2 = quote! {...};

let quote_combined = quote!{
    #quote1
    #quote2
};

Changing identifiers

Assuming you want to quote an identifier ident, but prepended with an underscore. If you naively do:

quote! {
    _#ident
}

If ident is foo this will lead to a space between the underscore and the identifier _ foo. You can create a new identifier that the compiler can trace back to the original identifier by creating a new identifier with the span of the previous one.

fn underscore_ident(ident: &syn::Ident) -> syn::Ident {
    syn::Ident::new(&format!("_{}", ident), ident.span())
}

which you can then use as:

let underscore_ident = underscore_ident(&ident);
quote! {
    #underscore_ident
}

Using `syn::Type`

Say the variable field_type contains the syn::Type of Vec<i32> of a struct_field from your derive macro. Using

quote!{
    let v: #field_type = some_collection.iter().collect();
}

will work. However if you want a new vector you'd usually use the turbofish operator Vec::<i32>::new(), e.g.

quote!{
    let v = #field_type::new();
}

This will expand to Vec<i32>::new(), so it won't work. However you can use the field_type directly in quote! when you use the fully qualified type notation.

quote! {
    let v = <#field_type>::new();
}

You can also use <Type as Trait> here, e.g. for the trait MyTrait you could do <#field_type as MyTrait<#field_type>>.

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().

A different span can be provided explicitly through the quote_spanned! macro.

Limitations

A non-repeating variable may not be interpolated inside of a repeating block (#7).
The same variable may not be interpolated more than once inside of a repeating block (#8).

Recursion limit

The quote! macro relies on deep recursion so some large invocations may fail with "recursion limit reached" when you compile. If it fails, bump up the recursion limit by adding #![recursion_limit = "128"] to your crate. An even higher limit may be necessary for especially large invocations. You don't need this unless the compiler tells you that you need it.

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

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.