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0.3.10 | Nov 26, 2016 |
#7 in Procedural macros
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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.
[dependencies]
quote = "1.0"
Version requirement: Quote supports rustc 1.56 and up.
Release notes
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 a Vec
, slice, BTreeSet
, or any 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
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.
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
.
// 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.
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.
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.
// 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.
quote! {
let value = <#field_type>::new();
}
This expands to <Vec<i32>>::new()
which behaves correctly.
A similar pattern is appropriate for trait methods.
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()
.
A different span can be provided explicitly through the quote_spanned!
macro.
Non-macro code generators
When using quote
in a build.rs or main.rs and writing the output out to a
file, consider having the code generator pass the tokens through prettyplease
before writing. This way if an error occurs in the generated code it is
convenient for a human to read and debug.
Be aware that no kind of hygiene or span information is retained when tokens are written to a file; the conversion from tokens to source code is lossy.
Example usage in build.rs:
let output = quote! { ... };
let syntax_tree = syn::parse2(output).unwrap();
let formatted = prettyplease::unparse(&syntax_tree);
let out_dir = env::var_os("OUT_DIR").unwrap();
let dest_path = Path::new(&out_dir).join("out.rs");
fs::write(dest_path, formatted).unwrap();
License
Licensed under either of Apache License, Version 2.0 or MIT license at your option.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.
Dependencies
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