Rust Quasiquoter
This crate implements the quote!
macro as a procedural macro, instead of
the original quote!
macro, implemented
with macro_rules!
.
The quote!
macro turns 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 it is a general-purpose Rust quasi-quoting library and is not specific to procedural macros.
Why use proc-quote?
This crate serves the same purpose as quote
however it is implemented with procedural macros rather than macro_rules!
. Switching
from quote
to the proc_quote
crate should not require any change in the code in most cases.
Besides, opting to proc_quote
has the advantage of lifting some of the limitations
of the original quasi-quoting crate:
- Interpolate a
ToTokens
variable inside of a repeating block - Interpolate the same
Iterator
more than once inside of the same repeating block - Use
Vec
, arrays and slices in a repeating block without consuming the variable
Interpolate a ToTokens
variable inside of a repeating block
In proc_quote
, this is now possible:
let i = 1..5; // Iterator<ToTokens>
let a = "a"; // ToTokens
let q = quote! ;
assert_eq!;
You may interpolate variables implementing ToTokens
inside repeating blocks.
The variable will be interpolated in every iteration.
Interpolate the same Iterator
more than once inside of the same repeating block
In the original quote!
macro, the same iterator could not be interpolated more than
once in repeating blocks. In other words, for a: impl Iterator<ToTokens>
,
quote!( #(#a #a)* )
didn't work.
This is fixed in proc-quote
.
Use Vec
, arrays and slices in a repeating block without consuming the variable
In proc-quote
, any type that implements Borrow<[T]>
inside a repeating pattern
will iterate without consuming the variable (using the .iter
method). This includes
Vec
, arrays and slices.
This means you no longer need to create ten different variables to iterate the same
elements in ten different places in your quote!
macros.
From quote
to proc-quote
If you are already using the quote
crate but you want to leverage the advantages of
proc-quote
, you can easily switch crates.
After changing your Cargo.toml
dependency, change the following:
extern crate quote;
use quote;
use quote_spanned;
respectively into:
extern crate proc_quote;
use quote;
use quote_spanned;
Note: You may also remove #![recursion_limit="..."]
as it is not needed in the
procedural macro.
And that's basically it, most of the times!
However, repeating patterns #(...)*
do NOT work directly with IntoIterator
in proc-quote
(see Repeat
trait). If your type was already an Iterator
or any
type implementing
Borrow<[T]>
(such as Vec
, slices, arrays, ...), your code will work the same
as in quote.
On the other hand, cases like TokenStream
(implements IntoIterator
but it is
not any of the cases stated above) must be explicitely turned into iterators before
being interpolated. That is, if you had:
let a: TokenStream = /* ... */;
let q = quote!;
You must change it to:
let a: TokenStream = /* ... */;
let a = a.into_iter;
let q = quote!;
This explicitness is necessary because of proc-quote
's new features. In this example,
TokenStream
would have be seen as a type implementing ToTokens
instead of an Iterator
of tokens, which makes sense because it is much more frequently used as the former when
interpolated.
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! ;
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.
#(#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#(let #var = self.#var;)*
- the same variable can be used more than once
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!
.
The proc_quote::Repeat
trait defines which types are allowed to be interpolated inside a repition pattern.
Which types do Repeat
:
Iterator<T>
consumes the iterator, iterating through every element.Borrow<[T]>
(includesVec
,array
, andslice
) iterates with theslice::iter
method, thus not consuming the original data.ToTokens
, interpolates the variable in every iteration.
Which types do NOT Repeat
:
IntoIterator
, to avoid ambiguity (Ex. "Which behavior would have been used forVec
, which implements bothIntoIterator
andBorrow<[T]>
?"; "Which behavior would have been used forTokenStream
, which implements bothIntoIterator
andToTokens
?"). To use the iterator, you may callIntoIterator::into_iter
explicitly.- Ambiguous types that implement at least two of the
Repeat
traits. In the very unlikely case this happens, disambiguate the type by wrapping it under some structure that only implements the trait you desire to use.
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! ;
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!
The solution is to perform token-level manipulations using the APIs provided by Syn and proc-macro2.
let concatenated = format!;
let varname = new;
quote!
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!
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!
This expands to <Vec<i32>>::new()
which behaves correctly.
A similar pattern is appropriate for trait methods.
quote!
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.
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.