4 releases
Uses old Rust 2015
0.0.5 | Oct 31, 2018 |
---|---|
0.0.4 | Oct 23, 2017 |
0.0.3 | Mar 2, 2017 |
0.0.1 | Feb 22, 2017 |
#6 in #proc-macro-hack
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Used in proc-macro-hack
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Procedural macros in expression position
Note: As of Rust 1.45 this crate is superseded by native support for #[proc_macro] in expression position. Only consider using this crate if you care about supporting compilers between 1.31 and 1.45. |
Since Rust 1.30, the language supports user-defined function-like procedural macros. However these can only be invoked in item position, not in statements or expressions.
This crate implements an alternative type of procedural macro that can be invoked in statement or expression position.
This approach works with any Rust version 1.31+.
Defining procedural macros
Two crates are required to define a procedural macro.
The implementation crate
This crate must contain nothing but procedural macros. Private helper functions and private modules are fine but nothing can be public.
» example of an implementation crate
Just like you would use a #[proc_macro] attribute to define a natively supported procedural macro, use proc-macro-hack's #[proc_macro_hack] attribute to define a procedural macro that works in expression position. The function signature is the same as for ordinary function-like procedural macros.
use proc_macro::TokenStream;
use proc_macro_hack::proc_macro_hack;
use quote::quote;
use syn::{parse_macro_input, Expr};
#[proc_macro_hack]
pub fn add_one(input: TokenStream) -> TokenStream {
let expr = parse_macro_input!(input as Expr);
TokenStream::from(quote! {
1 + (#expr)
})
}
The declaration crate
This crate is allowed to contain other public things if you need, for example traits or functions or ordinary macros.
» example of a declaration crate
Within the declaration crate there needs to be a re-export of your procedural macro from the implementation crate. The re-export also carries a #[proc_macro_hack] attribute.
use proc_macro_hack::proc_macro_hack;
/// Add one to an expression.
///
/// (Documentation goes here on the re-export, not in the other crate.)
#[proc_macro_hack]
pub use demo_hack_impl::add_one;
Both crates depend on proc-macro-hack
:
[dependencies]
proc-macro-hack = "0.5"
Additionally, your implementation crate (but not your declaration crate) is a proc macro crate:
[lib]
proc-macro = true
Using procedural macros
Users of your crate depend on your declaration crate (not your implementation crate), then use your procedural macros as usual.
» example of a downstream crate
use demo_hack::add_one;
fn main() {
let two = 2;
let nine = add_one!(two) + add_one!(2 + 3);
println!("nine = {}", nine);
}
Limitations
-
Only proc macros in expression position are supported. Proc macros in pattern position (#20) are not supported.
-
By default, nested invocations are not supported i.e. the code emitted by a proc-macro-hack macro invocation cannot contain recursive calls to the same proc-macro-hack macro nor calls to any other proc-macro-hack macros. Use
proc-macro-nested
if you require support for nested invocations. -
By default, hygiene is structured such that the expanded code can't refer to local variables other than those passed by name somewhere in the macro input. If your macro must refer to local variables that don't get named in the macro input, use
#[proc_macro_hack(fake_call_site)]
on the re-export in your declaration crate. Most macros won't need this. -
On compilers that are new enough to natively support proc macros in expression position, proc-macro-hack does not automatically use that support, since the hygiene can be subtly different between the two implementations. To opt in to compiling your macro to native
#[proc_macro]
on sufficiently new compilers, use#[proc_macro_hack(only_hack_old_rustc)]
on the re-export in your declaration crate.
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 hack by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.
Dependencies
~2MB
~48K SLoC