1 unstable release
0.1.0 | Apr 20, 2024 |
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#24 in #macro-rules
19KB
375 lines
The macro_rules_rec
crate enables recursive calls to itself within macro_rules!
in the Rust language.
In Rust, macros, especially defined with macro_rules!
expands the macro's body in-place when used. This means that all intermediate states produced during the expansion must adhere to Rust's syntax.
Namely, when a macro recursively calls itself within its body, the position where this call is written must be within a syntactic element that supports macro invocation according to Rust's syntax. Typically, many macros are intended to expand into expressions, statements, types, or declaration items, and this rarely poses a problem. However, in special cases like the following, the position of the macro call does not correspond to any syntactic element in Rust, which traditionally prevented the recursive invocation of the macro:
- When generating pairs of types and constraints simultaneously in the where clause of generics using a single macro.
- When generating pairs of field names and types simultaneously within a struct declaration using a single macro.
macro_rules_rec
extends the declarative macro writing with Rust's macro_rules!
and enables recursive macro calls that are not constrained by Rust's syntax. Additionally, the macros generated by macro_rules_rec
are pure macro_rules!
and do not depend on external crates (including macro_rules_rec
). This is very useful when creating libraries, as users of the library do not need to reference macro_rules_rec
when using macros generated with it.
Example
You can perform self-recursive call via $self!
sentence.
# use macro_rules_rec::recursive;
#[recursive]
macro_rules! m {
($($id:ident)*) => {
$(
$self!(@ $id);
)*
};
(@ $id:ident) => {
#[allow(unused)]
let $id = 123;
};
}
m!(a b c);
Dependencies
~1.5MB
~38K SLoC