Macro metavar expr concat doesn't work with nested repetitions #127723
Description
Is it a known issue that this doesn't seem to work with identifiers repeating at a depth?
#![feature(macro_metavar_expr_concat)] macro_rules! many_idents { ($a:ident, $c:ident) => { const ${concat($a, B, $c, D)}: i32 = 1; }; } // Paste implementation included for reference macro_rules! many_idents_paste { ($a:ident, $c:ident) => { paste::paste! { const [<$a B $c D>]: i32 = 2; } }; } macro_rules! many_idents_multi_metavar { ($($a:ident, $c:ident;)*) => { $( const ${concat($a, B, $c, D)}: i32 = 3; )* }; } // Paste implementation included for reference macro_rules! many_idents_multi_paste { ($($a:ident, $c:ident;)*) => { $( paste::paste! { const [<$a B $c D>]: i32 = 3; } )* }; } fn main() { many_idents!(A, C); assert_eq!(ABCD, 1); many_idents_paste!(F, G); assert_eq!(FBGD, 2); many_idents_multi_paste! { H, I; J, K; L, M; } assert_eq!(HBID, 3); assert_eq!(JBKD, 3); assert_eq!(LBMD, 3); many_idents_multi_metavar! { N, O; P, Q; R, S; } }error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth --> src/main.rs:19:10 | 19 | $( | __________^ 20 | | const ${concat($a, B, $c, D)}: i32 = 3; 21 | | )* | |_________^The paste version works without errors.
I can go a bit further with ignore
macro_rules! many_idents_multi_metavar { ($($a:ident, $c:ident;)*) => { $( ${ignore($a)} ${ignore($c)} const ${concat($a, B, $c, D)}: i32 = 3; )* }; }But then I get:
error: `${concat(..)}` currently only accepts identifiers or meta-variables as parameters --> src/main.rs:22:29 | 22 | const ${concat($a, B, $c, D)}: i32 = 3; | This seems like a major limitation.
Originally posted by @crumblingstatue in #124225 (comment)