Using `${index()}` inside `${concat()}` fails with "expected identifier or string literal" error

I tried this code:

#![feature(macro_metavar_expr)]
#![feature(macro_metavar_expr_concat)]
#![allow(dead_code)]

macro_rules! state_num {
    ($($state:ident),+) => {
        enum StateNum {
            $(${concat(S, ${index()})} ${ignore($state)}),+
        }
    };
}

state_num!(A, B, C);

fn main() {
    let _s = StateNum::S1;
}

I expected to see this happen: it compiling successfully.

Instead, this happened:

error: expected identifier or string literal
 --> src/main.rs:8:28
  |
8 |             $(${concat(S, ${index()})} ${ignore($state)}),+
  |                            ^^^^^^^^^

error: expected identifier, found `$`
  --> src/main.rs:8:15
   |
7  |         enum StateNum {
   |              -------- while parsing this enum
8  |             $(${concat(S, ${index()})} ${ignore($state)}),+
   |               ^ expected identifier
...
13 | state_num!(A, B, C);
   | ------------------- in this macro invocation
   |
   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
   = note: this error originates in the macro `state_num` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: use of undeclared type `StateNum`
  --> src/main.rs:16:14
   |
16 |     let _s = StateNum::S1;
   |              ^^^^^^^^ use of undeclared type `StateNum`

For more information about this error, try `rustc --explain E0433`.

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (b74da9613 2025-03-06)
binary: rustc
commit-hash: b74da9613a8cb5ba67a985f71325be0b7b16c0dd
commit-date: 2025-03-06
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

Start with the reproducer in src/main.rs, especially the nested ${concat(S, ${index()})} expression on line 8, and run it with the nightly compiler version reported in the issue. Investigate how nested macro metavariable expressions are parsed and expanded. Done means the example compiles and StateNum::S1 resolves successfully, with a regression test covering the case.