[DESIGN BUG] declarative macros lack of neat way to simulate lookahead within rust grammer syntax `const X: Y`

I tried this code:

macro match and echo non const X: Y pattern is fine.

    macro_rules! echo1 {
        (pub type $ident:ident<$($gi:ident),*> = $($tt:tt)*) => {
            pub type $ident<$($gi),*> = $($tt)*;
        };
    }

    echo1!(pub type Foo1<T, N> = (T, N));

But within current rust desgin, to let declarative macro match lookahead within rust grammer syntax const X: Y

• or input rust code, and do lookahead simulation

• or input lookahead free dsl, and output lookahead code

    macro_rules! echo2 {
        (@derive_foo pub type $ident:ident<$($gi:ident $(lookahead_qualifier=$gq:tt)? $(: $gt:ty)?),*> = $($tt:tt)*) => {
            pub type $ident<$($($gq)? $gi $(: $gt)?),*> = $($tt)*;
        };

        (@derive_bar pub type $ident:ident<$($gi:ident $(lookahead_qualifier=$gq:tt)? $(: $gt:ty)?),*> = $($tt:tt)*) => {
            pub type $ident<$($($gq)? $gi $(: $gt)?),*> = $($tt)*;
        };

        (@lookahead_workaround pub type $ident:ident<$($gi:ident $(lookahead_qualifier=$gq:tt)? $(: $gt:ty)?),*> = $($tt:tt)*) => {
            pub type $ident<$($($gq)? $gi $(: $gt)?),*> = $($tt)*;
        };

        (pub type $ident:ident<$($(const)? $gi:ident $(: $gt:ty)?),*> = $($tt:tt)*) => {
            pub type $ident<$($(const)? $gi $(: $gt)?),*> = $($tt)*;
        };
    }

    // // TODO: https://github.com/rust-lang/rust/issues/130928
    // echo2!(pub type Foo2<T, const N: usize> = [T; N]);

    echo2!(@lookahead_workaround pub type Foo2<T, N lookahead_qualifier=const: usize> = [T; N]);

I expected to see this happen:

• Neat way to match lookahead within rust grammer syntax const X: Y

Instead, this happened:

• Need to simulate lookahead within rust grammer syntax const X: Y

• Leads to ugly simulation code.

• Thus I think it's a design bug.

Meta

rustc --version --verbose:

rustc 1.83.0-nightly (9b72238eb 2024-09-14)
binary: rustc
commit-hash: 9b72238eb813e9d06e9e9d270168512fbffd7ee7
commit-date: 2024-09-14
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

<backtrace>

related

Also found similar issue back to 2021 in pin-project-lite.

So, I guess there is no way yet.

https://github.com/taiki-e/pin-project-lite/issues/62

Start with the macro_rules! examples in the issue and investigate how declarative macro matching handles generic parameters such as const N: usize. Read the related pin-project-lite issue for prior context. Done would require a clearly scoped, accepted design for matching or simulating this syntax, along with corresponding Rust compiler coverage.