Rollup merge of #95805 - c410-f3r:meta-vars, r=petrochenkov

Left overs of #95761

These are just nits. Feel free to close this PR if all modifications are not worth merging.

* `#![feature(decl_macro)]` is not needed anymore in `rustc_expand`
* `tuple_impls` does not require `$Tuple:ident`. I guess it is there to enhance readability?

r? ```@petrochenkov```