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

Left overs of rust-lang#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```

Author: Dylan-DPC

compiler/rustc_expand/src/lib.rs

@@ -2,7 +2,6 @@
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
 #![feature(crate_visibility_modifier)]
-#![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(let_else)]

library/core/src/tuple.rs

@@ -5,7 +5,7 @@ use crate::cmp::*;
 
 // macro for implementing n-ary tuple functions and operations
 macro_rules! tuple_impls {
-    ( $( $Tuple:ident( $( $T:ident )+ ) )+ ) => {
+    ( $( ( $( $T:ident )+ ) )+ ) => {
         $(
             #[stable(feature = "rust1", since = "1.0.0")]
             impl<$($T:PartialEq),+> PartialEq for ($($T,)+) where last_type!($($T,)+): ?Sized {
@@ -106,16 +106,16 @@ macro_rules! last_type {
 }
 
 tuple_impls! {
-    Tuple1(A)
-    Tuple2(A B)
-    Tuple3(A B C)
-    Tuple4(A B C D)
-    Tuple5(A B C D E)
-    Tuple6(A B C D E F)
-    Tuple7(A B C D E F G)
-    Tuple8(A B C D E F G H)
-    Tuple9(A B C D E F G H I)
-    Tuple10(A B C D E F G H I J)
-    Tuple11(A B C D E F G H I J K)
-    Tuple12(A B C D E F G H I J K L)
+    (A)
+    (A B)
+    (A B C)
+    (A B C D)
+    (A B C D E)
+    (A B C D E F)
+    (A B C D E F G)
+    (A B C D E F G H)
+    (A B C D E F G H I)
+    (A B C D E F G H I J)
+    (A B C D E F G H I J K)
+    (A B C D E F G H I J K L)
 }