Rollup merge of #95761 - c410-f3r:meta-var-stuff, r=petrochenkov

Kickstart the inner usage of `macro_metavar_expr`

There can be more use-cases but I am out of ideas.

cc #83527
r? ``@petrochenkov``

compiler/rustc_expand/src/expand.rs

@@ -83,9 +83,7 @@ macro_rules! ast_fragments {
                 }
                 match self {
                     $($(AstFragment::$Kind(ast) => ast.extend(placeholders.iter().flat_map(|id| {
-                        // We are repeating through arguments with `many`, to do that we have to
-                        // mention some macro variable from those arguments even if it's not used.
-                        macro _repeating($flat_map_ast_elt) {}
+                        ${ignore(flat_map_ast_elt)}
                         placeholder(AstFragmentKind::$Kind, *id, None).$make_ast()
                     })),)?)*
                     _ => panic!("unexpected AST fragment kind")

compiler/rustc_expand/src/lib.rs

@@ -1,3 +1,4 @@
+#![allow(rustc::potential_query_instability)]
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
 #![feature(crate_visibility_modifier)]
@@ -5,12 +6,12 @@
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(let_else)]
+#![feature(macro_metavar_expr)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_span)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 
 #[macro_use]
 extern crate rustc_macros;