feat: use maro_metavar_expr to count macro repetitions and be able to use with_capacity in HashMap

Author: stifskere

library/std/src/lib.rs

@@ -303,6 +303,7 @@
 #![feature(link_cfg)]
 #![feature(linkage)]
 #![feature(macro_metavar_expr_concat)]
+#![feature(macro_metavar_expr)]
 #![feature(maybe_uninit_fill)]
 #![feature(min_specialization)]
 #![feature(must_not_suspend)]

library/std/src/macros.rs

@@ -414,14 +414,15 @@ macro_rules! dbg {
 /// [`HashMap::new`]: crate::collections::HashMap::new
 /// [`HashMap::insert`]: crate::collections::HashMap::insert
 #[macro_export]
+#[allow_internal_unstable(macro_metavar_expr)]
 #[unstable(feature = "hash_map_macro", issue = "144032")]
 macro_rules! hash_map {
     () => {{
         ::std::collections::HashMap::new()
     }};
 
     ( $( $key:expr => $value:expr ),* $(,)? ) => {{
-        let mut map = ::std::collections::HashMap::new();
+        let mut map = ::std::collections::HashMap::with_capacity( ${count($expr)} );
         $( map.insert($key, $value); )*
         map
     }}