don't compile Regex in frequently called function get_rust_item (#971)

Author: tjhance

source/rust_verify/src/verus_items.rs

@@ -631,7 +631,9 @@ pub(crate) fn get_rust_item<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<Ru
     }
 
     if let Some(rust_path) = rust_path {
-        let num_re = Regex::new(r"^([A-Za-z0-9_]+)::(MIN|MAX|BITS)").unwrap();
+        static NUM_RE: std::sync::OnceLock<Regex> = std::sync::OnceLock::new();
+        let num_re =
+            NUM_RE.get_or_init(|| Regex::new(r"^([A-Za-z0-9_]+)::(MIN|MAX|BITS)").unwrap());
         if let Some(captures) = num_re.captures(rust_path) {
             let ty_name = captures.get(1).expect("invalid int intrinsic regex");
             let const_name = captures.get(2).expect("invalid int intrinsic regex");