diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index fe963a0ea3a..f7c7e00ab22 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -267,17 +267,17 @@ pub fn make_format_args(
         })
     };
 
+    let num_explicit_args = args.len();
+    let mut used = vec![false; num_explicit_args];
+    let mut invalid_refs = Vec::new();
+    let mut numeric_refences_to_named_arg = Vec::new();
+
     enum ArgRef<'a> {
         Index(usize),
         Name(&'a str, Option<Span>),
     }
     use ArgRef::*;
 
-    let num_explicit_args = args.len();
-    let mut used = vec![false; num_explicit_args];
-    let mut invalid_refs = Vec::new();
-    let mut numeric_refences_to_named_arg = Vec::new();
-
     let mut lookup_arg = |arg: ArgRef<'_>,
                           span: Option<Span>,
                           used_as: PositionUsedAs,