Make rustc linter happy with upcoming lints (#486)

- Satisfy a new `mismatched-lifetime-syntaxes` lint introduced in
rust-lang/rust#138677.
- Fix `dead_code` warnings probably since rust-lang/rust#142485.

Co-authored-by: Kai Ren <tyranron@gmail.com>

Author: kiendang

impl/src/fmt/parsing.rs

@@ -280,7 +280,7 @@ pub(crate) fn format(input: &str) -> Option<(LeftToParse<'_>, Format<'_>)> {
 /// ```
 ///
 /// [0]: std::fmt#syntax
-fn argument(input: &str) -> Option<(LeftToParse<'_>, Argument)> {
+fn argument(input: &str) -> Option<(LeftToParse<'_>, Argument<'_>)> {
     alt(&mut [
         &mut map(identifier, |(i, ident)| (i, Argument::Identifier(ident))),
         &mut map(integer, |(i, int)| (i, Argument::Integer(int))),

tests/lib.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
+#![allow(dead_code)] // some code is tested for type checking only
 
 #[cfg(not(feature = "std"))]
 #[macro_use]