rust: concat_idents: use format! for concatenation

This is part of the effort to minimize the differences of the `rust`
branch with respect to mainline in order to eventually drop it.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/macros/concat_idents.rs

@@ -18,6 +18,6 @@ pub(crate) fn concat_idents(ts: TokenStream) -> TokenStream {
     assert_eq!(expect_punct(&mut it), ',');
     let b = expect_ident(&mut it);
     assert!(it.next().is_none(), "only two idents can be concatenated");
-    let res = Ident::new(&(a.to_string() + &b.to_string()), b.span());
+    let res = Ident::new(&format!("{a}{b}"), b.span());
     TokenStream::from_iter([TokenTree::Ident(res)])
 }