Fix warnings introduced in newer Rust Nightly

This does not (yet) upgrade ./rust-toolchain

The warnings:

* dead_code "field is never read"
* redundant_semicolons "unnecessary trailing semicolon"
* non_fmt_panic "panic message is not a string literal, this is no longer accepted in Rust 2021"
* unstable_name_collisions "a method with this name may be added to the standard library in the future"
* legacy_derive_helpers "derive helper attribute is used before it is introduced" https://github.com/rust-lang/rust/issues/79202

components/config_plugins/lib.rs

@@ -132,12 +132,14 @@ impl Build {
             .get_field_name_mapping()
             .map(|pref_attr| pref_attr.value())
             .unwrap_or_else(|| {
-                path_stack
-                    .iter()
-                    .chain(iter::once(&field.name))
-                    .map(Ident::to_string)
-                    .intersperse(String::from("."))
-                    .collect()
+                Itertools::intersperse(
+                    path_stack
+                        .iter()
+                        .chain(iter::once(&field.name))
+                        .map(Ident::to_string),
+                    String::from("."),
+                )
+                .collect()
             })
     }