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/pref_util.rs

@@ -113,12 +113,7 @@ macro_rules! impl_from_pref {
                     if let $variant(value) = other {
                         value.into()
                     } else {
-                        panic!(
-                            format!("Cannot convert {:?} to {:?}",
-                                other,
-                                std::any::type_name::<$t>()
-                            )
-                        );
+                        panic!("Cannot convert {:?} to {:?}", other, std::any::type_name::<$t>())
                     }
                 }
             }