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/net/tests/data_loader.rs

@@ -44,7 +44,7 @@ fn assert_parse(
                     filtered: FilteredMetadata::Basic(m),
                     ..
                 }) => m,
-                result => panic!(result),
+                result => panic!("{:?}", result),
             };
             assert_eq!(metadata.content_type.map(Serde::into_inner), content_type);
             assert_eq!(metadata.charset.as_ref().map(String::deref), charset);