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/style/values/generics/image.rs

@@ -264,7 +264,6 @@ impl ToCss for PaintWorklet {
 ///
 /// `-moz-image-rect(<uri>, top, right, bottom, left);`
 #[allow(missing_docs)]
-#[css(comma, function = "-moz-image-rect")]
 #[derive(
     Clone,
     Debug,
@@ -276,6 +275,7 @@ impl ToCss for PaintWorklet {
     ToResolvedValue,
     ToShmem,
 )]
+#[css(comma, function = "-moz-image-rect")]
 #[repr(C)]
 pub struct GenericMozImageRect<NumberOrPercentage, MozImageRectUrl> {
     pub url: MozImageRectUrl,