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/transform.rs

@@ -47,7 +47,6 @@ pub use self::GenericMatrix as Matrix;
 
 #[allow(missing_docs)]
 #[cfg_attr(rustfmt, rustfmt_skip)]
-#[css(comma, function = "matrix3d")]
 #[derive(
     Clone,
     Copy,
@@ -62,6 +61,7 @@ pub use self::GenericMatrix as Matrix;
     ToResolvedValue,
     ToShmem,
 )]
+#[css(comma, function = "matrix3d")]
 #[repr(C)]
 pub struct GenericMatrix3D<T> {
     pub m11: T, pub m12: T, pub m13: T, pub m14: T,