[Rust] Syntax highlighting breaks in macros

[deathaxe]

What happened?

Source

https://github.com/WebAssembly/WASI/blob/01bb90d8b66cbc1d50349aaaab9ac5b143c9c98c/legacy/tools/witx/src/abi.rs#L64-L120

Screenshot

Code Snippet

// Helper macro for defining instructions without having to have tons of
// exhaustive `match` statements to update
macro_rules! def_instruction {
    (
        $( #[$enum_attr:meta] )*
        pub enum Instruction<'a> {
            $(
                $( #[$attr:meta] )*
                $variant:ident $( {
                    $($field:ident : $field_ty:ty $(,)* )*
                } )?
                    :
                [$num_popped:expr] => [$num_pushed:expr],
            )*
        }
    ) => {
        $( #[$enum_attr] )*
        pub enum Instruction<'a> {
            $(
                $( #[$attr] )*
                $variant $( {
                    $(
                        $field : $field_ty,
                    )*
                } )? ,
            )*
        }

        impl Instruction<'_> {
            /// How many operands does this instruction pop from the stack?
            #[allow(unused_variables)]
            pub fn operands_len(&self) -> usize {
                match self {
                    $(
                        Self::$variant $( {
                            $(
                                $field,
                            )*
                        } )? => $num_popped,
                    )*
                }
            }

            /// How many results does this instruction push onto the stack?
            #[allow(unused_variables)]
            pub fn results_len(&self) -> usize {
                match self {
                    $(
                        Self::$variant $( {
                            $(
                                $field,
                            )*
                        } )? => $num_pushed,
                    )*
                }
            }
        }
    };
}

[FichteFoll]

Probably the same issue as #3904 but with a different construct. #3912 might be able to help (haven't checked) but introduces a different issue.