Syntax Highlighting does not highlight where bound for tuple structs

[shingtaklam1324]

This is highlighted correctly by RLS-VSCode

pub trait UnitTr
where
    f64: Rem<Self, Output = Unit<Self>>,
    Self: Sized,
{
}

whereas this does not highlight the where even though it is a Rust keyword and valid syntax.

pub struct Unit<Type: UnitTr>(Prefix, f64, PhantomData<Type>)
where
    f64: Rem<Type, Output = Unit<Type>>;

I originally posted this on rust-lang-nursery/rls-vscode#328 and was recommended to post here instead