Proposal: const boolean restrictions in `where` clause

Currently the where clause supports only type: trait restrictions and works fine for the lifetime and type parameters. But for const generics, there is few ability to restrict those const parameters.

For example, if I want a small vec where the maximum on-stack size can be specified by a const generic parameter:

union Buf<T, const N: usize> {
    Stack([MaybeUninit<T>; N]),
    Heap(RawVec<T>),
}
pub struct SmallVec<T, const N: usize> {
    buf: Buf<T, N>,
    len: usize,
}

But, I need to keep N tiny to avoid too large stack occupying. How about introduce such where clause like this?

const MAX_ON_STACK_LEN: usize = 8;
union Buf<T, const N: usize>
    where const N < MAX_ON_STACK_LEN,
{ /* ... */ }

It might not violate the parser because const is not in the FIRST set of <ty>, and , is in the FOLLOW set of <expr>.

The only requirement of the <expr> is:

1. it must be const (compile-time computable),
2. it must have type bool.

No implementation files, tests, or entry points are named. Start by reviewing the proposed const-generic where syntax and the two stated requirements, then determine the Rust language-design and RFC work needed; done means the syntax and compile-time boolean and type rules are specified.