Add bound contraint for trait associated const

I recently post on rust forum about how to write constraint on associated const, and find out that there is no direct way to achieve this (even no RFC?).

For example, I have the following trait:

trait Foo {
    const FOO: usize;
}

And I want to have constraint for that like

fn foo<T: Foo>() where <T as Foo>::FOO < 4 {}

impl<T: Foo> SomeTrait for T where <T as Foo>::FOO.is_power_of_two() {}

const fn something_about_foo(foo: usize) -> bool { /* ... */ }
struct Bar<T: Foo> where something_about_foo(<T as Foo>::FOO) {}

Since we already have various mature bound constraint on associated type, there should be some bound constraint on associated const as well.

I also wonder how to design the syntax of associated const bound, since allowing any const expression in where clause may lead to hard parsing. Maybe a brace should be required?

Start with the issue's Rust forum discussion and the associated-const examples to understand the proposed constraint forms and unresolved syntax concerns. A completed contribution would need a decided design for associated-const bounds, including how expressions are represented in where clauses, suitable for an RFC.