Consider checking constants in public interfaces for privacy

[petrochenkov]

I.e. making something like

const PRIV: usize = 10;
pub fn f() -> [u8; PRIV] { ... };

a privacy error/warning.

Some discussion in #29973 (comment) and below.
There are unresolved questions - what exactly should be checked - all signatures, all signatures except for constant initializers, const function bodies?

[petrochenkov]

Personally, I don't think it's a good idea to make this a language rule, maybe a lint at best, due to leaky nature of this restriction (it gives no useful guarantees).

[petrochenkov]

@nikomatsakis
I think this can be closed now as not fitting into the privacy model and too breaking to change.

[nikomatsakis]

Agreed.