Remove impl_stable_hash_via_hash!()

[michaelwoerister]

I came across impl_stable_hash_via_hash!() the other day and it seems very problematic because one can easily generate invalid HashStable implementations without even noticing:

#[derive(Hash)]
struct Foo<'tcx> {
    def_id: DefId, // numeric value of def_id ends up in fingerprint,
    ptr: *const c_void, // memory address ends up in fingerprint,
    bar: Interned<'tcx, Bar>, // ditto
}

// Yay, three separate bugs in a single line!
impl_stable_hash_via_hash!(Foo);

I think we should just remove the macro (or keep it local to the module that implements HashStable for basic types). #[derive(HashStable_Generic)] is available as a safe alternative.

rustc_lint_defs::Level looks like a case where this is already a problem because of the HirId in the contained LintExpectationId.

cc @rust-lang/wg-incr-comp

[michaelwoerister]

I plan to fix this on Friday if nobody beats me to it.