Trait methods inherit trait const stability, do not inherit const stability from their own regular stability

[compiler-errors]

Needed to constify PartialEq:

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "none")]
#[const_trait]
pub trait PartialEq<Rhs: ?Sized = Self> {
    #[stable(feature = "rust1", since = "1.0.0")]
    fn eq(&self, other: &Rhs) -> bool;

    #[stable(feature = "rust1", since = "1.0.0")]
    fn ne(&self, other: &Rhs) -> bool {
        !self.eq(other)
    }
}

Since currently, we get const stability errors in fn ne, since it's currently considered to be const stable via inheriting const stability from its own regular stability, rather than from the parent trait, and thus we can't call Self::eq b/c it requires the const_trait_impl and const_cmp features.

We do two changes:

1. Do not inherit const stability from regular stability if there's parent stability.
2. Make sure to set InheritConstStability::Yes for trait items.

cc @rust-lang/project-const-generics
r? RalfJung

This stability computation code is kinda jank, maybe it should get rewritten :S

[RalfJung]

This stability computation code is kinda jank, maybe it should get rewritten :S

It definitely should get rewritten, this code is a mess and I was hoping I don't have to see it again.^^

[RalfJung]

@rustbot author

[compiler-errors]

gah rebase error

[compiler-errors]

@rustbot ready

[RalfJung]

Shouldn't this be const_stab = parent?

Right now, it seems like we could then enter the if below which actually sets const_stab to Some and then that gets inserted into the table.

[RalfJung]

Suggested change

	// For now, `const fn` in const traits/trait impls does not exist.
	// For now, `const fn` in const traits/trait impls does not exist.
	// If we ever support that, the logic here and also the const-stability enforcement
	// logic needs some re-thinking.

[RalfJung]

@rustbot author