rustc: do not inherit #[stable]

[aturon]

This patch tweaks the stability inheritance infrastructure so that
#{stable] attributes are not inherited. Doing so solves two problems:

1. It allows us to mark module names as stable without accidentally
   marking the items they contain as stable.
2. It means that a #[stable] attribution must always appear directly
   on the item it applies to, which makes it easier for reviewers to catch
   changes to stable APIs.

Fixes #17484

[seanmonstar]

This means I'll need to litter my library with stable attributes when I mark it 1.0?

[aturon]

@seanmonstar The intended use of stability attributes is to give finer-grained control over semver; not everything needs to (or should) be marked stable at a 1.0 release unless you really want to make all of those promises.

In general, the intended approach is to slowly annotate items over time with greater stability, even before 1.0 -- this was what we're doing for the standard library, for example.

By not inheriting #[stable], we remove two kinds of easy mistakes:

1. If you add an item to an already-stable module, generally that new item should not be immediately stable -- it should likely be experimental. With this approach, it worst it is considered unmarked (which counts as unstable.)
2. If you are changing a #[stable] API, you always know, because the attribute it local to the item. This makes it easier to catch stability regressions.

Hope that helps. This is definitely something we can tweak over time.

[Gankra]

Would it be worth considering a #[stable(all)]-type attribute to obtain the old behaviour?

[seanmonstar]

I'm more thinking of smaller libraries with a hand full of methods or structs. One that I'm pretty confident won't change, I'd just stick a #![stable] in lib.rs and commit.

[aturon]

@gankro

Would it be worth considering a #[stable(all)]-type attribute to obtain the old behaviour?

That's what I was going to do at first, but it seemed like overkill. Certainly something we can add. But we definitely want a way to not inherit #[stable] for std.

@seanmonstar

I'm more thinking of smaller libraries with a hand full of methods or structs. One that I'm pretty confident won't change, I'd just stick a #![stable] in lib.rs and commit.

Fair enough. We can take something like @gankro's suggestion if this turns out to be painful in practice.