Tidy feature: deny commits with a tracking issue of 0

[clarfonthey]

A reasonable number of commits get pushed that forget a tracking issue, so, I think that this would be useful to have. Additionally, it'd be nice if any #[unstable] attributes got checked to make sure that the tracking issues are all the same.

This should also check links in the unstable book.

[GuillaumeGomez]

I'm against it. A lot of commits/improvements don't have an issue because it has been discussed outside github.

[varkor]

As discussed in #49985 (comment), the way forward here is making the issue field optional. This should be easy to fix. You can probably just remove this branch:

rust/src/libsyntax/attr/builtin.rs

Lines 363 to 366 in 6d59933

	_ => {
	span_err!(diagnostic, attr.span, E0547, "missing 'issue'");
	continue
	}

and remove any existing issue = "0" fields.

[Julian-Wollersberger]

I have looked into this and the easiest way to do it is to internally use 0 as the issue number if it wasn't given.
This feels a bit hacky. Should I try to change the type in StabilityLevel::Unstable from issue: i32 to issue: Option<i32> instead?

[varkor]

Should I try to change the type in StabilityLevel::Unstable from issue: i32 to issue: Option<i32> instead?

Yes, that's right. 0 should not be used to encode an issue anywhere; we should be using Option instead.

[Julian-Wollersberger]

Ok, the types were changed to issue: Option<i32> in a few places and I added a basic ui test. It turned out that issue fields were already Option<i32> in most places.

Tomorrow I'll try to fix the 79 #[unstable(... , issue = "0")] I found. (And make a proper warning)