const expression can borrow static items

[dingxiangfei2009]

Tracked by rust-lang/rust#119618

Blocked on rust-lang/rust#129759

Stabilization report: rust-lang/rust#128183

This PR updates the documentation to mention that now static items can be used for borrowing, explicitly or implicitly, which permits general use of them in const context.

[ehuss]

Can you also update https://github.com/rust-lang/reference/blob/master/src/items/static-items.md which mentions this isn't possible?

I'm wondering if this might be confusion (and perhaps my understanding is missing) with regards to what counts as a borrow. IIUC, statics have an implied &, is that correct? For example, if you see const C: i32 = SOME_STATIC;, there's no obvious borrow there.

[dingxiangfei2009]

@ehuss I updated the wording to include simply "use of statics".

I have updated static-items page as well with tweaks on wording.

[traviscross]

We probably want to update the constant-items page that currently says:

Constants may refer to the address of other constants...

Since it'd now make sense for that to also address the fact that constants may refer to statics with some limitations.

[traviscross]

Also in const_eval.md, where it says:

Paths to statics. These are only allowed within the initializer of a static.

...that seems to need to change also.

[traviscross]

More generally, it seems what we need to do here is to loosen the various places in the Reference that have these restrictions, and in loosening them, add the necessary caveats, e.g. with respect to Freeze as described in rust-lang/rust#128183.