Add Unicode XID related functionalities to `proc_macro` crate

Proposal

Problem statement

When generating code in proc macros, sometimes one need to generate identifiers from user-provided strings. It's necessary to validate the validity of such strings to judge whether they can actually be used as identifiers.

One can use community-provided crates to do such, but they can easily go out of sync with later rustc updates, and needs to bump to new releases for updating such dependencies.

Motivating examples or use cases

Solution sketch

I'd propose adding new trait and implement on char within proc_macro crate.

Alternatives

• Add such methods on char as inherent methods. (But might not be worthwhile, because normal rust programs rarely needs to use xid-related checks.

• Leave things as is

Links and related work

• https://docs.rs/unicode-ident
• https://docs.rs/unicode-xid

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reviewing the proposal in issue #540 and the linked unicode-ident and unicode-xid crates, then read the proc_macro API and the feature lifecycle guidance. Done would require a resolved API design and libs-api team approval before an implementation can be scoped.