Regression: some macro invocations no longer work in Nightly.

[emilio]

This is a regression which also affects Firefox, though it can be worked around by updating the syn crate.

I'll post a repro in a bit. The repro builds with stable rust (rustc 1.46.0 (04488afe3 2020-08-24)) and fails to build with nightly rust (rustc 1.48.0-nightly (0e2c1281e 2020-09-07)).

[emilio]

Repro lives here: https://github.com/emilio/rustc-issue-76482

[Aaron1011]

This is caused by an outdated version of syn, which does not handle None-delimited groups properly. Running cargo update -p syn in your repro repository fixed the issue.

[emilio]

Yes, I mentioned that above:

it can be worked around by updating the syn crate

Still seems weird for it to regress like that...

[emilio]

To be clear, I'm a bit disappointed at breaking changes without even a warning cycle, but so it is I guess.

[Aaron1011]

@emilio: Unfortunately, the nature of this change makes emitting a warning impossible. The error is emitted by a proc macro, which is a black box as far as rustc is concerned. The only way to know if a proc-macro will produce an error is to run it, and the only way to 'recovery' or 'downgrade' such an error would be to run the proc-macro again with a different TokenStream. Since proc-macros are currently unsandboxed, this could cause all sorts of issues (duplicate network requests, overwriting files, etc).