Nightly builds failing due to proc_macro_span_shrink

[adetaylor]

As manifesting on CI:

error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /Users/liztaylor/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.59/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0635`.
error: could not compile `proc-macro2` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...