CI fails with

    Updating crates.io index
error: failed to select a version for the requirement `aho-corasick = "=1.0.1"`
candidate versions found which didn't match: 0.7.20, 0.7.19, 0.7.18, ...
location searched: crates.io index
required by package `regex v1.8.1`
    ... which satisfies dependency `regex = "=1.8.1"` of package `pulldown-cmark v0.9.2 (/home/vsts/work/1/s)`

I can reproduce this locally with Rust 1.56.0.

A cargo update is enough to get things working, but this changes the Cargo.lock file — and it changes it differently that what you get when running cargo update on nightly Rust.

Is the policy of the project to keep the code compatible with Rust 1.56? If so, we should probably commit the lock file from that version.