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When testing with older versions of rustc, there's a CI failure mode
when our dependency, which used to be compatible with that Rust version,
publishes a new version which can't be compiled using that old Rust
anymore. That's pretty unfortunate, as that means that third parties can
break our CI without any changes to the code in this repo.
To protect against that, let's use Cargo.lock on CI, but only when
testing against older versions. For stable Rust, we continue to generate
Cargo.lock with freshest dependencies. Implementation wise, we save
known-good Cargo.lock as `Cargo.lock.msrv`, and just copy that in every
relevant CI job.
To get a working `Cargo.lock.msrv`, I run `cargo +nightly
generate-lockfile -Zminimal-versions` -- that is guaranteed to support
the oldest Rust version we can.
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