Derive tests take too long to compile and run

[djkoloski]

Running cargo clean && cargo test -p zerocopy-derive takes upwards of a 1m20s on my relatively beefy machine. Compiling enum_from_bytes alone makes up 89% of that time - a full 1m11s. By comparison, cargo clean && cargo test -p zerocopy only takes 12s. Additionally, actually compiling zerocopy-derive only takes around 2.3s.

General slowness also makes rust-analyzer lag pretty far behind and compete over the lockfile with any command-line operations. This results in a pretty miserable development experience.

[joshlf]

Is there any way to get cargo test to skip compiling a test by default? Analogous to #[ignore], but for compilation rather than for running the test? If so, we could compile and run enum_from_bytes in CI but disable it by default for local development.

One hacky way of accomplishing that would be to put a #![cfg(zerocopy_compile_expensive_tests)] at the top of the file. I worry that it'd be yet one more thing that we have to keep track of wiring through our CI code, though. It'd be very easy to delete that at some point and not notice that we're silently ignoring that test.