Pass LexerKind by reference.

[ltratt]

At the moment the code happens to work only because of an oversight in rustc: that oversight is likely to be fixed soon (rust-lang/rust#150681).

@meithecatte If you're able to do so, I'd be grateful if you can confirm this addresses the rustc issue properly. If so, I will then make a new release shortly after so that we're not blocking the rustc PR.

[ratmice]

I'm happy to approve.

I was able to reproduce the error from the crater log in #620 on current rustc by adding a second variant to LexerKind and compiling with cargo build --all-targets since the error only occurs with cfg(test).

Edit: The test worked as expected (failed on the master branch, and worked with this one)

[ltratt]

@ratmice Cool, and thanks! So we're confident with this PR merged, we won't trigger the upcoming-rustc change?

[ratmice]

If my understanding of the problem is right, adding another variant it should behave the same on both the new and old compilers, since the change seems to be making single variant #[non_exhaustive]` behave as they had multiple variants on the new compiler.

I'd still wait for feedback from @meithecatte before release, in case they can conveniently test on the new compiler though, or my understanding is flawed.