Revisit "must camel case identifiers" lint warning on by default

[erickt]

I like camel casing types that came from #3060, but sometimes I feel the warning is a little too strict. I'm porting erickt/rust-crypto to 0.4, and it has complicated enum variants like AES_256_ECB, which don't read nicely as Aes256Ecb. I ran into the same thing updating rust-elasticsearch, where I converted DFS_QUERY_THEN_FETCH to DfsQueryThenFetch, which made me sad.

I heard @graydon has some reservations with this lint checker on by default, so I figure maybe we should talk this through if it's not a settled issue. What does the community think about turning this into an opt-in lint? Or if that ship has already sailed, what about allowing types, traits, and variants to have _ in them?

[brson]

Putting this on 0.4 to make sure we discuss it before releasing a potentially annoying feature to the wild.

[bblum]

There is #[allow(non_camel_case_types)]. If the problem types are truly the 'uncommon' case, then we should have to use that for them.

[brson]

This warning is disabled by default again. Can revisit later.

Not sure if there's a 'modern' issue tracking this, but, they seem to be enabled by default in 1.35.0.