loom dependency pulled in by default

[djc]

Since #392 (by @taiki-e), it looks like loom went from a dev-dependency to a default dependency. It's guarded by cfg(loom), but I'm not sure if that's working as intended. My application now seems to be pulling in loom as a dependency of bytes even though I didn't specify any loom cfg. As far as I understand, loom should only be used for testing?

[djc]

Ah, sorry, I got confused by cargo update calling this out, but it doesn't look like this gets built without --cfg loom enabled. It just clutters up Cargo.lock files, which had me confused for a moment.

[taiki-e]

Yeah, cargo writes all target.cfg(..).dependencies to Cargo.lock even if it never activates (this is the same as target.cfg(windows), target.cfg(unix), etc.). But unless you set RUSTFLAGS, they will never actually be compiled.

Also, target.cfg(..).dependencies are basically supported except cfg(feature = ..), cfg(test), etc (see rust-lang/cargo#7660 (comment))