code:

#![crate_type = "lib"]
#![feature(unboxed_closures)]

expected behavior: A warning that unboxed_closures is unused.
actual behavior: no warning.

this will be slightly tricky to track, but i think it's possible if tcx.features were better integrated with the query system. right now, that's a single query that returns all features at once. if there were instead one query per-feature, the query system would know in detail which features were used and emit a warning if the feature was enabled but never queried. (it might also be possible to do this in a simpler way using RefCell.)