Currently, if a future-incompatible lint is set to allow (either manually or via cap-lints), no warning is printed. This is bad because it means that if you author a library that is widely used, but which you are not actively using at the moment, you might not notice that it will break in the future -- moreover, your users won't either, since cargo will cap lints when it builds your library as a dependency.

So basically the behavior for future-incompatible lints would be that they can either be "warn" or "deny" but can never be completely silenced. If cap-lints is set to allow, then while other lints will be set to allow, the future-incompatible lints will be set to warn (but not deny).

cc @rust-lang/compiler @brson