Warnings about removed lints may want to itself become a lint

[alexcrichton]

Right now the warnings about a removed lint is a full-fledged warning in the compiler, which primarily means that it is not affected by --cap-lints allow. Note that -A warnings does indeed suppress the warning, but Cargo no longer pases -A warnings.

This ends up meaning that if you just updated to Rust 1.6 (where raw_pointer_derive was removed) and recompile your project, you're actually pretty likely to get tons of these warnings from all your upstream dependencies. (although you yourself are unlikely to have allow(raw_pointer_derive). Ideally these warnings would behave like other lint warnings in Cargo, which is that they are ignored in dependencies by default (e.g. suppressed via --cap-lints allow).

cc @wycats (reporter)
cc @brson (implementor)

[alexcrichton]

As another concrete example, the libc 0.1, openssl-sys 0.7.4, curl-sys, and curl crates all had the raw_pointer_derive lint explicitly as #[allow], and I found that at least a bunch of the projects I was working with depended on one of these at some point, which meant that there were lots of warnings when updating to 1.6

[alexcrichton]

cc #30878

[retep998]

Funnily enough, the warning for an invalid lint is actually a lint so you can disable warnings for invalid lints, but not removed lints.

[brson]

I agree.