cannot forbid `dead_code` since lint triggers for third party macros

[andrewbanchich]

if i, for example, forbid dead_code, clippy produces

error[E0453]: allow(dead_code) incompatible with previous forbid
  |
  | #[derive(Parser, Debug, Clone)]
  |          ^^^^^^ overruled by previous forbid
  |
  = note: `forbid` lint level was set on command line
  = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)

this is coming from clap's Parser derive macro, which i have no control over, so prevents me from forbidding dead_code in my own crate.

[veera-sivarajan]

@rustbot label -needs-triage +A-clippy +L-dead_code +A-lint

[andreaskrath]

I encountered this as well, also with the Parser trait from clap.

Although I was under the impression that this is the expected behaviour because of macro expansion, therefore making our lints affect code that was already affected by another lint (therein is the conflict).

Regardless, it would be nice if this could be changed, as anyone using a given crate, has no control over what lints the crate utilizes.