Undefined lints scoped under `rustc` don't result in lint unknown-lints getting emitted (unless `-Zunstable-options` is passed)

[fmease]

While undefined "unscoped" lints lead to the compiler emitting the warn-by-default lint unknown-lints

#![allow(undefined)] //~ WARN unknown lint `undefined`

warning: unknown lint: `undefined`
 --> src/lib.rs:1:10
  |
1 | #![allow(undefined)]
  |          ^^^^^^^^^
  |
  = note: `#[warn(unknown_lints)]` on by default

the same doesn't happen for undefined lints scoped under rustc:

//@ check-pass
#![deny(unknown_lints)]

#![allow(rustc::undefined)]

It would be beneficial if rustc would emit unknown_lints in these cases.

Built-in tools like rustfmt and clippy or tools defined via register_tool are responsible for dealing with undefined lints themselves. However, the ~~tool modules~ lint scope rustc is owned by rustc and should therefore do the checking.

[fmease]

Wait, locally it does result in an error but not so on the playground?

[fmease]

Ooh, it does result in it getting emitted if -Zunstable-options is passed. What on earth?

[fmease]

Actually lint scope rustdoc doesn't have this problem. Updated the issue description.