`cfg!($crate)`, `cfg!(crate)`, `cfg!(self)`, `cfg!(Self)`, and `cfg!(super)` should not be accepted

[theemathas]

I tried this code:

macro_rules! foo {
    () => {
        cfg!($crate)
    };
}

fn main() {
    let b = foo!();
    println!("{b}");
}

I expected a compile error. Instead, I got the following warning, but the code compiles fine and prints false.

warning: unexpected `cfg` condition name: `$crate`
 --> src/main.rs:3:14
  |
3 |         cfg!($crate)
  |              ^^^^^^
...
8 |     let b = foo!();
  |             ------ in this macro invocation
  |
  = help: expected names are: `docsrs`, `feature`, and `test` and 31 more
  = help: consider using a Cargo feature instead
  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
           [lints.rust]
           unexpected_cfgs = { level = "warn", check-cfg = ['cfg($crate)'] }
  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg($crate)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default
  = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)

Since it's impossible to set the value of this cfg, I think this should cause a compile error.

See also #146967 for more nonsense with $crate.

Meta

Reproducible on the playground with version 1.92.0-nightly (2025-09-23 975e6c8fec280816d24f)

[theemathas]

@danielhenrymantilla discovered that cfg!(crate), cfg!(self), cfg!(Self), and cfg!(super) are also accepted, even though other keywords are not accepted. I think this also seems wrong.

[petrochenkov]

Syntactically, cfg predicates are parsed as paths, because they are parsed as meta-items.
E.g. something like #[cfg(a::b)] also parses, it's just then rejected by a semantic check "cfg predicate key must be an identifier".
crate, self, etc are all valid paths, but there's no semantic check that rejects them.
Other (non-path) keywords are different because they are rejected at parsing time.

[petrochenkov]

FWIW, on command line the validation seems to be even weaker:

rustc main.rs --cfg struct --cfg crate // ok

[theemathas]

cfg!() can accept raw identifiers, so it makes sense that such values can be set via the command line

[mu001999]

@rustbot claim