#[cfg(unknown_option = "...") should produce a warning

[Wilfred]

The rust reference https://doc.rust-lang.org/reference.html#conditional-compilation states:

contains a string that can be checked against (#[cfg(bar = "baz")]). Currently, only compiler-defined configuration options can have the latter form.

Given that there's a fixed list of options, could rustc warn on invalid options? The following code compiles with no warnings:

#[cfg(i_dont_exist = "foo")]
extern crate bar;

fn main() {
}

This caught me out today when I wrote target = "macos" rather than target_os = "macos".

[durka]

Hmm, that line in the reference is a total lie.

$ rustc xx.rs --cfg 'i_dont_exist="foo"'
error[E0463]: can't find crate for `bar`
 --> xx.rs:2:1
  |
2 | extern crate bar;
  | ^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

[Wilfred]

Ah, sounds like I'm doomed and there's no way to warn on invalid option names. I will close this issue if there's nothing we can do to improve this.

[retep998]

There is something we can do. We can update the documentation so it no longer states "Currently, only compiler-defined configuration options can have the latter form."

[durka]

The reference should be fixed or we should add a warning, one or the other.