#[cfg(features = "something")] compiles, creating a potential foot-gun

[sam0x17]

I tried this code:

#[cfg(features = "my-feature")]
struct MyStruct {}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=256c4dfff8b9a14d10ea75cb4e4aa2f5

I expected to see this happen:

A compiler error, or at least a suppressable warning specific to this situation. The valid syntax is #[cfg(feature = ...], not #[cfg(features = ...], and as far as I know, there is no valid context where it would be #[cfg(features = [something]].

Instead, this happened:

Right now this compiles without any errors or warnings. This caused a huge foot-gun for me in a project where I accidentally had #[cfg(features = ...] in several places and the compiler didn't fail or warn me in any way.

Meta

rustc --version --verbose:

rustc 1.67.1 (d5a82bbd2 2023-02-07)
binary: rustc
commit-hash: d5a82bbd26e1ad8b7401f6a718a9c57c96905483
commit-date: 2023-02-07
host: aarch64-apple-darwin
release: 1.67.1
LLVM version: 15.0.6

Note: this behavior is consistent in stable, beta, and nightly on the Rust Playground as of 03/04/2023.

[Noratrieb]

This can't be a compiler error, because it's totally legal to pass a cfg with that name, like --cfg features="my-feature".

We already have an unstable option --check-cfg "names()", that will complain about unknown cfgs.

While this lint is useful, it does need some configuration to pass other known cfgs to avoid false positives. Maybe it would make sense to enable a subset of it by default, where it only checks whether an unknown cfg is similiar to a known one. This way something "exotic" like cfg(loom) wouldn't warn by default, but cfg(debug_asertions) or cfg(features = "whatever") would (because they are almost surely wrong).

[Noratrieb]

RFC 3013 describes this in more detail

[ehuss]

This has been implemented and is available on the nightly channel. See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg and rust-lang/cargo#10554 and #82450 for more information and tracking of the stabilization.