rustdoc doesn't gate `cfg(true/false)` within `doc(cfg(..))`

[Urgau]

I tried this code:

// #![feature(cfg_boolean_literals)] should be required
#![feature(doc_cfg)]

#[doc(cfg(false))]
pub fn foo() {}

#[doc(cfg(true))]
pub fn bar() {}

I expected to see this happen: An error requiring #![feature(cfg_boolean_literals)], as is the case currently when done #[cfg(true)].

Instead, this happened: The documenation is generated and no extra feature gate is required.

Meta

rustdoc --version --verbose:

rustdoc 1.87.0-nightly (e16a049ad 2025-03-03)
binary: rustdoc
commit-hash: e16a049adbf94d610787430b6efdf31d896dc5b6
commit-date: 2025-03-03
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

[Urgau]

The current feature gating is done (for rustc) when evaluating the cfg, maybe rustdoc should do the same, even through it will just discard the result.

rust/compiler/rustc_attr_parsing/src/attributes/cfg.rs

Lines 95 to 104 in 30f168e

	gate_cfg(
	&(
	if *b { kw::True } else { kw::False },
	sym::cfg_boolean_literals,
	|features: &Features| features.cfg_boolean_literals(),
	),
	cfg.span(),
	sess,
	features,
	);

[Urgau]

And if rustdoc where to call cfg_matches (one function upper), then #[doc(cfg(...))] would get check-cfg for free, as it's checked in that function.