Rollup merge of #138632 - clubby789:stabilize-cfg-boolean-lit, r=davidtwco,Urgau,traviscross

Stabilize `cfg_boolean_literals`

Closes #131204
`@rustbot` labels +T-lang +I-lang-nominated
This will end up conflicting with the test in #138293 so whichever doesn't land first will need updating

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# Stabilization Report

## General design

### What is the RFC for this feature and what changes have occurred to the user-facing design since the RFC was finalized?

[RFC 3695](https://github.com/rust-lang/rfcs/pull/3695), none.

### What behavior are we committing to that has been controversial? Summarize the major arguments pro/con.

None

### Are there extensions to this feature that remain unstable? How do we know that we are not accidentally committing to those?

None

## Has a call-for-testing period been conducted? If so, what feedback was received?

Yes; only positive feedback was received.

## Implementation quality

### Summarize the major parts of the implementation and provide links into the code (or to PRs)

Implemented in [#131034](https://github.com/rust-lang/rust/pull/131034).

### Summarize existing test coverage of this feature

- [Basic usage, including `#[cfg()]`, `cfg!()` and `#[cfg_attr()]`](6d71251cf9/tests/ui/cfg/true-false.rs)
- [`--cfg=true/false` on the command line being accessible via `r#true/r#false`](6d71251cf9/tests/ui/cfg/raw-true-false.rs)
- [Interaction with the unstable `#[doc(cfg(..))]` feature](6d71251/tests/rustdoc-ui/cfg-boolean-literal.rs)
- [Denying `--check-cfg=cfg(true/false)`](6d71251/tests/ui/check-cfg/invalid-arguments.rs)
- Ensuring `--cfg false` on the command line doesn't change the meaning of `cfg(false)`: `tests/ui/cfg/cmdline-false.rs`
- Ensuring both `cfg(true)` and `cfg(false)` on the same item result in it being disabled: `tests/ui/cfg/both-true-false.rs`

### What outstanding bugs in the issue tracker involve this feature? Are they stabilization-blocking?

The above mentioned issue; it should not block as it interacts with another unstable feature.

### What FIXMEs are still in the code for that feature and why is it ok to leave them there?

None

### Summarize contributors to the feature by name for recognition and assuredness that people involved in the feature agree with stabilization
- `@clubby789` (RFC)
- `@Urgau` (Implementation in rustc)

### Which tools need to be adjusted to support this feature. Has this work been done?

`rustdoc`'s  unstable`#[doc(cfg(..)]` has been updated to respect it. `cargo` has been updated with a forward compatibility lint to enable supporting it in cargo once stabilized.

## Type system and execution rules

### What updates are needed to the reference/specification? (link to PRs when they exist)

A few lines to be added to the reference for configuration predicates, specified in the RFC.

compiler/rustc_attr_parsing/src/attributes/cfg.rs

@@ -7,7 +7,6 @@ use rustc_session::config::ExpectedValues;
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::UNEXPECTED_CFGS;
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::kw;
 use rustc_span::{Span, Symbol, sym};
 
 use crate::session_diagnostics::{self, UnsupportedLiteralReason};
@@ -89,20 +88,6 @@ pub fn eval_condition(
     let cfg = match cfg {
         MetaItemInner::MetaItem(meta_item) => meta_item,
         MetaItemInner::Lit(MetaItemLit { kind: LitKind::Bool(b), .. }) => {
-            if let Some(features) = features {
-                // we can't use `try_gate_cfg` as symbols don't differentiate between `r#true`
-                // and `true`, and we want to keep the former working without feature gate
-                gate_cfg(
-                    &(
-                        if *b { kw::True } else { kw::False },
-                        sym::cfg_boolean_literals,
-                        |features: &Features| features.cfg_boolean_literals(),
-                    ),
-                    cfg.span(),
-                    sess,
-                    features,
-                );
-            }
             return *b;
         }
         _ => {

compiler/rustc_feature/src/accepted.rs

@@ -95,6 +95,8 @@ declare_features! (
     (accepted, c_unwind, "1.81.0", Some(74990)),
     /// Allows `#[cfg_attr(predicate, multiple, attributes, here)]`.
     (accepted, cfg_attr_multi, "1.33.0", Some(54881)),
+    /// Allows the use of `#[cfg(<true/false>)]`.
+    (accepted, cfg_boolean_literals, "CURRENT_RUSTC_VERSION", Some(131204)),
     /// Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests.
     (accepted, cfg_doctest, "1.40.0", Some(62210)),
     /// Enables `#[cfg(panic = "...")]` config key.

compiler/rustc_feature/src/unstable.rs

@@ -391,8 +391,6 @@ declare_features! (
     (unstable, async_trait_bounds, "1.85.0", Some(62290)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
-    /// Allows the use of `#[cfg(<true/false>)]`.
-    (unstable, cfg_boolean_literals, "1.83.0", Some(131204)),
     /// Allows the use of `#[cfg(contract_checks)` to check if contract checks are enabled.
     (unstable, cfg_contract_checks, "1.86.0", Some(128044)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.