Implement RFC3695 Allow boolean literals as cfg predicates

compiler/rustc_ast/src/attr/mod.rs

@@ -527,6 +527,16 @@ impl NestedMetaItem {
         }
     }
 
+    /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem` or if it's
+    /// `NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(_), .. })`.
+    pub fn meta_item_or_bool(&self) -> Option<&NestedMetaItem> {
+        match self {
+            NestedMetaItem::MetaItem(_item) => Some(self),
+            NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(_), .. }) => Some(self),
+            _ => None,
+        }
+    }
+
     /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem`.
     pub fn meta_item(&self) -> Option<&MetaItem> {
         match self {

compiler/rustc_attr/messages.ftl

@@ -107,6 +107,8 @@ attr_unknown_version_literal =
 attr_unstable_cfg_target_compact =
     compact `cfg(target(..))` is experimental and subject to change
 
+attr_unsupported_literal_cfg_boolean =
+    literal in `cfg` predicate value must be a boolean
 attr_unsupported_literal_cfg_string =
     literal in `cfg` predicate value must be a string
 attr_unsupported_literal_deprecated_kv_pair =

compiler/rustc_attr/src/builtin.rs

@@ -18,7 +18,7 @@ use rustc_session::parse::feature_err;
 use rustc_session::{RustcVersion, Session};
 use rustc_span::Span;
 use rustc_span::hygiene::Transparency;
-use rustc_span::symbol::{Symbol, sym};
+use rustc_span::symbol::{Symbol, kw, sym};
 
 use crate::fluent_generated;
 use crate::session_diagnostics::{self, IncorrectReprFormatGenericCause};
@@ -36,6 +36,7 @@ pub fn is_builtin_attr(attr: &Attribute) -> bool {
 pub(crate) enum UnsupportedLiteralReason {
     Generic,
     CfgString,
+    CfgBoolean,
     DeprecatedString,
     DeprecatedKvPair,
 }
@@ -523,7 +524,7 @@ pub struct Condition {
 
 /// Tests if a cfg-pattern matches the cfg set
 pub fn cfg_matches(
-    cfg: &ast::MetaItem,
+    cfg: &ast::NestedMetaItem,
     sess: &Session,
     lint_node_id: NodeId,
     features: Option<&Features>,
@@ -594,12 +595,43 @@ pub fn parse_version(s: Symbol) -> Option<RustcVersion> {
 /// Evaluate a cfg-like condition (with `any` and `all`), using `eval` to
 /// evaluate individual items.
 pub fn eval_condition(
-    cfg: &ast::MetaItem,
+    cfg: &ast::NestedMetaItem,
     sess: &Session,
     features: Option<&Features>,
     eval: &mut impl FnMut(Condition) -> bool,
 ) -> bool {
     let dcx = sess.dcx();
+
+    let cfg = match cfg {
+        ast::NestedMetaItem::MetaItem(meta_item) => meta_item,
+        ast::NestedMetaItem::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;
+        }
+        _ => {
+            dcx.emit_err(session_diagnostics::UnsupportedLiteral {
+                span: cfg.span(),
+                reason: UnsupportedLiteralReason::CfgBoolean,
+                is_bytestr: false,
+                start_point_span: sess.source_map().start_point(cfg.span()),
+            });
+            return false;
+        }
+    };
+
     match &cfg.kind {
         ast::MetaItemKind::List(mis) if cfg.name_or_empty() == sym::version => {
             try_gate_cfg(sym::version, cfg.span, sess, features);
@@ -635,7 +667,7 @@ pub fn eval_condition(
         }
         ast::MetaItemKind::List(mis) => {
             for mi in mis.iter() {
-                if !mi.is_meta_item() {
+                if mi.meta_item_or_bool().is_none() {
                     dcx.emit_err(session_diagnostics::UnsupportedLiteral {
                         span: mi.span(),
                         reason: UnsupportedLiteralReason::Generic,
@@ -653,23 +685,19 @@ pub fn eval_condition(
                     .iter()
                     // We don't use any() here, because we want to evaluate all cfg condition
                     // as eval_condition can (and does) extra checks
-                    .fold(false, |res, mi| {
-                        res | eval_condition(mi.meta_item().unwrap(), sess, features, eval)
-                    }),
+                    .fold(false, |res, mi| res | eval_condition(mi, sess, features, eval)),
                 sym::all => mis
                     .iter()
                     // We don't use all() here, because we want to evaluate all cfg condition
                     // as eval_condition can (and does) extra checks
-                    .fold(true, |res, mi| {
-                        res & eval_condition(mi.meta_item().unwrap(), sess, features, eval)
-                    }),
+                    .fold(true, |res, mi| res & eval_condition(mi, sess, features, eval)),
                 sym::not => {
                     let [mi] = mis.as_slice() else {
                         dcx.emit_err(session_diagnostics::ExpectedOneCfgPattern { span: cfg.span });
                         return false;
                     };
 
-                    !eval_condition(mi.meta_item().unwrap(), sess, features, eval)
+                    !eval_condition(mi, sess, features, eval)
                 }
                 sym::target => {
                     if let Some(features) = features
@@ -690,7 +718,12 @@ pub fn eval_condition(
                             seg.ident.name = Symbol::intern(&format!("target_{}", seg.ident.name));
                         }
 
-                        res & eval_condition(&mi, sess, features, eval)
+                        res & eval_condition(
+                            &ast::NestedMetaItem::MetaItem(mi),
+                            sess,
+                            features,
+                            eval,
+                        )
                     })
                 }
                 _ => {

compiler/rustc_attr/src/session_diagnostics.rs

@@ -206,6 +206,7 @@ impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for UnsupportedLiteral {
         let mut diag = Diag::new(dcx, level, match self.reason {
             UnsupportedLiteralReason::Generic => fluent::attr_unsupported_literal_generic,
             UnsupportedLiteralReason::CfgString => fluent::attr_unsupported_literal_cfg_string,
+            UnsupportedLiteralReason::CfgBoolean => fluent::attr_unsupported_literal_cfg_boolean,
             UnsupportedLiteralReason::DeprecatedString => {
                 fluent::attr_unsupported_literal_deprecated_string
             }

compiler/rustc_builtin_macros/src/cfg.rs

@@ -6,7 +6,6 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::PResult;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacEager, MacroExpanderResult};
-use rustc_parse::parser::attr::AllowLeadingUnsafe;
 use rustc_span::Span;
 use {rustc_ast as ast, rustc_attr as attr};
 
@@ -36,14 +35,18 @@ pub(crate) fn expand_cfg(
     })
 }
 
-fn parse_cfg<'a>(cx: &ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a, ast::MetaItem> {
+fn parse_cfg<'a>(
+    cx: &ExtCtxt<'a>,
+    span: Span,
+    tts: TokenStream,
+) -> PResult<'a, ast::NestedMetaItem> {
     let mut p = cx.new_parser_from_tts(tts);
 
     if p.token == token::Eof {
         return Err(cx.dcx().create_err(errors::RequiresCfgPattern { span }));
     }
 
-    let cfg = p.parse_meta_item(AllowLeadingUnsafe::No)?;
+    let cfg = p.parse_meta_item_inner()?;
 
     let _ = p.eat(&token::Comma);
 

compiler/rustc_codegen_ssa/src/lib.rs

@@ -156,7 +156,7 @@ pub struct NativeLib {
     pub kind: NativeLibKind,
     pub name: Symbol,
     pub filename: Option<Symbol>,
-    pub cfg: Option<ast::MetaItem>,
+    pub cfg: Option<ast::NestedMetaItem>,
     pub verbatim: bool,
     pub dll_imports: Vec<cstore::DllImport>,
 }

compiler/rustc_expand/src/config.rs

@@ -5,7 +5,9 @@ use rustc_ast::token::{Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::{
     AttrTokenStream, AttrTokenTree, LazyAttrTokenStream, Spacing, TokenTree,
 };
-use rustc_ast::{self as ast, AttrStyle, Attribute, HasAttrs, HasTokens, MetaItem, NodeId};
+use rustc_ast::{
+    self as ast, AttrStyle, Attribute, HasAttrs, HasTokens, MetaItem, NestedMetaItem, NodeId,
+};
 use rustc_attr as attr;
 use rustc_data_structures::flat_map_in_place::FlatMapInPlace;
 use rustc_feature::{
@@ -449,7 +451,7 @@ impl<'a> StripUnconfigured<'a> {
     }
 }
 
-pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a MetaItem> {
+pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a NestedMetaItem> {
     let span = meta_item.span;
     match meta_item.meta_item_list() {
         None => {
@@ -464,7 +466,7 @@ pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a Meta
             sess.dcx().emit_err(InvalidCfg::MultiplePredicates { span: l.span() });
             None
         }
-        Some([single]) => match single.meta_item() {
+        Some([single]) => match single.meta_item_or_bool() {
             Some(meta_item) => Some(meta_item),
             None => {
                 sess.dcx().emit_err(InvalidCfg::PredicateLiteral { span: single.span() });

compiler/rustc_feature/src/unstable.rs

@@ -371,6 +371,8 @@ declare_features! (
     (unstable, async_for_loop, "1.77.0", Some(118898)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
+    /// Allows the use of `#[cfg(<true/false>)]`.
+    (unstable, cfg_boolean_literals, "CURRENT_RUSTC_VERSION", Some(131204)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.
     (unstable, cfg_overflow_checks, "1.71.0", Some(111466)),
     /// Provides the relocation model information as cfg entry

compiler/rustc_metadata/src/native_libs.rs

@@ -1,7 +1,7 @@
 use std::ops::ControlFlow;
 use std::path::{Path, PathBuf};
 
-use rustc_ast::{CRATE_NODE_ID, NestedMetaItem};
+use rustc_ast::CRATE_NODE_ID;
 use rustc_attr as attr;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::query::LocalCrate;
@@ -304,7 +304,12 @@ impl<'tcx> Collector<'tcx> {
                             sess.dcx().emit_err(errors::LinkCfgForm { span: item.span() });
                             continue;
                         };
-                        let [NestedMetaItem::MetaItem(link_cfg)] = link_cfg else {
+                        let [link_cfg] = link_cfg else {
+                            sess.dcx()
+                                .emit_err(errors::LinkCfgSinglePredicate { span: item.span() });
+                            continue;
+                        };
+                        let Some(link_cfg) = link_cfg.meta_item_or_bool() else {
                             sess.dcx()
                                 .emit_err(errors::LinkCfgSinglePredicate { span: item.span() });
                             continue;

compiler/rustc_parse/src/lib.rs

@@ -18,7 +18,7 @@ use std::path::Path;
 
 use rustc_ast as ast;
 use rustc_ast::tokenstream::TokenStream;
-use rustc_ast::{AttrItem, Attribute, MetaItem, token};
+use rustc_ast::{AttrItem, Attribute, NestedMetaItem, token};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Diag, FatalError, PResult};
@@ -160,7 +160,7 @@ pub fn fake_token_stream_for_crate(psess: &ParseSess, krate: &ast::Crate) -> Tok
 pub fn parse_cfg_attr(
     cfg_attr: &Attribute,
     psess: &ParseSess,
-) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
+) -> Option<(NestedMetaItem, Vec<(AttrItem, Span)>)> {
     const CFG_ATTR_GRAMMAR_HELP: &str = "#[cfg_attr(condition, attribute, other_attribute, ...)]";
     const CFG_ATTR_NOTE_REF: &str = "for more information, visit \
         <https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute>";

compiler/rustc_parse/src/parser/attr.rs

@@ -356,8 +356,10 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `cfg_attr(pred, attr_item_list)` where `attr_item_list` is comma-delimited.
-    pub fn parse_cfg_attr(&mut self) -> PResult<'a, (ast::MetaItem, Vec<(ast::AttrItem, Span)>)> {
-        let cfg_predicate = self.parse_meta_item(AllowLeadingUnsafe::No)?;
+    pub fn parse_cfg_attr(
+        &mut self,
+    ) -> PResult<'a, (ast::NestedMetaItem, Vec<(ast::AttrItem, Span)>)> {
+        let cfg_predicate = self.parse_meta_item_inner()?;
         self.expect(&token::Comma)?;
 
         // Presumably, the majority of the time there will only be one attr.
@@ -452,7 +454,7 @@ impl<'a> Parser<'a> {
     /// ```ebnf
     /// MetaItemInner = UNSUFFIXED_LIT | MetaItem ;
     /// ```
-    fn parse_meta_item_inner(&mut self) -> PResult<'a, ast::NestedMetaItem> {
+    pub fn parse_meta_item_inner(&mut self) -> PResult<'a, ast::NestedMetaItem> {
         match self.parse_unsuffixed_meta_item_lit() {
             Ok(lit) => return Ok(ast::NestedMetaItem::Lit(lit)),
             Err(err) => err.cancel(), // we provide a better error below

compiler/rustc_session/src/cstore.rs

@@ -72,7 +72,7 @@ pub struct NativeLib {
     pub name: Symbol,
     /// If packed_bundled_libs enabled, actual filename of library is stored.
     pub filename: Option<Symbol>,
-    pub cfg: Option<ast::MetaItem>,
+    pub cfg: Option<ast::NestedMetaItem>,
     pub foreign_module: Option<DefId>,
     pub verbatim: Option<bool>,
     pub dll_imports: Vec<DllImport>,

compiler/rustc_span/src/symbol.rs

@@ -543,6 +543,7 @@ symbols! {
         cfg_accessible,
         cfg_attr,
         cfg_attr_multi,
+        cfg_boolean_literals,
         cfg_doctest,
         cfg_eval,
         cfg_fmt_debug,

compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

@@ -1,7 +1,7 @@
 use std::iter;
 use std::path::PathBuf;
 
-use rustc_ast::{AttrArgs, AttrArgsEq, AttrKind, Attribute, MetaItem, NestedMetaItem};
+use rustc_ast::{AttrArgs, AttrArgsEq, AttrKind, Attribute, NestedMetaItem};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::codes::*;
 use rustc_errors::{ErrorGuaranteed, struct_span_code_err};
@@ -282,7 +282,7 @@ pub struct OnUnimplementedFormatString {
 
 #[derive(Debug)]
 pub struct OnUnimplementedDirective {
-    pub condition: Option<MetaItem>,
+    pub condition: Option<NestedMetaItem>,
     pub subcommands: Vec<OnUnimplementedDirective>,
     pub message: Option<OnUnimplementedFormatString>,
     pub label: Option<OnUnimplementedFormatString>,
@@ -414,7 +414,7 @@ impl<'tcx> OnUnimplementedDirective {
             let cond = item_iter
                 .next()
                 .ok_or_else(|| tcx.dcx().emit_err(EmptyOnClauseInOnUnimplemented { span }))?
-                .meta_item()
+                .meta_item_or_bool()
                 .ok_or_else(|| tcx.dcx().emit_err(InvalidOnClauseInOnUnimplemented { span }))?;
             attr::eval_condition(cond, &tcx.sess, Some(tcx.features()), &mut |cfg| {
                 if let Some(value) = cfg.value
@@ -558,8 +558,8 @@ impl<'tcx> OnUnimplementedDirective {
                         IgnoredDiagnosticOption::maybe_emit_warning(
                             tcx,
                             item_def_id,
-                            directive.condition.as_ref().map(|i| i.span),
-                            aggr.condition.as_ref().map(|i| i.span),
+                            directive.condition.as_ref().map(|i| i.span()),
+                            aggr.condition.as_ref().map(|i| i.span()),
                             "condition",
                         );
                         IgnoredDiagnosticOption::maybe_emit_warning(

src/doc/unstable-book/src/language-features/cfg-boolean-literals.md

@@ -0,0 +1,22 @@
+# `cfg_boolean_literals`
+
+The tracking issue for this feature is: [#131204]
+
+[#131204]: https://github.com/rust-lang/rust/issues/131204
+
+------------------------
+
+The `cfg_boolean_literals` feature makes it possible to use the `true`/`false`
+literal as cfg predicate. They always evaluate to true/false respectively.
+
+## Examples
+
+```rust
+#![feature(cfg_boolean_literals)]
+
+#[cfg(true)]
+const A: i32 = 5;
+
+#[cfg(all(false))]
+const A: i32 = 58 * 89;
+```