Rollup merge of #131034 - Urgau:cfg-true-false, r=nnethercote

Implement RFC3695 Allow boolean literals as cfg predicates

This PR implements https://github.com/rust-lang/rfcs/pull/3695: allow boolean literals as cfg predicates, i.e. `cfg(true)` and `cfg(false)`.

r? `@nnethercote` *(or anyone with parser knowledge)*
cc `@clubby789`

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,
 }
@@ -533,7 +534,7 @@ 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>,
@@ -604,12 +605,43 @@ 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);
@@ -645,7 +677,7 @@ 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,
@@ -663,23 +695,19 @@ 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
@@ -700,7 +728,12 @@ 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
             }