make cfg_select a builtin macro

compiler/rustc_builtin_macros/messages.ftl

@@ -81,6 +81,12 @@ builtin_macros_cfg_accessible_literal_path = `cfg_accessible` path cannot be a l
 builtin_macros_cfg_accessible_multiple_paths = multiple `cfg_accessible` paths are specified
 builtin_macros_cfg_accessible_unspecified_path = `cfg_accessible` path is not specified
 
+builtin_macros_cfg_select_no_matches = none of the rules in this `cfg_select` evaluated to true
+
+builtin_macros_cfg_select_unreachable = unreachable rule
+    .label = always matches
+    .label2 = this rules is never reached
+
 builtin_macros_coerce_pointee_requires_maybe_sized = `derive(CoercePointee)` requires `{$name}` to be marked `?Sized`
 
 builtin_macros_coerce_pointee_requires_one_field = `CoercePointee` can only be derived on `struct`s with at least one field

compiler/rustc_builtin_macros/src/cfg_select.rs

@@ -0,0 +1,63 @@
+use rustc_ast::tokenstream::TokenStream;
+use rustc_attr_parsing as attr;
+use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
+use rustc_parse::parser::cfg_select::{CfgSelectBranches, CfgSelectRule, parse_cfg_select};
+use rustc_span::{Ident, Span, sym};
+
+use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
+
+/// Selects the first arm whose rule evaluates to true.
+fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
+    for (cfg, tt, arm_span) in branches.reachable {
+        if attr::cfg_matches(
+            &cfg,
+            &ecx.sess,
+            ecx.current_expansion.lint_node_id,
+            Some(ecx.ecfg.features),
+        ) {
+            return Some((tt, arm_span));
+        }
+    }
+
+    branches.wildcard.map(|(_, tt, span)| (tt, span))
+}
+
+pub(super) fn expand_cfg_select<'cx>(
+    ecx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> MacroExpanderResult<'cx> {
+    ExpandResult::Ready(match parse_cfg_select(&mut ecx.new_parser_from_tts(tts)) {
+        Ok(branches) => {
+            if let Some((underscore, _, _)) = branches.wildcard {
+                // Warn for every unreachable rule. We store the fully parsed branch for rustfmt.
+                for (rule, _, _) in &branches.unreachable {
+                    let span = match rule {
+                        CfgSelectRule::Wildcard(underscore) => underscore.span,
+                        CfgSelectRule::Cfg(cfg) => cfg.span(),
+                    };
+                    let err = CfgSelectUnreachable { span, wildcard_span: underscore.span };
+                    ecx.dcx().emit_warn(err);
+                }
+            }
+
+            if let Some((tts, arm_span)) = select_arm(ecx, branches) {
+                return ExpandResult::from_tts(
+                    ecx,
+                    tts,
+                    sp,
+                    arm_span,
+                    Ident::with_dummy_span(sym::cfg_select),
+                );
+            } else {
+                // Emit a compiler error when none of the rules matched.
+                let guar = ecx.dcx().emit_err(CfgSelectNoMatches { span: sp });
+                DummyResult::any(sp, guar)
+            }
+        }
+        Err(err) => {
+            let guar = err.emit();
+            DummyResult::any(sp, guar)
+        }
+    })
+}

compiler/rustc_builtin_macros/src/errors.rs

@@ -954,3 +954,21 @@ pub(crate) struct AsmExpectedOther {
     pub(crate) span: Span,
     pub(crate) is_inline_asm: bool,
 }
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_cfg_select_no_matches)]
+pub(crate) struct CfgSelectNoMatches {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_cfg_select_unreachable)]
+pub(crate) struct CfgSelectUnreachable {
+    #[primary_span]
+    #[label(builtin_macros_label2)]
+    pub span: Span,
+
+    #[label]
+    pub wildcard_span: Span,
+}

compiler/rustc_builtin_macros/src/lib.rs

@@ -33,6 +33,7 @@ mod autodiff;
 mod cfg;
 mod cfg_accessible;
 mod cfg_eval;
+mod cfg_select;
 mod compile_error;
 mod concat;
 mod concat_bytes;
@@ -79,6 +80,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         asm: asm::expand_asm,
         assert: assert::expand_assert,
         cfg: cfg::expand_cfg,
+        cfg_select: cfg_select::expand_cfg_select,
         column: source_util::expand_column,
         compile_error: compile_error::expand_compile_error,
         concat: concat::expand_concat,

compiler/rustc_expand/src/base.rs

@@ -34,6 +34,7 @@ use thin_vec::ThinVec;
 use crate::base::ast::MetaItemInner;
 use crate::errors;
 use crate::expand::{self, AstFragment, Invocation};
+use crate::mbe::macro_rules::ParserAnyMacro;
 use crate::module::DirOwnership;
 use crate::stats::MacroStat;
 
@@ -262,6 +263,25 @@ impl<T, U> ExpandResult<T, U> {
     }
 }
 
+impl<'cx> MacroExpanderResult<'cx> {
+    /// Creates a [`MacroExpanderResult::Ready`] from a [`TokenStream`].
+    ///
+    /// The `TokenStream` is forwarded without any expansion.
+    pub fn from_tts(
+        cx: &'cx mut ExtCtxt<'_>,
+        tts: TokenStream,
+        site_span: Span,
+        arm_span: Span,
+        macro_ident: Ident,
+    ) -> Self {
+        // Emit the SEMICOLON_IN_EXPRESSIONS_FROM_MACROS deprecation lint.
+        let is_local = true;
+
+        let parser = ParserAnyMacro::from_tts(cx, tts, site_span, arm_span, is_local, macro_ident);
+        ExpandResult::Ready(Box::new(parser))
+    }
+}
+
 pub trait MultiItemModifier {
     /// `meta_item` is the attribute, and `item` is the item being modified.
     fn expand(

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -96,6 +96,30 @@ impl<'a> ParserAnyMacro<'a> {
         ensure_complete_parse(parser, &path, kind.name(), site_span);
         fragment
     }
+
+    #[instrument(skip(cx, tts))]
+    pub(crate) fn from_tts<'cx>(
+        cx: &'cx mut ExtCtxt<'a>,
+        tts: TokenStream,
+        site_span: Span,
+        arm_span: Span,
+        is_local: bool,
+        macro_ident: Ident,
+    ) -> Self {
+        Self {
+            parser: Parser::new(&cx.sess.psess, tts, None),
+
+            // Pass along the original expansion site and the name of the macro
+            // so we can print a useful error message if the parse of the expanded
+            // macro leaves unparsed tokens.
+            site_span,
+            macro_ident,
+            lint_node_id: cx.current_expansion.lint_node_id,
+            is_trailing_mac: cx.current_expansion.is_trailing_mac,
+            arm_span,
+            is_local,
+        }
+    }
 }
 
 pub(super) struct MacroRule {
@@ -207,9 +231,6 @@ fn expand_macro<'cx>(
     rules: &[MacroRule],
 ) -> Box<dyn MacResult + 'cx> {
     let psess = &cx.sess.psess;
-    // Macros defined in the current crate have a real node id,
-    // whereas macros from an external crate have a dummy id.
-    let is_local = node_id != DUMMY_NODE_ID;
 
     if cx.trace_macros() {
         let msg = format!("expanding `{}! {{ {} }}`", name, pprust::tts_to_string(&arg));
@@ -220,7 +241,7 @@ fn expand_macro<'cx>(
     let try_success_result = try_match_macro(psess, name, &arg, rules, &mut NoopTracker);
 
     match try_success_result {
-        Ok((i, rule, named_matches)) => {
+        Ok((rule_index, rule, named_matches)) => {
             let mbe::TokenTree::Delimited(rhs_span, _, ref rhs) = rule.rhs else {
                 cx.dcx().span_bug(sp, "malformed macro rhs");
             };
@@ -241,27 +262,13 @@ fn expand_macro<'cx>(
                 trace_macros_note(&mut cx.expansions, sp, msg);
             }
 
-            let p = Parser::new(psess, tts, None);
-
+            let is_local = is_defined_in_current_crate(node_id);
             if is_local {
-                cx.resolver.record_macro_rule_usage(node_id, i);
+                cx.resolver.record_macro_rule_usage(node_id, rule_index);
             }
 
-            // Let the context choose how to interpret the result.
-            // Weird, but useful for X-macros.
-            Box::new(ParserAnyMacro {
-                parser: p,
-
-                // Pass along the original expansion site and the name of the macro
-                // so we can print a useful error message if the parse of the expanded
-                // macro leaves unparsed tokens.
-                site_span: sp,
-                macro_ident: name,
-                lint_node_id: cx.current_expansion.lint_node_id,
-                is_trailing_mac: cx.current_expansion.is_trailing_mac,
-                arm_span,
-                is_local,
-            })
+            // Let the context choose how to interpret the result. Weird, but useful for X-macros.
+            Box::new(ParserAnyMacro::from_tts(cx, tts, sp, arm_span, is_local, name))
         }
         Err(CanRetry::No(guar)) => {
             debug!("Will not retry matching as an error was emitted already");
@@ -373,9 +380,9 @@ pub fn compile_declarative_macro(
     node_id: NodeId,
     edition: Edition,
 ) -> (SyntaxExtension, usize) {
-    let is_local = node_id != DUMMY_NODE_ID;
     let mk_syn_ext = |expander| {
         let kind = SyntaxExtensionKind::LegacyBang(expander);
+        let is_local = is_defined_in_current_crate(node_id);
         SyntaxExtension::new(sess, kind, span, Vec::new(), edition, ident.name, attrs, is_local)
     };
     let dummy_syn_ext = |guar| (mk_syn_ext(Arc::new(DummyExpander(guar))), 0);
@@ -439,7 +446,7 @@ pub fn compile_declarative_macro(
     }
 
     // Return the number of rules for unused rule linting, if this is a local macro.
-    let nrules = if is_local { rules.len() } else { 0 };
+    let nrules = if is_defined_in_current_crate(node_id) { rules.len() } else { 0 };
 
     let expander =
         Arc::new(MacroRulesMacroExpander { name: ident, span, node_id, transparency, rules });
@@ -1034,9 +1041,7 @@ fn check_matcher_core<'tt>(
                     // definition of this macro_rules, not while (re)parsing
                     // the macro when compiling another crate that is using the
                     // macro. (See #86567.)
-                    // Macros defined in the current crate have a real node id,
-                    // whereas macros from an external crate have a dummy id.
-                    if node_id != DUMMY_NODE_ID
+                    if is_defined_in_current_crate(node_id)
                         && matches!(kind, NonterminalKind::Pat(PatParam { inferred: true }))
                         && matches!(
                             next_token,
@@ -1296,6 +1301,12 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
     }
 }
 
+fn is_defined_in_current_crate(node_id: NodeId) -> bool {
+    // Macros defined in the current crate have a real node id,
+    // whereas macros from an external crate have a dummy id.
+    node_id != DUMMY_NODE_ID
+}
+
 pub(super) fn parser_from_cx(
     psess: &ParseSess,
     mut tts: TokenStream,

compiler/rustc_parse/src/parser/cfg_select.rs

@@ -0,0 +1,73 @@
+use rustc_ast::token::Token;
+use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::{MetaItemInner, token};
+use rustc_errors::PResult;
+use rustc_span::Span;
+
+use crate::exp;
+use crate::parser::Parser;
+
+pub enum CfgSelectRule {
+    Cfg(MetaItemInner),
+    Wildcard(Token),
+}
+
+#[derive(Default)]
+pub struct CfgSelectBranches {
+    /// All the conditional branches.
+    pub reachable: Vec<(MetaItemInner, TokenStream, Span)>,
+    /// The first wildcard `_ => { ... }` branch.
+    pub wildcard: Option<(Token, TokenStream, Span)>,
+    /// All branches after the first wildcard, including further wildcards.
+    /// These branches are kept for formatting.
+    pub unreachable: Vec<(CfgSelectRule, TokenStream, Span)>,
+}
+
+/// Parses a `TokenTree` that must be of the form `{ /* ... */ }`, and returns a `TokenStream` where
+/// the surrounding braces are stripped.
+fn parse_token_tree<'a>(p: &mut Parser<'a>) -> PResult<'a, TokenStream> {
+    // Generate an error if the `=>` is not followed by `{`.
+    if p.token != token::OpenBrace {
+        p.expect(exp!(OpenBrace))?;
+    }
+
+    // Strip the outer '{' and '}'.
+    match p.parse_token_tree() {
+        TokenTree::Token(..) => unreachable!("because of the expect above"),
+        TokenTree::Delimited(.., tts) => Ok(tts),
+    }
+}
+
+pub fn parse_cfg_select<'a>(p: &mut Parser<'a>) -> PResult<'a, CfgSelectBranches> {
+    let mut branches = CfgSelectBranches::default();
+
+    while p.token != token::Eof {
+        if p.eat_keyword(exp!(Underscore)) {
+            let underscore = p.prev_token;
+            p.expect(exp!(FatArrow))?;
+
+            let tts = parse_token_tree(p)?;
+            let span = underscore.span.to(p.token.span);
+
+            match branches.wildcard {
+                None => branches.wildcard = Some((underscore, tts, span)),
+                Some(_) => {
+                    branches.unreachable.push((CfgSelectRule::Wildcard(underscore), tts, span))
+                }
+            }
+        } else {
+            let meta_item = p.parse_meta_item_inner()?;
+            p.expect(exp!(FatArrow))?;
+
+            let tts = parse_token_tree(p)?;
+            let span = meta_item.span().to(p.token.span);
+
+            match branches.wildcard {
+                None => branches.reachable.push((meta_item, tts, span)),
+                Some(_) => branches.unreachable.push((CfgSelectRule::Cfg(meta_item), tts, span)),
+            }
+        }
+    }
+
+    Ok(branches)
+}

compiler/rustc_parse/src/parser/mod.rs

@@ -1,4 +1,3 @@
-pub mod asm;
 pub mod attr;
 mod attr_wrapper;
 mod diagnostics;
@@ -12,6 +11,11 @@ mod stmt;
 pub mod token_type;
 mod ty;
 
+// Parsers for non-functionlike builtin macros are defined in rustc_parse so they can be used by
+// both rustc_builtin_macros and rustfmt.
+pub mod asm;
+pub mod cfg_select;
+
 use std::assert_matches::debug_assert_matches;
 use std::{fmt, mem, slice};
 

compiler/rustc_span/src/symbol.rs

@@ -624,6 +624,7 @@ symbols! {
         cfg_relocation_model,
         cfg_sanitize,
         cfg_sanitizer_cfi,
+        cfg_select,
         cfg_target_abi,
         cfg_target_compact,
         cfg_target_feature,