1 files changed, 69 insertions, 0 deletions
diff --git a/compiler/rustc_builtin_macros/src/cfg.rs b/compiler/rustc_builtin_macros/src/cfg.rs
new file mode 100644
index 000000000..aa355150b
--- /dev/null
+++ b/compiler/rustc_builtin_macros/src/cfg.rs
@@ -0,0 +1,69 @@
+//! The compiler code necessary to support the cfg! extension, which expands to
+//! a literal `true` or `false` based on whether the given cfg matches the
+//! current compilation environment.
+
+use rustc_ast as ast;
+use rustc_ast::token;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_attr as attr;
+use rustc_errors::PResult;
+use rustc_expand::base::{self, *};
+use rustc_macros::SessionDiagnostic;
+use rustc_span::Span;
+
+pub fn expand_cfg(
+    cx: &mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> Box<dyn base::MacResult + 'static> {
+    let sp = cx.with_def_site_ctxt(sp);
+
+    match parse_cfg(cx, sp, tts) {
+        Ok(cfg) => {
+            let matches_cfg = attr::cfg_matches(
+                &cfg,
+                &cx.sess.parse_sess,
+                cx.current_expansion.lint_node_id,
+                cx.ecfg.features,
+            );
+            MacEager::expr(cx.expr_bool(sp, matches_cfg))
+        }
+        Err(mut err) => {
+            err.emit();
+            DummyResult::any(sp)
+        }
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[error(builtin_macros::requires_cfg_pattern)]
+struct RequiresCfgPattern {
+    #[primary_span]
+    #[label]
+    span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[error(builtin_macros::expected_one_cfg_pattern)]
+struct OneCfgPattern {
+    #[primary_span]
+    span: Span,
+}
+
+fn parse_cfg<'a>(cx: &mut ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a, ast::MetaItem> {
+    let mut p = cx.new_parser_from_tts(tts);
+
+    if p.token == token::Eof {
+        return Err(cx.create_err(RequiresCfgPattern { span }));
+    }
+
+    let cfg = p.parse_meta_item()?;
+
+    let _ = p.eat(&token::Comma);
+
+    if !p.eat(&token::Eof) {
+        return Err(cx.create_err(OneCfgPattern { span }));
+    }
+
+    Ok(cfg)
+}