Implement built-in attribute macro #[cfg_eval]

compiler/rustc_builtin_macros/src/cfg_eval.rs

@@ -0,0 +1,29 @@
+use crate::util::check_builtin_macro_attribute;
+
+use rustc_ast::{self as ast, AstLike};
+use rustc_expand::base::{Annotatable, ExtCtxt};
+use rustc_expand::config::StripUnconfigured;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
+
+pub fn expand(
+    ecx: &mut ExtCtxt<'_>,
+    _span: Span,
+    meta_item: &ast::MetaItem,
+    item: Annotatable,
+) -> Vec<Annotatable> {
+    check_builtin_macro_attribute(ecx, meta_item, sym::cfg_eval);
+
+    let mut visitor =
+        StripUnconfigured { sess: ecx.sess, features: ecx.ecfg.features, modified: false };
+    let mut item = visitor.fully_configure(item);
+    if visitor.modified {
+        // Erase the tokens if cfg-stripping modified the item
+        // This will cause us to synthesize fake tokens
+        // when `nt_to_tokenstream` is called on this item.
+        if let Some(tokens) = item.tokens_mut() {
+            *tokens = None;
+        }
+    }
+    vec![item]
+}