1 files changed, 269 insertions, 0 deletions
diff --git a/compiler/rustc_builtin_macros/src/cfg_eval.rs b/compiler/rustc_builtin_macros/src/cfg_eval.rs
new file mode 100644
index 000000000..89b2c3292
--- /dev/null
+++ b/compiler/rustc_builtin_macros/src/cfg_eval.rs
@@ -0,0 +1,269 @@
+use crate::util::{check_builtin_macro_attribute, warn_on_duplicate_attribute};
+
+use rustc_ast as ast;
+use rustc_ast::mut_visit::MutVisitor;
+use rustc_ast::ptr::P;
+use rustc_ast::visit::Visitor;
+use rustc_ast::NodeId;
+use rustc_ast::{mut_visit, visit};
+use rustc_ast::{Attribute, HasAttrs, HasTokens};
+use rustc_expand::base::{Annotatable, ExtCtxt};
+use rustc_expand::config::StripUnconfigured;
+use rustc_expand::configure;
+use rustc_feature::Features;
+use rustc_parse::parser::{ForceCollect, Parser};
+use rustc_session::Session;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
+use smallvec::SmallVec;
+
+pub(crate) fn expand(
+    ecx: &mut ExtCtxt<'_>,
+    _span: Span,
+    meta_item: &ast::MetaItem,
+    annotatable: Annotatable,
+) -> Vec<Annotatable> {
+    check_builtin_macro_attribute(ecx, meta_item, sym::cfg_eval);
+    warn_on_duplicate_attribute(&ecx, &annotatable, sym::cfg_eval);
+    vec![cfg_eval(ecx.sess, ecx.ecfg.features, annotatable, ecx.current_expansion.lint_node_id)]
+}
+
+pub(crate) fn cfg_eval(
+    sess: &Session,
+    features: Option<&Features>,
+    annotatable: Annotatable,
+    lint_node_id: NodeId,
+) -> Annotatable {
+    CfgEval { cfg: &mut StripUnconfigured { sess, features, config_tokens: true, lint_node_id } }
+        .configure_annotatable(annotatable)
+        // Since the item itself has already been configured by the `InvocationCollector`,
+        // we know that fold result vector will contain exactly one element.
+        .unwrap()
+}
+
+struct CfgEval<'a, 'b> {
+    cfg: &'a mut StripUnconfigured<'b>,
+}
+
+fn flat_map_annotatable(
+    vis: &mut impl MutVisitor,
+    annotatable: Annotatable,
+) -> Option<Annotatable> {
+    match annotatable {
+        Annotatable::Item(item) => vis.flat_map_item(item).pop().map(Annotatable::Item),
+        Annotatable::TraitItem(item) => {
+            vis.flat_map_trait_item(item).pop().map(Annotatable::TraitItem)
+        }
+        Annotatable::ImplItem(item) => {
+            vis.flat_map_impl_item(item).pop().map(Annotatable::ImplItem)
+        }
+        Annotatable::ForeignItem(item) => {
+            vis.flat_map_foreign_item(item).pop().map(Annotatable::ForeignItem)
+        }
+        Annotatable::Stmt(stmt) => {
+            vis.flat_map_stmt(stmt.into_inner()).pop().map(P).map(Annotatable::Stmt)
+        }
+        Annotatable::Expr(mut expr) => {
+            vis.visit_expr(&mut expr);
+            Some(Annotatable::Expr(expr))
+        }
+        Annotatable::Arm(arm) => vis.flat_map_arm(arm).pop().map(Annotatable::Arm),
+        Annotatable::ExprField(field) => {
+            vis.flat_map_expr_field(field).pop().map(Annotatable::ExprField)
+        }
+        Annotatable::PatField(fp) => vis.flat_map_pat_field(fp).pop().map(Annotatable::PatField),
+        Annotatable::GenericParam(param) => {
+            vis.flat_map_generic_param(param).pop().map(Annotatable::GenericParam)
+        }
+        Annotatable::Param(param) => vis.flat_map_param(param).pop().map(Annotatable::Param),
+        Annotatable::FieldDef(sf) => vis.flat_map_field_def(sf).pop().map(Annotatable::FieldDef),
+        Annotatable::Variant(v) => vis.flat_map_variant(v).pop().map(Annotatable::Variant),
+        Annotatable::Crate(mut krate) => {
+            vis.visit_crate(&mut krate);
+            Some(Annotatable::Crate(krate))
+        }
+    }
+}
+
+struct CfgFinder {
+    has_cfg_or_cfg_attr: bool,
+}
+
+impl CfgFinder {
+    fn has_cfg_or_cfg_attr(annotatable: &Annotatable) -> bool {
+        let mut finder = CfgFinder { has_cfg_or_cfg_attr: false };
+        match annotatable {
+            Annotatable::Item(item) => finder.visit_item(&item),
+            Annotatable::TraitItem(item) => finder.visit_assoc_item(&item, visit::AssocCtxt::Trait),
+            Annotatable::ImplItem(item) => finder.visit_assoc_item(&item, visit::AssocCtxt::Impl),
+            Annotatable::ForeignItem(item) => finder.visit_foreign_item(&item),
+            Annotatable::Stmt(stmt) => finder.visit_stmt(&stmt),
+            Annotatable::Expr(expr) => finder.visit_expr(&expr),
+            Annotatable::Arm(arm) => finder.visit_arm(&arm),
+            Annotatable::ExprField(field) => finder.visit_expr_field(&field),
+            Annotatable::PatField(field) => finder.visit_pat_field(&field),
+            Annotatable::GenericParam(param) => finder.visit_generic_param(&param),
+            Annotatable::Param(param) => finder.visit_param(&param),
+            Annotatable::FieldDef(field) => finder.visit_field_def(&field),
+            Annotatable::Variant(variant) => finder.visit_variant(&variant),
+            Annotatable::Crate(krate) => finder.visit_crate(krate),
+        };
+        finder.has_cfg_or_cfg_attr
+    }
+}
+
+impl<'ast> visit::Visitor<'ast> for CfgFinder {
+    fn visit_attribute(&mut self, attr: &'ast Attribute) {
+        // We want short-circuiting behavior, so don't use the '|=' operator.
+        self.has_cfg_or_cfg_attr = self.has_cfg_or_cfg_attr
+            || attr
+                .ident()
+                .map_or(false, |ident| ident.name == sym::cfg || ident.name == sym::cfg_attr);
+    }
+}
+
+impl CfgEval<'_, '_> {
+    fn configure<T: HasAttrs + HasTokens>(&mut self, node: T) -> Option<T> {
+        self.cfg.configure(node)
+    }
+
+    fn configure_annotatable(&mut self, mut annotatable: Annotatable) -> Option<Annotatable> {
+        // Tokenizing and re-parsing the `Annotatable` can have a significant
+        // performance impact, so try to avoid it if possible
+        if !CfgFinder::has_cfg_or_cfg_attr(&annotatable) {
+            return Some(annotatable);
+        }
+
+        // The majority of parsed attribute targets will never need to have early cfg-expansion
+        // run (e.g. they are not part of a `#[derive]` or `#[cfg_eval]` macro input).
+        // Therefore, we normally do not capture the necessary information about `#[cfg]`
+        // and `#[cfg_attr]` attributes during parsing.
+        //
+        // Therefore, when we actually *do* run early cfg-expansion, we need to tokenize
+        // and re-parse the attribute target, this time capturing information about
+        // the location of `#[cfg]` and `#[cfg_attr]` in the token stream. The tokenization
+        // process is lossless, so this process is invisible to proc-macros.
+
+        let parse_annotatable_with: fn(&mut Parser<'_>) -> _ = match annotatable {
+            Annotatable::Item(_) => {
+                |parser| Annotatable::Item(parser.parse_item(ForceCollect::Yes).unwrap().unwrap())
+            }
+            Annotatable::TraitItem(_) => |parser| {
+                Annotatable::TraitItem(
+                    parser.parse_trait_item(ForceCollect::Yes).unwrap().unwrap().unwrap(),
+                )
+            },
+            Annotatable::ImplItem(_) => |parser| {
+                Annotatable::ImplItem(
+                    parser.parse_impl_item(ForceCollect::Yes).unwrap().unwrap().unwrap(),
+                )
+            },
+            Annotatable::ForeignItem(_) => |parser| {
+                Annotatable::ForeignItem(
+                    parser.parse_foreign_item(ForceCollect::Yes).unwrap().unwrap().unwrap(),
+                )
+            },
+            Annotatable::Stmt(_) => |parser| {
+                Annotatable::Stmt(P(parser.parse_stmt(ForceCollect::Yes).unwrap().unwrap()))
+            },
+            Annotatable::Expr(_) => {
+                |parser| Annotatable::Expr(parser.parse_expr_force_collect().unwrap())
+            }
+            _ => unreachable!(),
+        };
+
+        // 'Flatten' all nonterminals (i.e. `TokenKind::Interpolated`)
+        // to `None`-delimited groups containing the corresponding tokens. This
+        // is normally delayed until the proc-macro server actually needs to
+        // provide a `TokenKind::Interpolated` to a proc-macro. We do this earlier,
+        // so that we can handle cases like:
+        //
+        // ```rust
+        // #[cfg_eval] #[cfg] $item
+        //```
+        //
+        // where `$item` is `#[cfg_attr] struct Foo {}`. We want to make
+        // sure to evaluate *all* `#[cfg]` and `#[cfg_attr]` attributes - the simplest
+        // way to do this is to do a single parse of a stream without any nonterminals.
+        let orig_tokens = annotatable.to_tokens().flattened();
+
+        // Re-parse the tokens, setting the `capture_cfg` flag to save extra information
+        // to the captured `AttrAnnotatedTokenStream` (specifically, we capture
+        // `AttrAnnotatedTokenTree::AttributesData` for all occurrences of `#[cfg]` and `#[cfg_attr]`)
+        let mut parser =
+            rustc_parse::stream_to_parser(&self.cfg.sess.parse_sess, orig_tokens, None);
+        parser.capture_cfg = true;
+        annotatable = parse_annotatable_with(&mut parser);
+
+        // Now that we have our re-parsed `AttrAnnotatedTokenStream`, recursively configuring
+        // our attribute target will correctly the tokens as well.
+        flat_map_annotatable(self, annotatable)
+    }
+}
+
+impl MutVisitor for CfgEval<'_, '_> {
+    fn visit_expr(&mut self, expr: &mut P<ast::Expr>) {
+        self.cfg.configure_expr(expr);
+        mut_visit::noop_visit_expr(expr, self);
+    }
+
+    fn filter_map_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
+        let mut expr = configure!(self, expr);
+        mut_visit::noop_visit_expr(&mut expr, self);
+        Some(expr)
+    }
+
+    fn flat_map_generic_param(
+        &mut self,
+        param: ast::GenericParam,
+    ) -> SmallVec<[ast::GenericParam; 1]> {
+        mut_visit::noop_flat_map_generic_param(configure!(self, param), self)
+    }
+
+    fn flat_map_stmt(&mut self, stmt: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
+        mut_visit::noop_flat_map_stmt(configure!(self, stmt), self)
+    }
+
+    fn flat_map_item(&mut self, item: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
+        mut_visit::noop_flat_map_item(configure!(self, item), self)
+    }
+
+    fn flat_map_impl_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        mut_visit::noop_flat_map_assoc_item(configure!(self, item), self)
+    }
+
+    fn flat_map_trait_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        mut_visit::noop_flat_map_assoc_item(configure!(self, item), self)
+    }
+
+    fn flat_map_foreign_item(
+        &mut self,
+        foreign_item: P<ast::ForeignItem>,
+    ) -> SmallVec<[P<ast::ForeignItem>; 1]> {
+        mut_visit::noop_flat_map_foreign_item(configure!(self, foreign_item), self)
+    }
+
+    fn flat_map_arm(&mut self, arm: ast::Arm) -> SmallVec<[ast::Arm; 1]> {
+        mut_visit::noop_flat_map_arm(configure!(self, arm), self)
+    }
+
+    fn flat_map_expr_field(&mut self, field: ast::ExprField) -> SmallVec<[ast::ExprField; 1]> {
+        mut_visit::noop_flat_map_expr_field(configure!(self, field), self)
+    }
+
+    fn flat_map_pat_field(&mut self, fp: ast::PatField) -> SmallVec<[ast::PatField; 1]> {
+        mut_visit::noop_flat_map_pat_field(configure!(self, fp), self)
+    }
+
+    fn flat_map_param(&mut self, p: ast::Param) -> SmallVec<[ast::Param; 1]> {
+        mut_visit::noop_flat_map_param(configure!(self, p), self)
+    }
+
+    fn flat_map_field_def(&mut self, sf: ast::FieldDef) -> SmallVec<[ast::FieldDef; 1]> {
+        mut_visit::noop_flat_map_field_def(configure!(self, sf), self)
+    }
+
+    fn flat_map_variant(&mut self, variant: ast::Variant) -> SmallVec<[ast::Variant; 1]> {
+        mut_visit::noop_flat_map_variant(configure!(self, variant), self)
+    }
+}