derive: Do not configure or clone items unless necessary

compiler/rustc_builtin_macros/src/cfg_eval.rs

@@ -2,6 +2,7 @@ use crate::util::check_builtin_macro_attribute;
 
 use rustc_ast as ast;
 use rustc_ast::mut_visit::MutVisitor;
+use rustc_ast::ptr::P;
 use rustc_ast::tokenstream::CanSynthesizeMissingTokens;
 use rustc_ast::visit::Visitor;
 use rustc_ast::{mut_visit, visit};
@@ -9,10 +10,10 @@ use rustc_ast::{AstLike, Attribute};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_expand::config::StripUnconfigured;
 use rustc_expand::configure;
+use rustc_feature::Features;
 use rustc_parse::parser::ForceCollect;
 use rustc_session::utils::FlattenNonterminals;
-
-use rustc_ast::ptr::P;
+use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 use smallvec::SmallVec;
@@ -24,21 +25,19 @@ crate fn expand(
     annotatable: Annotatable,
 ) -> Vec<Annotatable> {
     check_builtin_macro_attribute(ecx, meta_item, sym::cfg_eval);
-    vec![cfg_eval(ecx, annotatable)]
+    vec![cfg_eval(ecx.sess, ecx.ecfg.features, annotatable)]
 }
 
-crate fn cfg_eval(ecx: &ExtCtxt<'_>, annotatable: Annotatable) -> Annotatable {
-    CfgEval {
-        cfg: &mut StripUnconfigured {
-            sess: ecx.sess,
-            features: ecx.ecfg.features,
-            config_tokens: true,
-        },
-    }
-    .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()
+crate fn cfg_eval(
+    sess: &Session,
+    features: Option<&Features>,
+    annotatable: Annotatable,
+) -> Annotatable {
+    CfgEval { cfg: &mut StripUnconfigured { sess, features, config_tokens: true } }
+        .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> {

compiler/rustc_builtin_macros/src/derive.rs

@@ -1,12 +1,13 @@
 use crate::cfg_eval::cfg_eval;
 
-use rustc_ast::{self as ast, attr, token, ItemKind, MetaItemKind, NestedMetaItem, StmtKind};
+use rustc_ast as ast;
+use rustc_ast::{attr, token, GenericParamKind, ItemKind, MetaItemKind, NestedMetaItem, StmtKind};
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_expand::base::{Annotatable, ExpandResult, ExtCtxt, Indeterminate, MultiItemModifier};
 use rustc_feature::AttributeTemplate;
 use rustc_parse::validate_attr;
 use rustc_session::Session;
-use rustc_span::symbol::sym;
+use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
 
 crate struct Expander;
@@ -26,8 +27,7 @@ impl MultiItemModifier for Expander {
             return ExpandResult::Ready(vec![item]);
         }
 
-        let configured_item = cfg_eval(ecx, item.clone());
-
+        let (sess, features) = (ecx.sess, ecx.ecfg.features);
         let result =
             ecx.resolver.resolve_derives(ecx.current_expansion.id, ecx.force_mode, &|| {
                 let template =
@@ -40,7 +40,8 @@ impl MultiItemModifier for Expander {
                     template,
                 );
 
-                attr.meta_item_list()
+                let mut resolutions: Vec<_> = attr
+                    .meta_item_list()
                     .unwrap_or_default()
                     .into_iter()
                     .filter_map(|nested_meta| match nested_meta {
@@ -56,8 +57,21 @@ impl MultiItemModifier for Expander {
                         report_path_args(sess, &meta);
                         meta.path
                     })
-                    .map(|path| (path, configured_item.clone(), None))
-                    .collect()
+                    .map(|path| (path, dummy_annotatable(), None))
+                    .collect();
+
+                // Do not configure or clone items unless necessary.
+                match &mut resolutions[..] {
+                    [] => {}
+                    [(_, first_item, _), others @ ..] => {
+                        *first_item = cfg_eval(sess, features, item.clone());
+                        for (_, item, _) in others {
+                            *item = first_item.clone();
+                        }
+                    }
+                }
+
+                resolutions
             });
 
         match result {
@@ -67,6 +81,18 @@ impl MultiItemModifier for Expander {
     }
 }
 
+// The cheapest `Annotatable` to construct.
+fn dummy_annotatable() -> Annotatable {
+    Annotatable::GenericParam(ast::GenericParam {
+        id: ast::DUMMY_NODE_ID,
+        ident: Ident::invalid(),
+        attrs: Default::default(),
+        bounds: Default::default(),
+        is_placeholder: false,
+        kind: GenericParamKind::Lifetime,
+    })
+}
+
 fn report_bad_target(sess: &Session, item: &Annotatable, span: Span) -> bool {
     let item_kind = match item {
         Annotatable::Item(item) => Some(&item.kind),