Auto merge of #34253 - jseyfried:improve_multi_modifiers, r=nrc

Allow `MultiItemModifier`s to expand into zero or many items

Fixes #34223.
r? @nrc

src/libsyntax/ext/base.rs

@@ -12,6 +12,7 @@ pub use self::SyntaxExtension::*;
 
 use ast;
 use ast::{Name, PatKind};
+use attr::HasAttrs;
 use codemap;
 use codemap::{CodeMap, Span, ExpnId, ExpnInfo, NO_EXPANSION};
 use errors::DiagnosticBuilder;
@@ -40,29 +41,31 @@ pub enum Annotatable {
     ImplItem(P<ast::ImplItem>),
 }
 
-impl Annotatable {
-    pub fn attrs(&self) -> &[ast::Attribute] {
+impl HasAttrs for Annotatable {
+    fn attrs(&self) -> &[ast::Attribute] {
         match *self {
-            Annotatable::Item(ref i) => &i.attrs,
-            Annotatable::TraitItem(ref ti) => &ti.attrs,
-            Annotatable::ImplItem(ref ii) => &ii.attrs,
+            Annotatable::Item(ref item) => &item.attrs,
+            Annotatable::TraitItem(ref trait_item) => &trait_item.attrs,
+            Annotatable::ImplItem(ref impl_item) => &impl_item.attrs,
         }
     }
 
-    pub fn fold_attrs(self, attrs: Vec<ast::Attribute>) -> Annotatable {
+    fn map_attrs<F: FnOnce(Vec<ast::Attribute>) -> Vec<ast::Attribute>>(self, f: F) -> Self {
         match self {
-            Annotatable::Item(i) => Annotatable::Item(i.map(|i| ast::Item {
-                attrs: attrs,
-                ..i
-            })),
-            Annotatable::TraitItem(i) => Annotatable::TraitItem(i.map(|ti| {
-                ast::TraitItem { attrs: attrs, ..ti }
-            })),
-            Annotatable::ImplItem(i) => Annotatable::ImplItem(i.map(|ii| {
-                ast::ImplItem { attrs: attrs, ..ii }
-            })),
+            Annotatable::Item(item) => Annotatable::Item(item.map_attrs(f)),
+            Annotatable::TraitItem(trait_item) => Annotatable::TraitItem(trait_item.map_attrs(f)),
+            Annotatable::ImplItem(impl_item) => Annotatable::ImplItem(impl_item.map_attrs(f)),
         }
     }
+}
+
+impl Annotatable {
+    pub fn attrs(&self) -> &[ast::Attribute] {
+        HasAttrs::attrs(self)
+    }
+    pub fn fold_attrs(self, attrs: Vec<ast::Attribute>) -> Annotatable {
+        self.map_attrs(|_| attrs)
+    }
 
     pub fn expect_item(self) -> P<ast::Item> {
         match self {
@@ -129,9 +132,7 @@ impl<F> MultiItemDecorator for F
     }
 }
 
-// A more flexible ItemKind::Modifier (ItemKind::Modifier should go away, eventually, FIXME).
-// meta_item is the annotation, item is the item being modified, parent_item
-// is the impl or trait item is declared in if item is part of such a thing.
+// `meta_item` is the annotation, and `item` is the item being modified.
 // FIXME Decorators should follow the same pattern too.
 pub trait MultiItemModifier {
     fn expand(&self,
@@ -139,22 +140,26 @@ pub trait MultiItemModifier {
               span: Span,
               meta_item: &ast::MetaItem,
               item: Annotatable)
-              -> Annotatable;
+              -> Vec<Annotatable>;
 }
 
-impl<F> MultiItemModifier for F
-    where F: Fn(&mut ExtCtxt,
-                Span,
-                &ast::MetaItem,
-                Annotatable) -> Annotatable
+impl<F, T> MultiItemModifier for F
+    where F: Fn(&mut ExtCtxt, Span, &ast::MetaItem, Annotatable) -> T,
+          T: Into<Vec<Annotatable>>,
 {
     fn expand(&self,
               ecx: &mut ExtCtxt,
               span: Span,
               meta_item: &ast::MetaItem,
               item: Annotatable)
-              -> Annotatable {
-        (*self)(ecx, span, meta_item, item)
+              -> Vec<Annotatable> {
+        (*self)(ecx, span, meta_item, item).into()
+    }
+}
+
+impl Into<Vec<Annotatable>> for Annotatable {
+    fn into(self) -> Vec<Annotatable> {
+        vec![self]
     }
 }