Rollup merge of #49350 - abonander:macros-in-extern, r=petrochenkov

Expand macros in `extern {}` blocks

This permits macro and proc-macro and attribute invocations (the latter only with the `proc_macro` feature of course) in `extern {}` blocks, gated behind a new `macros_in_extern` feature.

A tracking issue is now open at #49476

closes #48747

src/libsyntax/fold.rs

@@ -60,10 +60,14 @@ pub trait Folder : Sized {
         noop_fold_use_tree(use_tree, self)
     }
 
-    fn fold_foreign_item(&mut self, ni: ForeignItem) -> ForeignItem {
+    fn fold_foreign_item(&mut self, ni: ForeignItem) -> SmallVector<ForeignItem> {
         noop_fold_foreign_item(ni, self)
     }
 
+    fn fold_foreign_item_simple(&mut self, ni: ForeignItem) -> ForeignItem {
+        noop_fold_foreign_item_simple(ni, self)
+    }
+
     fn fold_item(&mut self, i: P<Item>) -> SmallVector<P<Item>> {
         noop_fold_item(i, self)
     }
@@ -414,7 +418,7 @@ pub fn noop_fold_foreign_mod<T: Folder>(ForeignMod {abi, items}: ForeignMod,
                                         fld: &mut T) -> ForeignMod {
     ForeignMod {
         abi,
-        items: items.move_map(|x| fld.fold_foreign_item(x)),
+        items: items.move_flat_map(|x| fld.fold_foreign_item(x)),
     }
 }
 
@@ -648,6 +652,10 @@ pub fn noop_fold_interpolated<T: Folder>(nt: token::Nonterminal, fld: &mut T)
         token::NtArg(arg) => token::NtArg(fld.fold_arg(arg)),
         token::NtVis(vis) => token::NtVis(fld.fold_vis(vis)),
         token::NtLifetime(lifetime) => token::NtLifetime(fld.fold_lifetime(lifetime)),
+        token::NtForeignItem(ni) =>
+            token::NtForeignItem(fld.fold_foreign_item(ni)
+                                 // see reasoning above
+                                 .expect_one("expected fold to produce exactly one item")),
     }
 }
 
@@ -1072,7 +1080,12 @@ pub fn noop_fold_item_simple<T: Folder>(Item {id, ident, attrs, node, vis, span,
     }
 }
 
-pub fn noop_fold_foreign_item<T: Folder>(ni: ForeignItem, folder: &mut T) -> ForeignItem {
+pub fn noop_fold_foreign_item<T: Folder>(ni: ForeignItem, folder: &mut T)
+-> SmallVector<ForeignItem> {
+    SmallVector::one(folder.fold_foreign_item_simple(ni))
+}
+
+pub fn noop_fold_foreign_item_simple<T: Folder>(ni: ForeignItem, folder: &mut T) -> ForeignItem {
     ForeignItem {
         id: folder.new_id(ni.id),
         vis: folder.fold_vis(ni.vis),
@@ -1086,6 +1099,7 @@ pub fn noop_fold_foreign_item<T: Folder>(ni: ForeignItem, folder: &mut T) -> For
                 ForeignItemKind::Static(folder.fold_ty(t), m)
             }
             ForeignItemKind::Ty => ForeignItemKind::Ty,
+            ForeignItemKind::Macro(mac) => ForeignItemKind::Macro(folder.fold_mac(mac)),
         },
         span: folder.new_span(ni.span)
     }