Support nested use closures

Author: spastorino

compiler/rustc_parse/src/parser/item.rs

@@ -209,7 +209,10 @@ impl<'a> Parser<'a> {
         let check_pub = def == &Defaultness::Final;
         let mut def_ = || mem::replace(def, Defaultness::Final);
 
-        let info = if self.eat_keyword_case(exp!(Use), case) {
+        let info = if !self
+            .look_ahead(1, |t| [token::OrOr, token::BinOp(token::BinOpToken::Or)].contains(&t.kind))
+            && self.eat_keyword_case(exp!(Use), case)
+        {
             self.parse_use_item()?
         } else if self.check_fn_front_matter(check_pub, case) {
             // FUNCTION ITEM

tests/ui/ergonomic-clones/closure/nested.rs

@@ -0,0 +1,20 @@
+//@ run-pass
+
+#![feature(ergonomic_clones)]
+
+use std::marker::UseCloned;
+
+#[derive(Clone)]
+struct Foo;
+
+impl UseCloned for Foo {}
+
+fn work(_: Box<Foo>) {}
+fn foo<F:FnOnce()>(_: F) {}
+
+pub fn main() {
+  let a = Box::new(Foo);
+  foo(use || { foo(use || { work(a) }) });
+  let x = use || { use || { Foo } };
+  let _y = x();
+}