Parse and allow const use closures

Author: spastorino

compiler/rustc_parse/src/parser/mod.rs

@@ -813,9 +813,9 @@ impl<'a> Parser<'a> {
         self.is_keyword_ahead(0, &[kw::Const])
             && self.look_ahead(1, |t| match &t.kind {
                 // async closures do not work with const closures, so we do not parse that here.
-                token::Ident(kw::Move | kw::Static, IdentIsRaw::No) | token::OrOr | token::Or => {
-                    true
-                }
+                token::Ident(kw::Move | kw::Use | kw::Static, IdentIsRaw::No)
+                | token::OrOr
+                | token::Or => true,
                 _ => false,
             })
     }

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

@@ -0,0 +1,11 @@
+//@ check-pass
+
+#![feature(const_closures)]
+#![feature(ergonomic_clones)]
+#![allow(incomplete_features)]
+
+const fn foo() {
+    let cl = const use || {};
+}
+
+fn main() {}