Remove unnecessary unboxed_closures feature usage

It has been possible to clone closures for a while now

Author: bjorn3

compiler/rustc_data_structures/src/lib.rs

@@ -10,9 +10,7 @@
 #![feature(array_windows)]
 #![feature(control_flow_enum)]
 #![feature(in_band_lifetimes)]
-#![feature(unboxed_closures)]
 #![feature(generator_trait)]
-#![feature(fn_traits)]
 #![feature(min_specialization)]
 #![feature(auto_traits)]
 #![feature(nll)]

compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -597,7 +597,7 @@ impl<O: ForestObligation> ObligationForest<O> {
                 Some(rpos) => {
                     // Cycle detected.
                     processor.process_backedge(
-                        stack[rpos..].iter().map(GetObligation(&self.nodes)),
+                        stack[rpos..].iter().map(|&i| &self.nodes[i].obligation),
                         PhantomData,
                     );
                 }
@@ -705,20 +705,3 @@ impl<O: ForestObligation> ObligationForest<O> {
         });
     }
 }
-
-// I need a Clone closure.
-#[derive(Clone)]
-struct GetObligation<'a, O>(&'a [Node<O>]);
-
-impl<'a, 'b, O> FnOnce<(&'b usize,)> for GetObligation<'a, O> {
-    type Output = &'a O;
-    extern "rust-call" fn call_once(self, args: (&'b usize,)) -> &'a O {
-        &self.0[*args.0].obligation
-    }
-}
-
-impl<'a, 'b, O> FnMut<(&'b usize,)> for GetObligation<'a, O> {
-    extern "rust-call" fn call_mut(&mut self, args: (&'b usize,)) -> &'a O {
-        &self.0[*args.0].obligation
-    }
-}