Mark closures returned via impl-trait as reachable.

src/librustc/middle/reachable.rs

@@ -296,6 +296,9 @@ impl<'a, 'tcx> ReachableContext<'a, 'tcx> {
  hir::ImplItemKind::Type(_) => {}
  }
  }
+ hir_map::NodeExpr(&hir::Expr { node: hir::ExprClosure(.., body, _), .. }) => {
+ self.visit_nested_body(body);
+ }
  // Nothing to recurse on for these
  hir_map::NodeForeignItem(_) |
  hir_map::NodeVariant(_) |

src/librustc_privacy/lib.rs

@@ -448,6 +448,7 @@ impl<'b, 'a, 'tcx> TypeVisitor<'tcx> for ReachEverythingInTheInterfaceVisitor<'b
  ty::TyDynamic(ref obj, ..) => obj.principal().map(|p| p.def_id()),
  ty::TyProjection(ref proj) => Some(proj.item_def_id),
  ty::TyFnDef(def_id, ..) |
+ ty::TyClosure(def_id, ..) |
  ty::TyAnon(def_id, _) => Some(def_id),
  _ => None
  };

src/test/run-pass/impl-trait/auxiliary/xcrate.rs

@@ -13,3 +13,14 @@
 pub fn fourway_add(a: i32) -> impl Fn(i32) -> impl Fn(i32) -> impl Fn(i32) -> i32 {
  move |b| move |c| move |d| a + b + c + d
 }
+
+fn some_internal_fn() -> u32 {
+ 1
+}
+
+// See #40839
+pub fn return_closure_accessing_internal_fn() -> impl Fn() -> u32 {
+ || {
+ some_internal_fn() + 1
+ }
+}

src/test/run-pass/impl-trait/xcrate.rs

@@ -14,4 +14,5 @@ extern crate xcrate;
 
 fn main() {
  assert_eq!(xcrate::fourway_add(1)(2)(3)(4), 10);
+ xcrate::return_closure_accessing_internal_fn()();
 }