Fix obscure compilation error

src/librustc/diagnostics.rs

@@ -1544,5 +1544,5 @@ register_diagnostics! {
  E0490, // a value of type `..` is borrowed for too long
  E0491, // in type `..`, reference has a longer lifetime than the data it...
  E0495, // cannot infer an appropriate lifetime due to conflicting requirements
- E0524, // the closure implements `..` but not `..`
+ E0524, // expected a closure that implements `..` but this closure only implements `..`
 }

src/librustc/traits/error_reporting.rs

@@ -471,9 +471,10 @@ pub fn report_selection_error<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
  let closure_span = infcx.tcx.map.span_if_local(closure_def_id).unwrap();
  let mut err = struct_span_err!(
  infcx.tcx.sess, closure_span, E0524,
- "the closure implements `{}` but not `{}`",
- found_kind,
- kind);
+ "expected a closure that implements the `{}` trait, but this closure \
+ only implements `{}`",
+ kind,
+ found_kind);
  err.span_note(
  obligation.cause.span,
  &format!("the requirement to implement `{}` derives from here", kind));

src/librustc_typeck/check/closure.rs

@@ -179,9 +179,16 @@ fn deduce_expectations_from_obligations<'a,'tcx>(
  ty::Predicate::TypeOutlives(..) => None,
  ty::Predicate::WellFormed(..) => None,
  ty::Predicate::ObjectSafe(..) => None,
- ty::Predicate::ClosureKind(_closure_def_id, kind) => {
- return Some(kind);
- }
+
+ // NB: This predicate is created by breaking down a
+ // `ClosureType: FnFoo()` predicate, where
+ // `ClosureType` represents some `TyClosure`. It can't
+ // possibly be referring to the current closure,
+ // because we haven't produced the `TyClosure` for
+ // this closure yet; this is exactly why the other
+ // code is looking for a self type of a unresolved
+ // inference variable.
+ ty::Predicate::ClosureKind(..) => None,
  };
  opt_trait_ref
  .and_then(|trait_ref| self_type_matches_expected_vid(fcx, trait_ref, expected_vid))

src/test/compile-fail/closure-wrong-kind.rs

@@ -17,7 +17,6 @@ fn bar<T: Fn(u32)>(_: T) {}
 
 fn main() {
  let x = X;
- let closure = |_| foo(x);
- //~^ ERROR the closure implements `FnOnce` but not `Fn`
+ let closure = |_| foo(x); //~ ERROR E0524
  bar(closure);
 }