consider closures/ty-fn-defs when making trait selection keys

Fixes #42602.

src/librustc/ty/mod.rs

@@ -944,7 +944,9 @@ impl<'tcx> TraitPredicate<'tcx> {
             self.input_types()
                 .flat_map(|t| t.walk())
                 .filter_map(|t| match t.sty {
-                    ty::TyAdt(adt_def, _) => Some(adt_def.did),
+                    ty::TyAdt(adt_def, ..) => Some(adt_def.did),
+                    ty::TyClosure(def_id, ..) => Some(def_id),
+                    ty::TyFnDef(def_id, ..) => Some(def_id),
                     _ => None
                 })
                 .next()

src/test/incremental/issue-42602.rs

@@ -0,0 +1,45 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #42602. It used to be that we had
+// a dep-graph like
+//
+//     typeck(foo) -> FnOnce -> typeck(bar)
+//
+// This was fixed by improving the resolution of the `FnOnce` trait
+// selection node.
+
+// revisions:cfail1
+// compile-flags:-Zquery-dep-graph
+
+#![feature(rustc_attrs)]
+
+fn main() {
+    a::foo();
+    b::bar();
+}
+
+mod a {
+    #[rustc_if_this_changed(HirBody)]
+    pub fn foo() {
+        let x = vec![1, 2, 3];
+        let v = || ::std::mem::drop(x);
+        v();
+    }
+}
+
+mod b {
+    #[rustc_then_this_would_need(TypeckTables)] //[cfail1]~ ERROR no path
+    pub fn bar() {
+        let x = vec![1, 2, 3];
+        let v = || ::std::mem::drop(x);
+        v();
+    }
+}