Rollup merge of #53545 - FelixMcFelix:fix-50865-beta, r=petrochenkov

Fix #50865: ICE on impl-trait returning functions reaching private items

Adds a test case as suggested in #50865, and implements @petrochenkov's suggestion. Fixes #50865.

Impl-trait-returning functions are marked under a new (low) access level, which they propagate rather than `AccessLevels::Reachable`. `AccessLevels::is_reachable` returns false for such items (leaving stability analysis unaffected), these items may still be visible to the lints phase however.

src/test/run-pass/issue-50865-private-impl-trait/auxiliary/lib.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+#![crate_type = "lib"]
+
+pub fn bar<P>( // Error won't happen if "bar" is not generic
+    _baz: P,
+) {
+    hide_foo()();
+}
+
+fn hide_foo() -> impl Fn() { // Error won't happen if "iterate" hasn't impl Trait or has generics
+    foo
+}
+
+fn foo() { // Error won't happen if "foo" isn't used in "iterate" or has generics
+}

src/test/run-pass/issue-50865-private-impl-trait/main.rs

@@ -0,0 +1,25 @@
+// Copyright 2018 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.
+
+// aux-build:lib.rs
+
+// Regression test for #50865.
+// When using generics or specifying the type directly, this example
+// codegens `foo` internally. However, when using a private `impl Trait`
+// function which references another private item, `foo` (in this case)
+// wouldn't be codegenned until main.rs used `bar`, as with impl Trait
+// it is not cast to `fn()` automatically to satisfy e.g.
+// `fn foo() -> fn() { ... }`.
+
+extern crate lib;
+
+fn main() {
+    lib::bar(()); // Error won't happen if bar is called from same crate
+}