introduce new subtyping

src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs

@@ -0,0 +1,27 @@
+// Copyright 2017 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.
+
+// Test that the NLL `relate_tys` code correctly deduces that a
+// function returning either argument CANNOT be upcast to one
+// that returns always its first argument.
+//
+// compile-flags:-Zno-leak-check
+
+#![feature(nll)]
+
+fn make_it() -> for<'a> fn(&'a u32, &'a u32) -> &'a u32 {
+    panic!()
+}
+
+fn main() {
+    let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
+    //~^ ERROR higher-ranked subtype error
+    drop(a);
+}

src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr

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+error: higher-ranked subtype error
+  --> $DIR/hr-fn-aaa-as-aba.rs:24:58
+   |
+LL |     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
+   |                                                          ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/nll/relate_tys/hr-fn-aau-eq-abu.rs

@@ -0,0 +1,38 @@
+// Copyright 2017 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.
+
+// Test an interesting corner case: a fn that takes two arguments that
+// are references with the same lifetime is in fact equivalent to a fn
+// that takes two references with distinct lifetimes. This is true
+// because the two functions can call one another -- effectively, the
+// single lifetime `'a` is just inferred to be the intersection of the
+// two distinct lifetimes.
+//
+// compile-flags:-Zno-leak-check
+
+#![feature(nll)]
+
+use std::cell::Cell;
+
+fn make_cell_aa() -> Cell<for<'a> fn(&'a u32, &'a u32)> {
+    panic!()
+}
+
+fn aa_eq_ab() {
+    let a: Cell<for<'a, 'b> fn(&'a u32, &'b u32)> = make_cell_aa();
+    //~^ ERROR higher-ranked subtype error
+    //
+    // FIXME -- this .. arguably ought to be accepted. However, the
+    // current leak check also rejects this example, and it's kind of
+    // a pain to support it. There are some comments in `relate_tys`
+    drop(a);
+}
+
+fn main() { }

src/test/ui/nll/relate_tys/hr-fn-aau-eq-abu.stderr

@@ -0,0 +1,8 @@
+error: higher-ranked subtype error
+  --> $DIR/hr-fn-aau-eq-abu.rs:29:53
+   |
+LL |     let a: Cell<for<'a, 'b> fn(&'a u32, &'b u32)> = make_cell_aa();
+   |                                                     ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/nll/relate_tys/hr-fn-aba-as-aaa.rs

@@ -0,0 +1,27 @@
+// Copyright 2017 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.
+
+// Test that the NLL `relate_tys` code correctly deduces that a
+// function returning always its first argument can be upcast to one
+// that returns either first or second argument.
+//
+// compile-pass
+// compile-flags:-Zno-leak-check
+
+#![feature(nll)]
+
+fn make_it() -> for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 {
+    panic!()
+}
+
+fn main() {
+    let a: for<'a> fn(&'a u32, &'a u32) -> &'a u32 = make_it();
+    drop(a);
+}