From f71de45b230954c25f2337272852be1146d26136 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Thu, 15 Mar 2018 10:49:05 -0400
Subject: [PATCH] use subtyping when we create a closure instead of for upvar
 types

We used to make the upvar types in the closure `==` but that was
stronger than we needed. Subtyping suffices, since we are copying the
upvar value into the closure field. This in turn allows us to infer
smaller lifetimes in captured values in some cases (like the example
here), avoiding errors.
---
 src/librustc_typeck/check/upvar.rs            |  2 +-
 .../ui/generator/auto-trait-regions.stderr    |  6 ++--
 ...-escape-via-bound-contravariant-closure.rs | 31 +++++++++++++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 src/test/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs

diff --git a/src/librustc_typeck/check/upvar.rs b/src/librustc_typeck/check/upvar.rs
index ab148afafbe..4fc3344dab2 100644
--- a/src/librustc_typeck/check/upvar.rs
+++ b/src/librustc_typeck/check/upvar.rs
@@ -217,7 +217,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             .upvar_tys(closure_def_id, self.tcx)
             .zip(final_upvar_tys)
         {
-            self.demand_eqtype(span, final_upvar_ty, upvar_ty);
+            self.demand_suptype(span, upvar_ty, final_upvar_ty);
         }
 
         // If we are also inferred the closure kind here,
diff --git a/src/test/ui/generator/auto-trait-regions.stderr b/src/test/ui/generator/auto-trait-regions.stderr
index cd83915fe82..dd78baf9275 100644
--- a/src/test/ui/generator/auto-trait-regions.stderr
+++ b/src/test/ui/generator/auto-trait-regions.stderr
@@ -1,15 +1,15 @@
-error[E0277]: the trait bound `No: Foo` is not satisfied in `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
+error[E0277]: the trait bound `No: Foo` is not satisfied in `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
   --> $DIR/auto-trait-regions.rs:40:5
    |
 LL |     assert_foo(gen); //~ ERROR the trait bound `No: Foo` is not satisfied
-   |     ^^^^^^^^^^ within `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`, the trait `Foo` is not implemented for `No`
+   |     ^^^^^^^^^^ within `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`, the trait `Foo` is not implemented for `No`
    |
    = help: the following implementations were found:
              <No as Foo>
    = note: required because it appears within the type `OnlyFooIfStaticRef`
    = note: required because it appears within the type `&OnlyFooIfStaticRef`
    = note: required because it appears within the type `for<'r> {&'r OnlyFooIfStaticRef, ()}`
-   = note: required because it appears within the type `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
+   = note: required because it appears within the type `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
 note: required by `assert_foo`
   --> $DIR/auto-trait-regions.rs:30:1
    |
diff --git a/src/test/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs b/src/test/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs
new file mode 100644
index 00000000000..f554efe9036
--- /dev/null
+++ b/src/test/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs
@@ -0,0 +1,31 @@
+// 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.
+
+// In contrast to `region-escape-via-bound-invariant`, in this case we
+// *can* return a value of type `&'x u32`, even though `'x` does not
+// appear in the bounds. This is because `&` is contravariant, and so
+// we are *actually* returning a `&'y u32`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+// run-pass
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+fn foo(x: &'x u32) -> impl Fn() -> &'y u32
+where 'x: 'y
+{
+    move || x
+}
+
+fn main() { }