From c59fe8bde2be55c46f627277e2cc37515fb7165e Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Wed, 25 Mar 2015 10:42:38 -0400
Subject: [PATCH] Drive-by fix for incorrect variance rule that I noticed.

---
 src/librustc_typeck/variance.rs               | 24 +++++++++++++++---
 .../compile-fail/variance-region-bounds.rs    | 25 +++++++++++++++++++
 2 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 src/test/compile-fail/variance-region-bounds.rs

diff --git a/src/librustc_typeck/variance.rs b/src/librustc_typeck/variance.rs
index ac1ff29e7f5..3a716b28e73 100644
--- a/src/librustc_typeck/variance.rs
+++ b/src/librustc_typeck/variance.rs
@@ -1059,14 +1059,29 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                 }
 
                 ty::Predicate::Equate(ty::Binder(ref data)) => {
-                    self.add_constraints_from_ty(generics, data.0, variance);
-                    self.add_constraints_from_ty(generics, data.1, variance);
+                    // A == B is only true if A and B are the same
+                    // types, not subtypes of one another, so this is
+                    // an invariant position:
+                    self.add_constraints_from_ty(generics, data.0, self.invariant);
+                    self.add_constraints_from_ty(generics, data.1, self.invariant);
                 }
 
                 ty::Predicate::TypeOutlives(ty::Binder(ref data)) => {
-                    self.add_constraints_from_ty(generics, data.0, variance);
+                    // Why contravariant on both? Let's consider:
+                    //
+                    // Under what conditions is `(T:'t) <: (U:'u)`,
+                    // meaning that `(T:'t) => (U:'u)`. The answer is
+                    // if `U <: T` or `'u <= 't`. Let's see some examples:
+                    //
+                    //   (T: 'big) => (T: 'small)
+                    //   where 'small <= 'big
+                    //
+                    //   (&'small Foo: 't) => (&'big Foo: 't)
+                    //   where 'small <= 'big
+                    //   note that &'big Foo <: &'small Foo
 
                     let variance_r = self.xform(variance, self.contravariant);
+                    self.add_constraints_from_ty(generics, data.0, variance_r);
                     self.add_constraints_from_region(generics, data.1, variance_r);
                 }
 
@@ -1084,6 +1099,9 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                                                         &*data.projection_ty.trait_ref,
                                                         variance);
 
+                    // as the equality predicate above, a binder is a
+                    // type equality relation, not a subtyping
+                    // relation
                     self.add_constraints_from_ty(generics, data.ty, self.invariant);
                 }
             }
diff --git a/src/test/compile-fail/variance-region-bounds.rs b/src/test/compile-fail/variance-region-bounds.rs
new file mode 100644
index 00000000000..96ae201f6ae
--- /dev/null
+++ b/src/test/compile-fail/variance-region-bounds.rs
@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// Check that `T:'a` is contravariant in T.
+
+#![feature(rustc_attrs)]
+
+#[rustc_variance]
+trait Foo: 'static { //~ ERROR types=[[];[-];[]]
+}
+
+#[rustc_variance]
+trait Bar<T> { //~ ERROR types=[[+];[-];[]]
+    fn do_it(&self)
+        where T: 'static;
+}
+
+fn main() { }