do not fetch variance for items when equating

Fixes #41849. Problem was that evaluating the constant expression
required evaluating a trait, which would equate types, which would
request variance information, which it would then discard. However,
computing the variance information would require determining the type of
a field, which would evaluate the constant expression.

(This problem will potentially arise *later* as we move to more sophisticated
constants, however, where we need to check subtyping. We can tackle that
when we come to it.)

src/librustc/infer/combine.rs

@@ -350,8 +350,14 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
             // (e.g. #41849).
             relate::relate_substs(self, None, a_subst, b_subst)
         } else {
-            let opt_variances = self.tcx().variances_of(item_def_id);
-            relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
+            let variances;
+            let opt_variances = if self.tcx().variance_computed.get() {
+                variances = self.tcx().item_variances(item_def_id);
+                Some(&*variances)
+            } else {
+                None
+            };
+            relate::relate_substs(self, opt_variances, a_subst, b_subst)
         }
     }
 

src/librustc/infer/equate.rs

@@ -11,9 +11,12 @@
 use super::combine::{CombineFields, RelationDir};
 use super::{Subtype};
 
+use hir::def_id::DefId;
+
 use ty::{self, Ty, TyCtxt};
 use ty::TyVar;
-use ty::relate::{Relate, RelateResult, TypeRelation};
+use ty::subst::Substs;
+use ty::relate::{self, Relate, RelateResult, TypeRelation};
 
 /// Ensures `a` is made equal to `b`. Returns `a` on success.
 pub struct Equate<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
@@ -38,6 +41,22 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
 
     fn a_is_expected(&self) -> bool { self.a_is_expected }
 
+    fn relate_item_substs(&mut self,
+                          _item_def_id: DefId,
+                          a_subst: &'tcx Substs<'tcx>,
+                          b_subst: &'tcx Substs<'tcx>)
+                          -> RelateResult<'tcx, &'tcx Substs<'tcx>>
+    {
+        // NB: Once we are equating types, we don't care about
+        // variance, so don't try to lookup the variance here. This
+        // also avoids some cycles (e.g. #41849) since looking up
+        // variance requires computing types which can require
+        // performing trait matching (which then performs equality
+        // unification).
+
+        relate::relate_substs(self, None, a_subst, b_subst)
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              _: ty::Variance,
                                              a: &T,

src/librustc/ty/relate.rs

@@ -51,6 +51,30 @@ pub trait TypeRelation<'a, 'gcx: 'a+'tcx, 'tcx: 'a> : Sized {
         Relate::relate(self, a, b)
     }
 
+    /// Relate the two substitutions for the given item. The default
+    /// is to look up the variance for the item and proceed
+    /// accordingly.
+    fn relate_item_substs(&mut self,
+                          item_def_id: DefId,
+                          a_subst: &'tcx Substs<'tcx>,
+                          b_subst: &'tcx Substs<'tcx>)
+                          -> RelateResult<'tcx, &'tcx Substs<'tcx>>
+    {
+        debug!("relate_item_substs(item_def_id={:?}, a_subst={:?}, b_subst={:?})",
+               item_def_id,
+               a_subst,
+               b_subst);
+
+        let variances;
+        let opt_variances = if self.tcx().variance_computed.get() {
+            variances = self.tcx().item_variances(item_def_id);
+            Some(&*variances)
+        } else {
+            None
+        };
+        relate_substs(self, opt_variances, a_subst, b_subst)
+    }
+
     /// Switch variance for the purpose of relating `a` and `b`.
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              variance: ty::Variance,
@@ -109,31 +133,6 @@ impl<'tcx> Relate<'tcx> for ty::TypeAndMut<'tcx> {
     }
 }
 
-// substitutions are not themselves relatable without more context,
-// but they is an important subroutine for things that ARE relatable,
-// like traits etc.
-fn relate_item_substs<'a, 'gcx, 'tcx, R>(relation: &mut R,
-                                         item_def_id: DefId,
-                                         a_subst: &'tcx Substs<'tcx>,
-                                         b_subst: &'tcx Substs<'tcx>)
-                                         -> RelateResult<'tcx, &'tcx Substs<'tcx>>
-    where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
-{
-    debug!("substs: item_def_id={:?} a_subst={:?} b_subst={:?}",
-           item_def_id,
-           a_subst,
-           b_subst);
-
-    let variances;
-    let opt_variances = if relation.tcx().variance_computed.get() {
-        variances = relation.tcx().item_variances(item_def_id);
-        Some(&*variances)
-    } else {
-        None
-    };
-    relate_substs(relation, opt_variances, a_subst, b_subst)
-}
-
 pub fn relate_substs<'a, 'gcx, 'tcx, R>(relation: &mut R,
                                         variances: Option<&Vec<ty::Variance>>,
                                         a_subst: &'tcx Substs<'tcx>,
@@ -297,7 +296,7 @@ impl<'tcx> Relate<'tcx> for ty::TraitRef<'tcx> {
         if a.def_id != b.def_id {
             Err(TypeError::Traits(expected_found(relation, &a.def_id, &b.def_id)))
         } else {
-            let substs = relate_item_substs(relation, a.def_id, a.substs, b.substs)?;
+            let substs = relation.relate_item_substs(a.def_id, a.substs, b.substs)?;
             Ok(ty::TraitRef { def_id: a.def_id, substs: substs })
         }
     }
@@ -314,7 +313,7 @@ impl<'tcx> Relate<'tcx> for ty::ExistentialTraitRef<'tcx> {
         if a.def_id != b.def_id {
             Err(TypeError::Traits(expected_found(relation, &a.def_id, &b.def_id)))
         } else {
-            let substs = relate_item_substs(relation, a.def_id, a.substs, b.substs)?;
+            let substs = relation.relate_item_substs(a.def_id, a.substs, b.substs)?;
             Ok(ty::ExistentialTraitRef { def_id: a.def_id, substs: substs })
         }
     }
@@ -378,7 +377,7 @@ pub fn super_relate_tys<'a, 'gcx, 'tcx, R>(relation: &mut R,
         (&ty::TyAdt(a_def, a_substs), &ty::TyAdt(b_def, b_substs))
             if a_def == b_def =>
         {
-            let substs = relate_item_substs(relation, a_def.did, a_substs, b_substs)?;
+            let substs = relation.relate_item_substs(a_def.did, a_substs, b_substs)?;
             Ok(tcx.mk_adt(a_def, substs))
         }
 

src/test/run-pass/issue-41849-variance-req.rs

@@ -0,0 +1,43 @@
+// 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 #41849.
+
+use std::ops::Mul;
+
+const C: usize = 1;
+const CAPACITY: usize = 1 * C;
+
+struct A<X> {
+    f: [X; CAPACITY],
+}
+
+struct B<T> {
+    f: T,
+}
+
+impl<T> Mul for B<T> {
+    type Output = Self;
+    fn mul(self, _rhs: B<T>) -> Self::Output {
+        self
+    }
+}
+
+impl<T> Mul<usize> for B<T> {
+    type Output = Self;
+    fn mul(self, _rhs: usize) -> Self::Output {
+        self
+    }
+}
+
+fn main() {
+    let a = A { f: [1] };
+    let _ = B { f: a };
+}