diff --git a/src/librustc/infer/combine.rs b/src/librustc/infer/combine.rs
index 1bac512e209..82578f6aa61 100644
--- a/src/librustc/infer/combine.rs
+++ b/src/librustc/infer/combine.rs
@@ -42,9 +42,8 @@ use super::{MiscVariable, TypeTrace};
use ty::{IntType, UintType};
use ty::{self, Ty, TyCtxt};
use ty::error::TypeError;
-use ty::fold::TypeFoldable;
-use ty::relate::{RelateResult, TypeRelation};
-use traits::PredicateObligations;
+use ty::relate::{self, Relate, RelateResult, TypeRelation};
+use traits::{Obligation, PredicateObligations};
use syntax::ast;
use syntax_pos::Span;
@@ -207,11 +206,16 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
// `'?2` and `?3` are fresh region/type inference
// variables. (Down below, we will relate `a_ty <: b_ty`,
// adding constraints like `'x: '?2` and `?1 <: ?3`.)
- let b_ty = self.generalize(a_ty, b_vid, dir == EqTo)?;
+ let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
a_ty, dir, b_vid, b_ty);
self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
+ if needs_wf {
+ self.obligations.push(Obligation::new(self.trace.cause.clone(),
+ ty::Predicate::WellFormed(b_ty)));
+ }
+
// Finally, relate `b_ty` to `a_ty`, as described in previous comment.
//
// FIXME(#16847): This code is non-ideal because all these subtype
@@ -230,10 +234,9 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
/// Attempts to generalize `ty` for the type variable `for_vid`.
/// This checks for cycle -- that is, whether the type `ty`
- /// references `for_vid`. If `is_eq_relation` is false, it will
- /// also replace all regions/unbound-type-variables with fresh
- /// variables. Returns `TyError` in the case of a cycle, `Ok`
- /// otherwise.
+ /// references `for_vid`. The `dir` is the "direction" for which we
+ /// a performing the generalization (i.e., are we producing a type
+ /// that can be used as a supertype etc).
///
/// Preconditions:
///
@@ -241,22 +244,33 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
fn generalize(&self,
ty: Ty<'tcx>,
for_vid: ty::TyVid,
- is_eq_relation: bool)
- -> RelateResult<'tcx, Ty<'tcx>>
+ dir: RelationDir)
+ -> RelateResult<'tcx, Generalization<'tcx>>
{
+ // Determine the ambient variance within which `ty` appears.
+ // The surrounding equation is:
+ //
+ // ty [op] ty2
+ //
+ // where `op` is either `==`, `<:`, or `:>`. This maps quite
+ // naturally.
+ let ambient_variance = match dir {
+ RelationDir::EqTo => ty::Invariant,
+ RelationDir::SubtypeOf => ty::Covariant,
+ RelationDir::SupertypeOf => ty::Contravariant,
+ };
+
let mut generalize = Generalizer {
infcx: self.infcx,
span: self.trace.cause.span,
for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
- is_eq_relation: is_eq_relation,
- cycle_detected: false
+ ambient_variance: ambient_variance,
+ needs_wf: false,
};
- let u = ty.fold_with(&mut generalize);
- if generalize.cycle_detected {
- Err(TypeError::CyclicTy)
- } else {
- Ok(u)
- }
+
+ let ty = generalize.relate(&ty, &ty)?;
+ let needs_wf = generalize.needs_wf;
+ Ok(Generalization { ty, needs_wf })
}
}
@@ -264,16 +278,81 @@ struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
span: Span,
for_vid_sub_root: ty::TyVid,
- is_eq_relation: bool,
- cycle_detected: bool,
+ ambient_variance: ty::Variance,
+ needs_wf: bool, // see the field `needs_wf` in `Generalization`
}
-impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
- fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
+/// Result from a generalization operation. This includes
+/// not only the generalized type, but also a bool flag
+/// indicating whether further WF checks are needed.q
+struct Generalization<'tcx> {
+ ty: Ty<'tcx>,
+
+ /// If true, then the generalized type may not be well-formed,
+ /// even if the source type is well-formed, so we should add an
+ /// additional check to enforce that it is. This arises in
+ /// particular around 'bivariant' type parameters that are only
+ /// constrained by a where-clause. As an example, imagine a type:
+ ///
+ /// struct Foo<A, B> where A: Iterator<Item=B> {
+ /// data: A
+ /// }
+ ///
+ /// here, `A` will be covariant, but `B` is
+ /// unconstrained. However, whatever it is, for `Foo` to be WF, it
+ /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
+ /// then after generalization we will wind up with a type like
+ /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
+ /// ?D>` (or `>:`), we will wind up with the requirement that `?A
+ /// <: ?C`, but no particular relationship between `?B` and `?D`
+ /// (after all, we do not know the variance of the normalized form
+ /// of `A::Item` with respect to `A`). If we do nothing else, this
+ /// may mean that `?D` goes unconstrained (as in #41677). So, in
+ /// this scenario where we create a new type variable in a
+ /// bivariant context, we set the `needs_wf` flag to true. This
+ /// will force the calling code to check that `WF(Foo<?C, ?D>)`
+ /// holds, which in turn implies that `?C::Item == ?D`. So once
+ /// `?C` is constrained, that should suffice to restrict `?D`.
+ needs_wf: bool,
+}
+
+impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
+ fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
self.infcx.tcx
}
- fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+ fn tag(&self) -> &'static str {
+ "Generalizer"
+ }
+
+ fn a_is_expected(&self) -> bool {
+ true
+ }
+
+ fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+ -> RelateResult<'tcx, ty::Binder<T>>
+ where T: Relate<'tcx>
+ {
+ Ok(ty::Binder(self.relate(a.skip_binder(), b.skip_binder())?))
+ }
+
+ fn relate_with_variance<T: Relate<'tcx>>(&mut self,
+ variance: ty::Variance,
+ a: &T,
+ b: &T)
+ -> RelateResult<'tcx, T>
+ {
+ let old_ambient_variance = self.ambient_variance;
+ self.ambient_variance = self.ambient_variance.xform(variance);
+
+ let result = self.relate(a, b);
+ self.ambient_variance = old_ambient_variance;
+ result
+ }
+
+ fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+ assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
+
// Check to see whether the type we are genealizing references
// any other type variable related to `vid` via
// subtyping. This is basically our "occurs check", preventing
@@ -286,41 +365,63 @@ impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx
if sub_vid == self.for_vid_sub_root {
// If sub-roots are equal, then `for_vid` and
// `vid` are related via subtyping.
- self.cycle_detected = true;
- self.tcx().types.err
+ return Err(TypeError::CyclicTy);
} else {
match variables.probe_root(vid) {
Some(u) => {
drop(variables);
- self.fold_ty(u)
+ self.relate(&u, &u)
}
None => {
- if !self.is_eq_relation {
- let origin = variables.origin(vid);
- let new_var_id = variables.new_var(false, origin, None);
- let u = self.tcx().mk_var(new_var_id);
- debug!("generalize: replacing original vid={:?} with new={:?}",
- vid, u);
- u
- } else {
- t
+ match self.ambient_variance {
+ // Invariant: no need to make a fresh type variable.
+ ty::Invariant => return Ok(t),
+
+ // Bivariant: make a fresh var, but we
+ // may need a WF predicate. See
+ // comment on `needs_wf` field for
+ // more info.
+ ty::Bivariant => self.needs_wf = true,
+
+ // Co/contravariant: this will be
+ // sufficiently constrained later on.
+ ty::Covariant | ty::Contravariant => (),
}
+
+ let origin = variables.origin(vid);
+ let new_var_id = variables.new_var(false, origin, None);
+ let u = self.tcx().mk_var(new_var_id);
+ debug!("generalize: replacing original vid={:?} with new={:?}",
+ vid, u);
+ return Ok(u);
}
}
}
}
+ ty::TyInfer(ty::IntVar(_)) |
+ ty::TyInfer(ty::FloatVar(_)) => {
+ // No matter what mode we are in,
+ // integer/floating-point types must be equal to be
+ // relatable.
+ Ok(t)
+ }
_ => {
- t.super_fold_with(self)
+ relate::super_relate_tys(self, t, t)
}
}
}
- fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+ fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
+ -> RelateResult<'tcx, ty::Region<'tcx>> {
+ assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
+
match *r {
// Never make variables for regions bound within the type itself,
// nor for erased regions.
ty::ReLateBound(..) |
- ty::ReErased => { return r; }
+ ty::ReErased => {
+ return Ok(r);
+ }
// Early-bound regions should really have been substituted away before
// we get to this point.
@@ -342,15 +443,16 @@ impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx
ty::ReScope(..) |
ty::ReVar(..) |
ty::ReFree(..) => {
- if self.is_eq_relation {
- return r;
+ match self.ambient_variance {
+ ty::Invariant => return Ok(r),
+ ty::Bivariant | ty::Covariant | ty::Contravariant => (),
}
}
}
// FIXME: This is non-ideal because we don't give a
// very descriptive origin for this region variable.
- self.infcx.next_region_var(MiscVariable(self.span))
+ Ok(self.infcx.next_region_var(MiscVariable(self.span)))
}
}
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 51cdba56cf1..f5d510c11ae 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -330,6 +330,66 @@ pub struct CrateVariancesMap {
pub empty_variance: Rc<Vec<ty::Variance>>,
}
+impl Variance {
+ /// `a.xform(b)` combines the variance of a context with the
+ /// variance of a type with the following meaning. If we are in a
+ /// context with variance `a`, and we encounter a type argument in
+ /// a position with variance `b`, then `a.xform(b)` is the new
+ /// variance with which the argument appears.
+ ///
+ /// Example 1:
+ ///
+ /// *mut Vec<i32>
+ ///
+ /// Here, the "ambient" variance starts as covariant. `*mut T` is
+ /// invariant with respect to `T`, so the variance in which the
+ /// `Vec<i32>` appears is `Covariant.xform(Invariant)`, which
+ /// yields `Invariant`. Now, the type `Vec<T>` is covariant with
+ /// respect to its type argument `T`, and hence the variance of
+ /// the `i32` here is `Invariant.xform(Covariant)`, which results
+ /// (again) in `Invariant`.
+ ///
+ /// Example 2:
+ ///
+ /// fn(*const Vec<i32>, *mut Vec<i32)
+ ///
+ /// The ambient variance is covariant. A `fn` type is
+ /// contravariant with respect to its parameters, so the variance
+ /// within which both pointer types appear is
+ /// `Covariant.xform(Contravariant)`, or `Contravariant`. `*const
+ /// T` is covariant with respect to `T`, so the variance within
+ /// which the first `Vec<i32>` appears is
+ /// `Contravariant.xform(Covariant)` or `Contravariant`. The same
+ /// is true for its `i32` argument. In the `*mut T` case, the
+ /// variance of `Vec<i32>` is `Contravariant.xform(Invariant)`,
+ /// and hence the outermost type is `Invariant` with respect to
+ /// `Vec<i32>` (and its `i32` argument).
+ ///
+ /// Source: Figure 1 of "Taming the Wildcards:
+ /// Combining Definition- and Use-Site Variance" published in PLDI'11.
+ pub fn xform(self, v: ty::Variance) -> ty::Variance {
+ match (self, v) {
+ // Figure 1, column 1.
+ (ty::Covariant, ty::Covariant) => ty::Covariant,
+ (ty::Covariant, ty::Contravariant) => ty::Contravariant,
+ (ty::Covariant, ty::Invariant) => ty::Invariant,
+ (ty::Covariant, ty::Bivariant) => ty::Bivariant,
+
+ // Figure 1, column 2.
+ (ty::Contravariant, ty::Covariant) => ty::Contravariant,
+ (ty::Contravariant, ty::Contravariant) => ty::Covariant,
+ (ty::Contravariant, ty::Invariant) => ty::Invariant,
+ (ty::Contravariant, ty::Bivariant) => ty::Bivariant,
+
+ // Figure 1, column 3.
+ (ty::Invariant, _) => ty::Invariant,
+
+ // Figure 1, column 4.
+ (ty::Bivariant, _) => ty::Bivariant,
+ }
+ }
+}
+
#[derive(Clone, Copy, Debug, RustcDecodable, RustcEncodable)]
pub struct MethodCallee<'tcx> {
/// Impl method ID, for inherent methods, or trait method ID, otherwise.
diff --git a/src/librustc_typeck/variance/constraints.rs b/src/librustc_typeck/variance/constraints.rs
index e986a381cd9..cb2ee7dd1bc 100644
--- a/src/librustc_typeck/variance/constraints.rs
+++ b/src/librustc_typeck/variance/constraints.rs
@@ -27,7 +27,6 @@ use rustc_data_structures::transitive_relation::TransitiveRelation;
use super::terms::*;
use super::terms::VarianceTerm::*;
-use super::xform::*;
pub struct ConstraintContext<'a, 'tcx: 'a> {
pub terms_cx: TermsContext<'a, 'tcx>,
diff --git a/src/librustc_typeck/variance/xform.rs b/src/librustc_typeck/variance/xform.rs
index 507734ce35e..7106ca4d420 100644
--- a/src/librustc_typeck/variance/xform.rs
+++ b/src/librustc_typeck/variance/xform.rs
@@ -10,35 +10,6 @@
use rustc::ty;
-pub trait Xform {
- fn xform(self, v: Self) -> Self;
-}
-
-impl Xform for ty::Variance {
- fn xform(self, v: ty::Variance) -> ty::Variance {
- // "Variance transformation", Figure 1 of The Paper
- match (self, v) {
- // Figure 1, column 1.
- (ty::Covariant, ty::Covariant) => ty::Covariant,
- (ty::Covariant, ty::Contravariant) => ty::Contravariant,
- (ty::Covariant, ty::Invariant) => ty::Invariant,
- (ty::Covariant, ty::Bivariant) => ty::Bivariant,
-
- // Figure 1, column 2.
- (ty::Contravariant, ty::Covariant) => ty::Contravariant,
- (ty::Contravariant, ty::Contravariant) => ty::Covariant,
- (ty::Contravariant, ty::Invariant) => ty::Invariant,
- (ty::Contravariant, ty::Bivariant) => ty::Bivariant,
-
- // Figure 1, column 3.
- (ty::Invariant, _) => ty::Invariant,
-
- // Figure 1, column 4.
- (ty::Bivariant, _) => ty::Bivariant,
- }
- }
-}
-
pub fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance {
// Greatest lower bound of the variance lattice as
// defined in The Paper:
diff --git a/src/test/compile-fail/region-invariant-static-error-reporting.rs b/src/test/compile-fail/region-invariant-static-error-reporting.rs
index d25674a74b1..646ae8183a2 100644
--- a/src/test/compile-fail/region-invariant-static-error-reporting.rs
+++ b/src/test/compile-fail/region-invariant-static-error-reporting.rs
@@ -13,9 +13,8 @@
// over time, but this test used to exhibit some pretty bogus messages
// that were not remotely helpful.
-// error-pattern:cannot infer
-// error-pattern:cannot outlive the lifetime 'a
-// error-pattern:must be valid for the static lifetime
+// error-pattern:the lifetime 'a
+// error-pattern:the static lifetime
struct Invariant<'a>(Option<&'a mut &'a mut ()>);
diff --git a/src/test/compile-fail/regions-trait-object-subtyping.rs b/src/test/compile-fail/regions-trait-object-subtyping.rs
index e8ada6a1755..948fb7e1ef6 100644
--- a/src/test/compile-fail/regions-trait-object-subtyping.rs
+++ b/src/test/compile-fail/regions-trait-object-subtyping.rs
@@ -22,7 +22,7 @@ fn foo2<'a:'b,'b>(x: &'b mut (Dummy+'a)) -> &'b mut (Dummy+'b) {
fn foo3<'a,'b>(x: &'a mut Dummy) -> &'b mut Dummy {
// Without knowing 'a:'b, we can't coerce
- x //~ ERROR cannot infer an appropriate lifetime
+ x //~ ERROR lifetime bound not satisfied
//~^ ERROR cannot infer an appropriate lifetime
}
diff --git a/src/test/compile-fail/variance-contravariant-arg-object.rs b/src/test/compile-fail/variance-contravariant-arg-object.rs
index d3bf92e85f4..1795ac95358 100644
--- a/src/test/compile-fail/variance-contravariant-arg-object.rs
+++ b/src/test/compile-fail/variance-contravariant-arg-object.rs
@@ -21,7 +21,7 @@ fn get_min_from_max<'min, 'max>(v: Box<Get<&'max i32>>)
-> Box<Get<&'min i32>>
where 'max : 'min
{
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn get_max_from_min<'min, 'max, G>(v: Box<Get<&'min i32>>)
@@ -29,7 +29,7 @@ fn get_max_from_min<'min, 'max, G>(v: Box<Get<&'min i32>>)
where 'max : 'min
{
// Previously OK:
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn main() { }
diff --git a/src/test/compile-fail/variance-covariant-arg-object.rs b/src/test/compile-fail/variance-covariant-arg-object.rs
index 0e94e35df28..ad059a467f5 100644
--- a/src/test/compile-fail/variance-covariant-arg-object.rs
+++ b/src/test/compile-fail/variance-covariant-arg-object.rs
@@ -22,14 +22,14 @@ fn get_min_from_max<'min, 'max>(v: Box<Get<&'max i32>>)
where 'max : 'min
{
// Previously OK, now an error as traits are invariant.
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn get_max_from_min<'min, 'max, G>(v: Box<Get<&'min i32>>)
-> Box<Get<&'max i32>>
where 'max : 'min
{
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn main() { }
diff --git a/src/test/compile-fail/variance-invariant-arg-object.rs b/src/test/compile-fail/variance-invariant-arg-object.rs
index aa3e06c015d..9edb510b826 100644
--- a/src/test/compile-fail/variance-invariant-arg-object.rs
+++ b/src/test/compile-fail/variance-invariant-arg-object.rs
@@ -18,14 +18,14 @@ fn get_min_from_max<'min, 'max>(v: Box<Get<&'max i32>>)
-> Box<Get<&'min i32>>
where 'max : 'min
{
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn get_max_from_min<'min, 'max, G>(v: Box<Get<&'min i32>>)
-> Box<Get<&'max i32>>
where 'max : 'min
{
- v //~ ERROR cannot infer an appropriate lifetime
+ v //~ ERROR mismatched types
}
fn main() { }
diff --git a/src/test/run-pass/issue-41677.rs b/src/test/run-pass/issue-41677.rs
new file mode 100644
index 00000000000..d014382ca39
--- /dev/null
+++ b/src/test/run-pass/issue-41677.rs
@@ -0,0 +1,37 @@
+// 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 #41677. The local variable was winding up with
+// a type `Receiver<?T, H>` where `?T` was unconstrained, because we
+// failed to enforce the WF obligations and `?T` is a bivariant type
+// parameter position.
+
+#![allow(unused_variables, dead_code)]
+
+use std::marker::PhantomData;
+
+trait Handle {
+ type Inner;
+}
+
+struct ResizingHandle<H>(PhantomData<H>);
+impl<H> Handle for ResizingHandle<H> {
+ type Inner = H;
+}
+
+struct Receiver<T, H: Handle<Inner=T>>(PhantomData<H>);
+
+fn channel<T>(size: usize) -> Receiver<T, ResizingHandle<T>> {
+ let rx = Receiver(PhantomData);
+ rx
+}
+
+fn main() {
+}