1 files changed, 145 insertions, 0 deletions
diff --git a/src/librustc/middle/infer/bivariate.rs b/src/librustc/middle/infer/bivariate.rs
new file mode 100644
index 00000000000..93c80fb754f
--- /dev/null
+++ b/src/librustc/middle/infer/bivariate.rs
@@ -0,0 +1,145 @@
+// 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.
+
+//! Applies the "bivariance relationship" to two types and/or regions.
+//! If (A,B) are bivariant then either A <: B or B <: A. It occurs
+//! when type/lifetime parameters are unconstrained. Usually this is
+//! an error, but we permit it in the specific case where a type
+//! parameter is constrained in a where-clause via an associated type.
+//!
+//! There are several ways one could implement bivariance. You could
+//! just do nothing at all, for example, or you could fully verify
+//! that one of the two subtyping relationships hold. We choose to
+//! thread a middle line: we relate types up to regions, but ignore
+//! all region relationships.
+//!
+//! At one point, handling bivariance in this fashion was necessary
+//! for inference, but I'm actually not sure if that is true anymore.
+//! In particular, it might be enough to say (A,B) are bivariant for
+//! all (A,B).
+
+use middle::ty::{BuiltinBounds};
+use middle::ty::{self, Ty};
+use middle::ty::TyVar;
+use middle::infer::combine::*;
+use middle::infer::{cres};
+use middle::infer::type_variable::{BiTo};
+use util::ppaux::{Repr};
+
+use syntax::ast::{Unsafety};
+
+pub struct Bivariate<'f, 'tcx: 'f> {
+    fields: CombineFields<'f, 'tcx>
+}
+
+#[allow(non_snake_case)]
+pub fn Bivariate<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Bivariate<'f, 'tcx> {
+    Bivariate { fields: cf }
+}
+
+impl<'f, 'tcx> Combine<'tcx> for Bivariate<'f, 'tcx> {
+    fn tag(&self) -> String { "Bivariate".to_string() }
+    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
+
+    fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
+                         -> cres<'tcx, Ty<'tcx>>
+    {
+        match v {
+            ty::Invariant => self.equate().tys(a, b),
+            ty::Covariant => self.tys(a, b),
+            ty::Contravariant => self.tys(a, b),
+            ty::Bivariant => self.tys(a, b),
+        }
+    }
+
+    fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
+                             -> cres<'tcx, ty::Region>
+    {
+        match v {
+            ty::Invariant => self.equate().regions(a, b),
+            ty::Covariant => self.regions(a, b),
+            ty::Contravariant => self.regions(a, b),
+            ty::Bivariant => self.regions(a, b),
+        }
+    }
+
+    fn regions(&self, a: ty::Region, _: ty::Region) -> cres<'tcx, ty::Region> {
+        Ok(a)
+    }
+
+    fn mts(&self, a: &ty::mt<'tcx>, b: &ty::mt<'tcx>) -> cres<'tcx, ty::mt<'tcx>> {
+        debug!("mts({} <: {})",
+               a.repr(self.fields.infcx.tcx),
+               b.repr(self.fields.infcx.tcx));
+
+        if a.mutbl != b.mutbl { return Err(ty::terr_mutability); }
+        let t = try!(self.tys(a.ty, b.ty));
+        Ok(ty::mt { mutbl: a.mutbl, ty: t })
+    }
+
+    fn unsafeties(&self, a: Unsafety, b: Unsafety) -> cres<'tcx, Unsafety> {
+        if a != b {
+            Err(ty::terr_unsafety_mismatch(expected_found(self, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+
+    fn builtin_bounds(&self,
+                      a: BuiltinBounds,
+                      b: BuiltinBounds)
+                      -> cres<'tcx, BuiltinBounds>
+    {
+        if a != b {
+            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+
+    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
+        debug!("{}.tys({}, {})", self.tag(),
+               a.repr(self.fields.infcx.tcx), b.repr(self.fields.infcx.tcx));
+        if a == b { return Ok(a); }
+
+        let infcx = self.fields.infcx;
+        let a = infcx.type_variables.borrow().replace_if_possible(a);
+        let b = infcx.type_variables.borrow().replace_if_possible(b);
+        match (&a.sty, &b.sty) {
+            (&ty::ty_infer(TyVar(a_id)), &ty::ty_infer(TyVar(b_id))) => {
+                infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
+                Ok(a)
+            }
+
+            (&ty::ty_infer(TyVar(a_id)), _) => {
+                try!(self.fields.instantiate(b, BiTo, a_id));
+                Ok(a)
+            }
+
+            (_, &ty::ty_infer(TyVar(b_id))) => {
+                try!(self.fields.instantiate(a, BiTo, b_id));
+                Ok(a)
+            }
+
+            _ => {
+                super_tys(self, a, b)
+            }
+        }
+    }
+
+    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
+        where T : Combineable<'tcx>
+    {
+        let a1 = ty::erase_late_bound_regions(self.tcx(), a);
+        let b1 = ty::erase_late_bound_regions(self.tcx(), b);
+        let c = try!(Combineable::combine(self, &a1, &b1));
+        Ok(ty::Binder(c))
+    }
+}