Add a "match" relation that can be used to make recursion check during

trait matching more tailored. We now detect recursion where the
obligations "match" -- meaning basically that they are the same for some
substitution of any unbound type variables.

src/librustc/lib.rs

@@ -122,6 +122,7 @@ pub mod middle {
     pub mod traits;
     pub mod ty;
     pub mod ty_fold;
+    pub mod ty_match;
     pub mod ty_relate;
     pub mod ty_walk;
     pub mod weak_lang_items;

src/librustc/middle/traits/select.rs

@@ -38,6 +38,8 @@ use middle::ty::{self, RegionEscape, ToPolyTraitRef, Ty};
 use middle::infer;
 use middle::infer::{InferCtxt, TypeFreshener};
 use middle::ty_fold::TypeFoldable;
+use middle::ty_match;
+use middle::ty_relate::TypeRelation;
 use std::cell::RefCell;
 use std::rc::Rc;
 use syntax::{abi, ast};
@@ -472,7 +474,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             unbound_input_types &&
              (self.intercrate ||
               stack.iter().skip(1).any(
-                  |prev| stack.fresh_trait_ref.def_id() == prev.fresh_trait_ref.def_id()))
+                  |prev| self.match_fresh_trait_refs(&stack.fresh_trait_ref,
+                                                     &prev.fresh_trait_ref)))
         {
             debug!("evaluate_stack({}) --> unbound argument, recursion -->  ambiguous",
                    stack.fresh_trait_ref.repr(self.tcx()));
@@ -2475,6 +2478,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ///////////////////////////////////////////////////////////////////////////
     // Miscellany
 
+    fn match_fresh_trait_refs(&self,
+                              previous: &ty::PolyTraitRef<'tcx>,
+                              current: &ty::PolyTraitRef<'tcx>)
+                              -> bool
+    {
+        let mut matcher = ty_match::Match::new(self.tcx());
+        matcher.relate(previous, current).is_ok()
+    }
+
     fn push_stack<'o,'s:'o>(&mut self,
                             previous_stack: Option<&'s TraitObligationStack<'s, 'tcx>>,
                             obligation: &'o TraitObligation<'tcx>)

src/librustc/middle/ty_match.rs

@@ -0,0 +1,95 @@
+// Copyright 2012 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.
+
+use middle::ty::{self, Ty};
+use middle::ty_relate::{self, Relate, TypeRelation, RelateResult};
+use util::ppaux::Repr;
+
+/// A type "A" *matches* "B" if the fresh types in B could be
+/// substituted with values so as to make it equal to A. Matching is
+/// intended to be used only on freshened types, and it basically
+/// indicates if the non-freshened versions of A and B could have been
+/// unified.
+///
+/// It is only an approximation. If it yields false, unification would
+/// definitely fail, but a true result doesn't mean unification would
+/// succeed. This is because we don't track the "side-constraints" on
+/// type variables, nor do we track if the same freshened type appears
+/// more than once. To some extent these approximations could be
+/// fixed, given effort.
+///
+/// Like subtyping, matching is really a binary relation, so the only
+/// important thing about the result is Ok/Err. Also, matching never
+/// affects any type variables or unification state.
+pub struct Match<'a, 'tcx: 'a> {
+    tcx: &'a ty::ctxt<'tcx>
+}
+
+impl<'a, 'tcx> Match<'a, 'tcx> {
+    pub fn new(tcx: &'a ty::ctxt<'tcx>) -> Match<'a, 'tcx> {
+        Match { tcx: tcx }
+    }
+}
+
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Match<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Match" }
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.tcx }
+    fn a_is_expected(&self) -> bool { true } // irrelevant
+
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               _: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
+    {
+        self.relate(a, b)
+    }
+
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
+        debug!("{}.regions({}, {})",
+               self.tag(),
+               a.repr(self.tcx()),
+               b.repr(self.tcx()));
+        Ok(a)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        debug!("{}.tys({}, {})", self.tag(),
+               a.repr(self.tcx()), b.repr(self.tcx()));
+        if a == b { return Ok(a); }
+
+        match (&a.sty, &b.sty) {
+            (_, &ty::ty_infer(ty::FreshTy(_))) |
+            (_, &ty::ty_infer(ty::FreshIntTy(_))) => {
+                Ok(a)
+            }
+
+            (&ty::ty_infer(_), _) |
+            (_, &ty::ty_infer(_)) => {
+                Err(ty::terr_sorts(ty_relate::expected_found(self, &a, &b)))
+            }
+
+            (&ty::ty_err, _) | (_, &ty::ty_err) => {
+                Ok(self.tcx().types.err)
+            }
+
+            _ => {
+                ty_relate::super_relate_tys(self, a, b)
+            }
+        }
+    }
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a,'tcx>
+    {
+        Ok(ty::Binder(try!(self.relate(a.skip_binder(), b.skip_binder()))))
+    }
+}