Check and extract bindings from trait definitions.

Fixes #21636.

Author: nikomatsakis

src/librustc/middle/traits/project.rs

@@ -401,6 +401,11 @@ fn project_type<'cx,'tcx>(
                                        &obligation_trait_ref,
                                        &mut candidates);
 
+    assemble_candidates_from_trait_def(selcx,
+                                       obligation,
+                                       &obligation_trait_ref,
+                                       &mut candidates);
+
     if let Err(e) = assemble_candidates_from_impls(selcx,
                                                    obligation,
                                                    &obligation_trait_ref,
@@ -446,6 +451,41 @@ fn assemble_candidates_from_param_env<'cx,'tcx>(
                                         candidate_set, env_predicates);
 }
 
+/// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
+/// that the definition of `Foo` has some clues:
+///
+/// ```rust
+/// trait Foo {
+///     type FooT : Bar<BarT=i32>
+/// }
+/// ```
+///
+/// Here, for example, we could conclude that the result is `i32`.
+fn assemble_candidates_from_trait_def<'cx,'tcx>(
+    selcx: &mut SelectionContext<'cx,'tcx>,
+    obligation: &ProjectionTyObligation<'tcx>,
+    obligation_trait_ref: &Rc<ty::TraitRef<'tcx>>,
+    candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
+{
+    // Check whether the self-type is itself a projection.
+    let trait_ref = match obligation_trait_ref.self_ty().sty {
+        ty::ty_projection(ref data) => data.trait_ref.clone(),
+        ty::ty_infer(ty::TyVar(_)) => {
+            // If the self-type is an inference variable, then it MAY wind up
+            // being a projected type, so induce an ambiguity.
+            candidate_set.ambiguous = true;
+            return;
+        }
+        _ => { return; }
+    };
+
+    // If so, extract what we know from the trait and try to come up with a good answer.
+    let trait_def = ty::lookup_trait_def(selcx.tcx(), trait_ref.def_id);
+    let bounds = trait_def.generics.to_bounds(selcx.tcx(), trait_ref.substs);
+    assemble_candidates_from_predicates(selcx, obligation, obligation_trait_ref,
+                                        candidate_set, bounds.predicates.into_vec());
+}
+
 fn assemble_candidates_from_predicates<'cx,'tcx>(
     selcx: &mut SelectionContext<'cx,'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,

src/test/run-pass/associated-types-binding-in-trait.rs

@@ -0,0 +1,44 @@
+// Copyright 2015 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.
+
+// Test a case where the associated type binding (to `bool`, in this
+// case) is derived from the trait definition. Issue #21636.
+
+use std::vec;
+
+pub trait BitIter {
+    type Iter: Iterator<Item=bool>;
+    fn bit_iter(self) -> <Self as BitIter>::Iter;
+}
+
+impl BitIter for Vec<bool> {
+    type Iter = vec::IntoIter<bool>;
+    fn bit_iter(self) -> <Self as BitIter>::Iter {
+        self.into_iter()
+    }
+}
+
+fn count<T>(arg: T) -> usize
+    where T: BitIter
+{
+    let mut sum = 0;
+    for i in arg.bit_iter() {
+        if i {
+            sum += 1;
+        }
+    }
+    sum
+}
+
+fn main() {
+    let v = vec![true, false, true];
+    let c = count(v);
+    assert_eq!(c, 2);
+}