auto merge of #20572 : nikomatsakis/rust/assoc-supertrait-stuff, r=brson

The first few commits in the PR are just general refactoring. I was intending them for some other code I didn't get around to writing yet, but might as well land them now. 

cc @japaric

Fixes #19541

src/test/compile-fail/associated-type-projection-from-multiple-supertraits.rs

@@ -0,0 +1,47 @@
+// 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.
+
+// Test equality constraints in a where clause where the type being
+// equated appears in a supertrait.
+
+#![feature(associated_types)]
+
+pub trait Vehicle {
+    type Color;
+
+    fn go(&self) {  }
+}
+
+pub trait Box {
+    type Color;
+
+    fn mail(&self) {  }
+}
+
+pub trait BoxCar : Box + Vehicle {
+}
+
+fn dent<C:BoxCar>(c: C, color: C::Color) {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+    //~| NOTE could derive from `Vehicle`
+    //~| NOTE could derive from `Box`
+}
+
+fn dent_object<COLOR>(c: BoxCar<Color=COLOR>) {
+    //~^ ERROR ambiguous associated type
+}
+
+fn paint<C:BoxCar>(c: C, d: C::Color) {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+    //~| NOTE could derive from `Vehicle`
+    //~| NOTE could derive from `Box`
+}
+
+pub fn main() { }

src/test/compile-fail/associated-type-projection-from-supertrait.rs

@@ -0,0 +1,56 @@
+// 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.
+
+// Test equality constraints in a where clause where the type being
+// equated appears in a supertrait.
+
+#![feature(associated_types)]
+
+pub trait Vehicle {
+    type Color;
+
+    fn go(&self) {  }
+}
+
+pub trait Car : Vehicle {
+    fn honk(&self) { }
+    fn chip_paint(&self, c: Self::Color) { }
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+struct Black;
+struct ModelT;
+impl Vehicle for ModelT { type Color = Black; }
+impl Car for ModelT { }
+
+///////////////////////////////////////////////////////////////////////////
+
+struct Blue;
+struct ModelU;
+impl Vehicle for ModelU { type Color = Blue; }
+impl Car for ModelU { }
+
+///////////////////////////////////////////////////////////////////////////
+
+fn dent<C:Car>(c: C, color: C::Color) { c.chip_paint(color) }
+fn a() { dent(ModelT, Black); }
+fn b() { dent(ModelT, Blue); } //~ ERROR type mismatch
+fn c() { dent(ModelU, Black); } //~ ERROR type mismatch
+fn d() { dent(ModelU, Blue); }
+
+///////////////////////////////////////////////////////////////////////////
+
+fn e() { ModelT.chip_paint(Black); }
+fn f() { ModelT.chip_paint(Blue); } //~ ERROR mismatched types
+fn g() { ModelU.chip_paint(Black); } //~ ERROR mismatched types
+fn h() { ModelU.chip_paint(Blue); }
+
+pub fn main() { }

src/test/compile-fail/associated-types-binding-to-type-defined-in-supertrait.rs

@@ -0,0 +1,53 @@
+// 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.
+
+// Test equality constraints in a where clause where the type being
+// equated appears in a supertrait.
+
+#![feature(associated_types)]
+
+pub trait Vehicle {
+    type Color;
+
+    fn go(&self) {  }
+}
+
+pub trait Car : Vehicle {
+    fn honk(&self) { }
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+struct Black;
+struct ModelT;
+impl Vehicle for ModelT { type Color = Black; }
+impl Car for ModelT { }
+
+///////////////////////////////////////////////////////////////////////////
+
+struct Blue;
+struct ModelU;
+impl Vehicle for ModelU { type Color = Blue; }
+impl Car for ModelU { }
+
+///////////////////////////////////////////////////////////////////////////
+
+fn black_car<C:Car<Color=Black>>(c: C) {
+}
+
+fn blue_car<C:Car<Color=Blue>>(c: C) {
+}
+
+fn a() { black_car(ModelT); }
+fn b() { blue_car(ModelT); } //~ ERROR type mismatch
+fn c() { black_car(ModelU); } //~ ERROR type mismatch
+fn d() { blue_car(ModelU); }
+
+pub fn main() { }

src/test/run-pass/associated-type-doubleendediterator-object.rs

@@ -0,0 +1,27 @@
+// 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.
+
+fn pairwise_sub(mut t: Box<DoubleEndedIterator<Item=int>>) -> int {
+    let mut result = 0;
+    loop {
+        let front = t.next();
+        let back = t.next_back();
+        match (front, back) {
+            (Some(f), Some(b)) => { result += b - f; }
+            _ => { return result; }
+        }
+    }
+}
+
+fn main() {
+    let v = vec!(1, 2, 3, 4, 5, 6);
+    let r = pairwise_sub(box v.into_iter());
+    assert_eq!(r, 9);
+}

src/test/run-pass/associated-types-iterator-binding.rs

@@ -0,0 +1,27 @@
+// 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.
+
+fn pairwise_sub<T:DoubleEndedIterator<Item=int>>(mut t: T) -> int {
+    let mut result = 0;
+    loop {
+        let front = t.next();
+        let back = t.next_back();
+        match (front, back) {
+            (Some(f), Some(b)) => { result += b - f; }
+            _ => { return result; }
+        }
+    }
+}
+
+fn main() {
+    let v = vec!(1, 2, 3, 4, 5, 6);
+    let r =pairwise_sub(v.into_iter());
+    assert_eq!(r, 9);
+}