src/test/run-pass/associated-types-conditional-dispatch.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 // Test that we evaluate projection predicates to winnow out
  12 // candidates during trait selection and method resolution (#20296).
  13 // If we don't properly winnow out candidates based on the output type
  14 // `Target=[A]`, then the impl marked with `(*)` is seen to conflict
  15 // with all the others.
  16
  17 // pretty-expanded FIXME #23616
  18
  19 use std::marker::PhantomData;
  20 use std::ops::Deref;
  21
  22 pub trait MyEq<U: ?Sized=Self> {
  23     fn eq(&self, u: &U) -> bool;
  24 }
  25
  26 impl<A, B> MyEq<[B]> for [A]
  27     where A : MyEq<B>
  28 {
  29     fn eq(&self, other: &[B]) -> bool {
  30         self.len() == other.len() &&
  31             self.iter().zip(other).all(|(a, b)| MyEq::eq(a, b))
  32     }
  33 }
  34
  35 // (*) This impl conflicts with everything unless the `Target=[A]`
  36 // constraint is considered.
  37 impl<'a, A, B, Lhs> MyEq<[B; 0]> for Lhs
  38     where A: MyEq<B>, Lhs: Deref<Target=[A]>
  39 {
  40     fn eq(&self, other: &[B; 0]) -> bool {
  41         MyEq::eq(&**self, other)
  42     }
  43 }
  44
  45 struct DerefWithHelper<H, T> {
  46     pub helper: H,
  47     pub marker: PhantomData<T>,
  48 }
  49
  50 trait Helper<T> {
  51     fn helper_borrow(&self) -> &T;
  52 }
  53
  54 impl<T> Helper<T> for Option<T> {
  55     fn helper_borrow(&self) -> &T {
  56         self.as_ref().unwrap()
  57     }
  58 }
  59
  60 impl<T, H: Helper<T>> Deref for DerefWithHelper<H, T> {
  61     type Target = T;
  62
  63     fn deref(&self) -> &T {
  64         self.helper.helper_borrow()
  65     }
  66 }
  67
  68 pub fn check<T: MyEq>(x: T, y: T) -> bool {
  69     let d: DerefWithHelper<Option<T>, T> = DerefWithHelper { helper: Some(x),
  70                                                              marker: PhantomData };
  71     d.eq(&y)
  72 }
  73
  74 pub fn main() {
  75 }