Auto merge of #45404 - giannicic:defaultimpl2, r=nikomatsakis

#37653 support `default impl` for specialization

this commit implements the second part of the `default impl` feature:

>  - a `default impl` need not include all items from the trait
>  - a `default impl` alone does not mean that a type implements the trait

The first point allows rustc to compile and run something like this:

```
trait Foo {
    fn foo_one(&self) -> &'static str;
    fn foo_two(&self) -> &'static str;
}

default impl<T> Foo for T {
    fn foo_one(&self) -> &'static str {
        "generic"
    }
}

struct MyStruct;

fn  main() {
    assert!(MyStruct.foo_one() == "generic");
}
```

but it shows a proper error if trying to call `MyStruct.foo_two()`

The second point allows a `default impl` to be considered as not implementing the `Trait` if it doesn't implement all the trait items.
The tests provided (in the compile-fail section) should cover all the possible trait resolutions.
Let me know if some tests is missed.

See [referenced ](#37653) issue for further info

r? @nikomatsakis

src/libcore/iter/mod.rs

@@ -595,15 +595,15 @@ impl<'a, I, T: 'a> FusedIterator for Cloned<I>
 {}
 
 #[doc(hidden)]
-default unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
+unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
     where I: TrustedRandomAccess<Item=&'a T>, T: Clone
 {
-    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
+    default unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
         self.it.get_unchecked(i).clone()
     }
 
     #[inline]
-    fn may_have_side_effect() -> bool { true }
+    default fn may_have_side_effect() -> bool { true }
 }
 
 #[doc(hidden)]

src/librustc_typeck/check/mod.rs

@@ -1395,7 +1395,7 @@ fn check_impl_items_against_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             .map(|node_item| !node_item.node.is_from_trait())
             .unwrap_or(false);
 
-        if !is_implemented {
+        if !is_implemented && !tcx.impl_is_default(impl_id) {
             if !trait_item.defaultness.has_value() {
                 missing_items.push(trait_item);
             } else if associated_type_overridden {

src/librustc_typeck/collect.rs

@@ -1364,6 +1364,7 @@ fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let node = tcx.hir.get(node_id);
 
     let mut is_trait = None;
+    let mut is_default_impl_trait = None;
 
     let icx = ItemCtxt::new(tcx, def_id);
     let no_generics = hir::Generics::empty();
@@ -1373,8 +1374,13 @@ fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
         NodeItem(item) => {
             match item.node {
+                ItemImpl(_, _, defaultness, ref generics, ..) => {
+                    if defaultness.is_default() {
+                        is_default_impl_trait = tcx.impl_trait_ref(def_id);
+                    }
+                    generics
+                }
                 ItemFn(.., ref generics, _) |
-                ItemImpl(_, _, _, ref generics, ..) |
                 ItemTy(_, ref generics) |
                 ItemEnum(_, ref generics) |
                 ItemStruct(_, ref generics) |
@@ -1446,6 +1452,18 @@ fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         predicates.push(trait_ref.to_poly_trait_ref().to_predicate());
     }
 
+    // In default impls, we can assume that the self type implements
+    // the trait. So in:
+    //
+    //     default impl Foo for Bar { .. }
+    //
+    // we add a default where clause `Foo: Bar`. We do a similar thing for traits
+    // (see below). Recall that a default impl is not itself an impl, but rather a
+    // set of defaults that can be incorporated into another impl.
+    if let Some(trait_ref) = is_default_impl_trait {
+        predicates.push(trait_ref.to_poly_trait_ref().to_predicate());
+    }
+
     // Collect the region predicates that were declared inline as
     // well. In the case of parameters declared on a fn or method, we
     // have to be careful to only iterate over early-bound regions.

src/test/run-pass/specialization/defaultimpl/assoc-fns.rs → src/test/compile-fail/specialization/defaultimpl/specialization-trait-item-not-implemented.rs

@@ -8,30 +8,26 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Test that non-method associated functions can be specialized
+// Tests that default impls do not have to supply all items but regular impls do.
 
 #![feature(specialization)]
 
 trait Foo {
-    fn mk() -> Self;
+    fn foo_one(&self) -> &'static str;
+    fn foo_two(&self) -> &'static str;
 }
 
-default impl<T: Default> Foo for T {
-    fn mk() -> T {
-        T::default()
-    }
-}
+struct MyStruct;
 
-impl Foo for Vec<u8> {
-    fn mk() -> Vec<u8> {
-        vec![0]
+default impl<T> Foo for T {
+    fn foo_one(&self) -> &'static str {
+        "generic"
     }
 }
 
-fn main() {
-    let v1: Vec<i32> = Foo::mk();
-    let v2: Vec<u8> = Foo::mk();
+impl Foo for MyStruct {}
+//~^ ERROR not all trait items implemented, missing: `foo_two` [E0046]
 
-    assert!(v1.len() == 0);
-    assert!(v2.len() == 1);
+fn main() {
+    println!("{}", MyStruct.foo_one());
 }

src/test/compile-fail/specialization/defaultimpl/specialization-trait-not-implemented.rs

@@ -0,0 +1,34 @@
+// 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.
+
+// Tests that:
+// - default impls do not have to supply all items and
+// - a default impl does not count as an impl (in this case, an incomplete default impl).
+
+#![feature(specialization)]
+
+trait Foo {
+    fn foo_one(&self) -> &'static str;
+    fn foo_two(&self) -> &'static str;
+}
+
+struct MyStruct;
+
+default impl<T> Foo for T {
+    fn foo_one(&self) -> &'static str {
+        "generic"
+    }
+}
+
+
+fn main() {
+    println!("{}", MyStruct.foo_one());
+    //~^ ERROR no method named `foo_one` found for type `MyStruct` in the current scope
+}

src/test/run-pass/specialization/defaultimpl/projection-alias.rs → src/test/compile-fail/specialization/defaultimpl/specialization-wfcheck.rs

@@ -1,4 +1,4 @@
-// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -8,25 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(specialization)]
-
-// Regression test for ICE when combining specialized associated types and type
-// aliases
-
-trait Id_ {
-    type Out;
-}
+// Tests that a default impl still has to have a WF trait ref.
 
-type Id<T> = <T as Id_>::Out;
-
-default impl<T> Id_ for T {
-    type Out = T;
-}
+#![feature(specialization)]
 
-fn test_proection() {
-    let x: Id<bool> = panic!();
-}
+trait Foo<'a, T: Eq + 'a> { }
 
-fn main() {
+default impl<U> Foo<'static, U> for () {}
+//~^ ERROR the trait bound `U: std::cmp::Eq` is not satisfied
 
-}
+fn main(){}