From 476ea9ef1c0ba7250e369a2ec7205506b09474b7 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Mon, 13 May 2019 01:37:04 -0400
Subject: Always try to project predicates when finding auto traits in rustdoc

Fixes #60726

Previous, AutoTraitFinder would only try to project predicates when the
predicate type contained an inference variable. When finding auto
traits, we only project to try to unify inference variables - we don't
otherwise learn any new information about the required bounds.

However, this lead to failing to properly generate a negative auto trait
impl (indicating that a type never implements a certain auto trait) in
the following unusual scenario:

In almost all cases, a type has an (implicit) negative impl of an auto
trait due some other type having an explicit *negative* impl of that
auto trait. For example:

struct MyType<T> {
    field: *const T
}

has an implicit 'impl<T> !Send for MyType<T>', due to the explicit
negative impl (in libcore) 'impl<T: ?Sized> !Send for *const T'.

However, as exposed by the 'abi_stable' crate, this isn't always the
case. This minimzed example shows how a type can never implement
'Send', due to a projection error:

```
pub struct True;
pub struct False;

pub trait MyTrait {
    type Project;
}

pub struct MyStruct<T> {
    field: T
}

impl MyTrait for u8 {
    type Project = False;
}

unsafe impl<T> Send for MyStruct<T>
    where T: MyTrait<Project=True> {}

pub struct Wrapper {
    inner: MyStruct<u8>
}
```

In this example, `<u8 as MyTrait>::Project == True'
must hold for 'MyStruct<u8>: Send' to hold.
However, '<u8 as MyTrait>::Project == False' holds instead

To properly account for this unusual case, we need to call
'poly_project_and_unify' on *all* predicates, not just those with
inference variables. This ensures that we catch the projection error
that occurs above, and don't incorrectly determine that 'Wrapper: Send'
holds.
---
 src/test/rustdoc/issue-60726.rs | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)
 create mode 100644 src/test/rustdoc/issue-60726.rs

(limited to 'src/test/rustdoc')

diff --git a/src/test/rustdoc/issue-60726.rs b/src/test/rustdoc/issue-60726.rs
new file mode 100644
index 00000000000..6acc8627738
--- /dev/null
+++ b/src/test/rustdoc/issue-60726.rs
@@ -0,0 +1,35 @@
+use std::marker::PhantomData;
+
+pub struct True;
+pub struct False;
+
+pub trait InterfaceType{
+    type Send;
+}
+
+
+pub struct FooInterface<T>(PhantomData<fn()->T>);
+
+impl<T> InterfaceType for FooInterface<T> {
+    type Send=False;
+}
+
+
+pub struct DynTrait<I>{
+    _interface:PhantomData<fn()->I>,
+    _unsync_unsend:PhantomData<::std::rc::Rc<()>>,
+}
+
+unsafe impl<I> Send for DynTrait<I>
+where
+    I:InterfaceType<Send=True>
+{}
+
+// @has issue_60726/struct.IntoIter.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//code' "impl<T> !Send for \
+// IntoIter<T>"
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//code' "impl<T> !Sync for \
+// IntoIter<T>"
+pub struct IntoIter<T>{
+    hello:DynTrait<FooInterface<T>>,
+}
-- 
cgit 1.4.1-3-g733a5