Auto merge of #52990 - Aaron1011:fix/rustdoc-auto-trait-static, r=eddyb

Fix ICE when rustdoc encounters certain usages of HRTBs

Fixes #51236

Under certain circumstances, `AutoTraitFinder` could end up computing a `ParamEnv` involving two trait predicates that differed only in the region parameters involved. One of these parameters would be a HRTB, while the other would be a normal region parameter.

When this `ParamEnv` was later passed to `SelectionContext`, an `Ambiguity` error would occur, since the erased versions of these predicates would be identical. To solve the issue, we de-duplicate our list of predicates as we build it up. Whenever we encounter two predicates that differ only in their assignment of region parameters (a HRTB vs a normal lifetime parameter), we pick the HRTB. This corresponds to selecting a 'stricter' bound to display in the generated documentation: we're requiring that a particular type works for all possible lifetime parameters if it's going to implement a particular auto trait.

2 files changed, 44 insertions, 0 deletions

diff --git a/src/test/rustdoc/issue-51236.rs b/src/test/rustdoc/issue-51236.rs
new file mode 100644
index 00000000000..541a1c5e19f
--- /dev/null
+++ b/src/test/rustdoc/issue-51236.rs
@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+use std::marker::PhantomData;
+
+pub mod traits {
+    pub trait Owned<'a> {
+        type Reader;
+    }
+}
+
+// @has issue_51236/struct.Owned.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<T> Send for \
+// Owned<T> where <T as Owned<'static>>::Reader: Send"
+pub struct Owned<T> where T: for<'a> ::traits::Owned<'a> {
+    marker: PhantomData<<T as ::traits::Owned<'static>>::Reader>,
+}
diff --git a/src/test/rustdoc/synthetic_auto/static-region.rs b/src/test/rustdoc/synthetic_auto/static-region.rs
new file mode 100644
index 00000000000..96e8b8ed5f6
--- /dev/null
+++ b/src/test/rustdoc/synthetic_auto/static-region.rs
@@ -0,0 +1,20 @@
+// Copyright 2018 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.
+
+pub trait OwnedTrait<'a> {
+    type Reader;
+}
+
+// @has static_region/struct.Owned.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<T> Send for \
+// Owned<T> where <T as OwnedTrait<'static>>::Reader: Send"
+pub struct Owned<T> where T: OwnedTrait<'static> {
+    marker: <T as OwnedTrait<'static>>::Reader,
+}