From 6c61db44073ce96176850ea5db5dc99624823d91 Mon Sep 17 00:00:00 2001
From: Ilya Yanok <ilya.yanok@gmail.com>
Date: Thu, 28 Oct 2021 12:49:46 +0000
Subject: [PATCH 1/3] Use `is_global` in
 `candidate_should_be_dropped_in_favor_of`

This manifistated in #90195 with compiler being unable to keep
one candidate for a trait impl, if where is a global impl and more
than one trait bound in the where clause.

Before #87280 `candidate_should_be_dropped_in_favor_of` was using
`TypeFoldable::is_global()` that was enough to discard the two
`ParamCandidate`s. But #87280 changed it to use
`TypeFoldable::is_known_global()` instead, which is pessimistic, so
now the compiler drops the global impl instead (because
`is_known_global` is not sure) and then can't decide between the
two `ParamCandidate`s.

Switching it to use `is_global` again solves the issue.

Fixes #90195.
---
 compiler/rustc_trait_selection/src/traits/select/mod.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 1b26e38fe0e..8bd842a5e06 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1548,7 +1548,7 @@ fn candidate_should_be_dropped_in_favor_of(
         // the param_env so that it can be given the lowest priority. See
         // #50825 for the motivation for this.
         let is_global =
-            |cand: &ty::PolyTraitRef<'_>| cand.is_known_global() && !cand.has_late_bound_regions();
+            |cand: &ty::PolyTraitRef<'tcx>| cand.is_global(self.infcx.tcx) && !cand.has_late_bound_regions();
 
         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
         // and `DiscriminantKindCandidate` to anything else.

From 6f942a2f4a0487a7428a9aba8f018eb45cdb924e Mon Sep 17 00:00:00 2001
From: Ilya Yanok <ilya.yanok@gmail.com>
Date: Thu, 28 Oct 2021 13:23:49 +0000
Subject: [PATCH 2/3] Reformat the changed line to make tidy happy

---
 compiler/rustc_trait_selection/src/traits/select/mod.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 8bd842a5e06..60676ad3f4f 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1547,8 +1547,9 @@ fn candidate_should_be_dropped_in_favor_of(
         // Check if a bound would previously have been removed when normalizing
         // the param_env so that it can be given the lowest priority. See
         // #50825 for the motivation for this.
-        let is_global =
-            |cand: &ty::PolyTraitRef<'tcx>| cand.is_global(self.infcx.tcx) && !cand.has_late_bound_regions();
+        let is_global = |cand: &ty::PolyTraitRef<'tcx>| {
+            cand.is_global(self.infcx.tcx) && !cand.has_late_bound_regions()
+        };
 
         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
         // and `DiscriminantKindCandidate` to anything else.

From 9a0a622a042d0ed04ad349d8bb778e40dd417a16 Mon Sep 17 00:00:00 2001
From: Ilya Yanok <ilya.yanok@gmail.com>
Date: Thu, 28 Oct 2021 14:25:46 +0000
Subject: [PATCH 3/3] Add test cases

---
 src/test/ui/traits/issue-90195-2.rs | 20 ++++++++++++++++++++
 src/test/ui/traits/issue-90195.rs   | 21 +++++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 src/test/ui/traits/issue-90195-2.rs
 create mode 100644 src/test/ui/traits/issue-90195.rs

diff --git a/src/test/ui/traits/issue-90195-2.rs b/src/test/ui/traits/issue-90195-2.rs
new file mode 100644
index 00000000000..b739dc46e4e
--- /dev/null
+++ b/src/test/ui/traits/issue-90195-2.rs
@@ -0,0 +1,20 @@
+// check-pass
+pub trait Archive {
+    type Archived;
+}
+
+impl<T> Archive for Option<T> {
+    type Archived = ();
+}
+pub type Archived<T> = <T as Archive>::Archived;
+
+pub trait Deserialize<D> {}
+
+const ARRAY_SIZE: usize = 32;
+impl<__D> Deserialize<__D> for ()
+where
+    Option<[u8; ARRAY_SIZE]>: Archive,
+    Archived<Option<[u8; ARRAY_SIZE]>>: Deserialize<__D>,
+{
+}
+fn main() {}
diff --git a/src/test/ui/traits/issue-90195.rs b/src/test/ui/traits/issue-90195.rs
new file mode 100644
index 00000000000..543c9f197e1
--- /dev/null
+++ b/src/test/ui/traits/issue-90195.rs
@@ -0,0 +1,21 @@
+// check-pass
+pub trait Archive {
+    type Archived;
+}
+
+impl<T> Archive for Option<T> {
+    type Archived = ();
+}
+pub type Archived<T> = <T as Archive>::Archived;
+
+pub trait Deserialize<D> {}
+
+const ARRAY_SIZE: usize = 32;
+impl<__D> Deserialize<__D> for ()
+where
+    Option<[u8; ARRAY_SIZE]>: Archive,
+    Option<[u8; ARRAY_SIZE]>: Archive,
+    Archived<Option<[u8; ARRAY_SIZE]>>: Deserialize<__D>,
+{
+}
+fn main() {}