From d688b86c3df18c76de65966d5355e19a7df5deb4 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Thu, 16 Nov 2023 22:50:59 +0000
Subject: [PATCH] Ignore but do not assume region obligations from unifying
 headers in negative coherence

---
 .../src/traits/coherence.rs                   | 12 ++++-----
 ...constraints-on-unification.explicit.stderr | 19 ++++++++++++++
 ...older-region-constraints-on-unification.rs | 25 +++++++++++++++++++
 3 files changed, 50 insertions(+), 6 deletions(-)
 create mode 100644 tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.explicit.stderr
 create mode 100644 tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.rs

diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 7ca33ae41c0ce..faded18582f70 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -542,14 +542,14 @@ fn try_prove_negated_where_clause<'tcx>(
     };
 
     // FIXME(with_negative_coherence): the infcx has region contraints from equating
-    // the impl headers as requirements. Given that the only region constraints we
-    // get are involving inference regions in the root, it shouldn't matter, but
-    // still sus.
+    // the impl headers as requirements.
     //
-    // We probably should just throw away the region obligations registered up until
-    // now, or ideally use them as assumptions when proving the region obligations
-    // that we get from proving the negative predicate below.
+    // We ideally should use these region constraints as assumptions when proving
+    // the region obligations that we get from proving the negative predicate below.
     let ref infcx = root_infcx.fork();
+    let _ = infcx.take_registered_region_obligations();
+    let _ = infcx.take_and_reset_region_constraints();
+
     let ocx = ObligationCtxt::new(infcx);
 
     ocx.register_obligation(Obligation::new(
diff --git a/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.explicit.stderr b/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.explicit.stderr
new file mode 100644
index 0000000000000..34f3904443c31
--- /dev/null
+++ b/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.explicit.stderr
@@ -0,0 +1,19 @@
+error: conflicting implementations of trait `FnMarker` for type `fn(&_)`
+  --> $DIR/negative-coherence-placeholder-region-constraints-on-unification.rs:21:1
+   |
+LL | impl<T: ?Sized + Marker> FnMarker for fn(T) {}
+   | ------------------------------------------- first implementation here
+LL | impl<T: ?Sized> FnMarker for fn(&T) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `fn(&_)`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
+   = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
+note: the lint level is defined here
+  --> $DIR/negative-coherence-placeholder-region-constraints-on-unification.rs:4:11
+   |
+LL | #![forbid(coherence_leak_check)]
+   |           ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.rs b/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.rs
new file mode 100644
index 0000000000000..26d9d84d8f0c9
--- /dev/null
+++ b/tests/ui/coherence/negative-coherence-placeholder-region-constraints-on-unification.rs
@@ -0,0 +1,25 @@
+// revisions: explicit implicit
+//[implicit] check-pass
+
+#![forbid(coherence_leak_check)]
+#![feature(negative_impls, with_negative_coherence)]
+
+pub trait Marker {}
+
+#[cfg(implicit)]
+impl<T: ?Sized> !Marker for &T {}
+
+#[cfg(explicit)]
+impl<'a, T: ?Sized + 'a> !Marker for &'a T {}
+
+trait FnMarker {}
+
+// Unifying these two impls below results in a `T: '!0` obligation
+// that we shouldn't need to care about. Ideally, we'd treat that
+// as an assumption when proving `&'!0 T: Marker`...
+impl<T: ?Sized + Marker> FnMarker for fn(T) {}
+impl<T: ?Sized> FnMarker for fn(&T) {}
+//[explicit]~^ ERROR conflicting implementations of trait `FnMarker` for type `fn(&_)`
+//[explicit]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+fn main() {}