From ba7272959df7447c7393a14f475663de1f250f8f Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sat, 20 Aug 2022 03:23:23 +0000
Subject: [PATCH] Use separate infcx to solve obligations during negative
 coherence

---
 .../src/traits/coherence.rs                   | 21 +++++++------------
 .../coherence-negative-outlives-lifetimes.rs  |  7 ++++++-
 ...-negative-outlives-lifetimes.stock.stderr} |  2 +-
 ...s-lifetimes.with_negative_coherence.stderr | 11 ++++++++++
 4 files changed, 26 insertions(+), 15 deletions(-)
 rename src/test/ui/coherence/{coherence-negative-outlives-lifetimes.stderr => coherence-negative-outlives-lifetimes.stock.stderr} (88%)
 create mode 100644 src/test/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 985600d9ebc..3bc08fba91a 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -342,10 +342,8 @@ fn equate<'cx, 'tcx>(
     };
 
     let selcx = &mut SelectionContext::new(&infcx);
-    let opt_failing_obligation = obligations
-        .into_iter()
-        .chain(more_obligations)
-        .find(|o| negative_impl_exists(selcx, impl_env, o));
+    let opt_failing_obligation =
+        obligations.into_iter().chain(more_obligations).find(|o| negative_impl_exists(selcx, o));
 
     if let Some(failing_obligation) = opt_failing_obligation {
         debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
@@ -359,18 +357,15 @@ fn equate<'cx, 'tcx>(
 #[instrument(level = "debug", skip(selcx))]
 fn negative_impl_exists<'cx, 'tcx>(
     selcx: &SelectionContext<'cx, 'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
     o: &PredicateObligation<'tcx>,
 ) -> bool {
-    let infcx = &selcx.infcx().fork();
-
-    if resolve_negative_obligation(infcx, param_env, o) {
+    if resolve_negative_obligation(selcx.infcx().fork(), o) {
         return true;
     }
 
     // Try to prove a negative obligation exists for super predicates
-    for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
-        if resolve_negative_obligation(infcx, param_env, &o) {
+    for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
+        if resolve_negative_obligation(selcx.infcx().fork(), &o) {
             return true;
         }
     }
@@ -380,8 +375,7 @@ fn negative_impl_exists<'cx, 'tcx>(
 
 #[instrument(level = "debug", skip(infcx))]
 fn resolve_negative_obligation<'cx, 'tcx>(
-    infcx: &InferCtxt<'cx, 'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
+    infcx: InferCtxt<'cx, 'tcx>,
     o: &PredicateObligation<'tcx>,
 ) -> bool {
     let tcx = infcx.tcx;
@@ -390,7 +384,8 @@ fn resolve_negative_obligation<'cx, 'tcx>(
         return false;
     };
 
-    let errors = super::fully_solve_obligation(infcx, o);
+    let param_env = o.param_env;
+    let errors = super::fully_solve_obligation(&infcx, o);
     if !errors.is_empty() {
         return false;
     }
diff --git a/src/test/ui/coherence/coherence-negative-outlives-lifetimes.rs b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.rs
index 159788b1b77..221c1bc23b3 100644
--- a/src/test/ui/coherence/coherence-negative-outlives-lifetimes.rs
+++ b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.rs
@@ -1,10 +1,15 @@
+// revisions: stock with_negative_coherence
 #![feature(negative_impls)]
+#![cfg_attr(with_negative_coherence, feature(with_negative_coherence))]
 
 // FIXME: this should compile
 
 trait MyPredicate<'a> {}
-impl<'a, T> !MyPredicate<'a> for &T where T: 'a {}
+
+impl<'a, T> !MyPredicate<'a> for &'a T where T: 'a {}
+
 trait MyTrait<'a> {}
+
 impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
 impl<'a, T> MyTrait<'a> for &'a T {}
 //~^ ERROR: conflicting implementations of trait `MyTrait<'_>` for type `&_`
diff --git a/src/test/ui/coherence/coherence-negative-outlives-lifetimes.stderr b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.stock.stderr
similarity index 88%
rename from src/test/ui/coherence/coherence-negative-outlives-lifetimes.stderr
rename to src/test/ui/coherence/coherence-negative-outlives-lifetimes.stock.stderr
index 263bd19b424..097cc4e0fe3 100644
--- a/src/test/ui/coherence/coherence-negative-outlives-lifetimes.stderr
+++ b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.stock.stderr
@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `MyTrait<'_>` for type `&_`
-  --> $DIR/coherence-negative-outlives-lifetimes.rs:9:1
+  --> $DIR/coherence-negative-outlives-lifetimes.rs:14:1
    |
 LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
    | ---------------------------------------------- first implementation here
diff --git a/src/test/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr
new file mode 100644
index 00000000000..097cc4e0fe3
--- /dev/null
+++ b/src/test/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr
@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `MyTrait<'_>` for type `&_`
+  --> $DIR/coherence-negative-outlives-lifetimes.rs:14:1
+   |
+LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
+   | ---------------------------------------------- first implementation here
+LL | impl<'a, T> MyTrait<'a> for &'a T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.