From 5cdbe83af82e0e48a05bc87ff41ef381e45391dc Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Tue, 27 Feb 2024 23:30:07 +0000
Subject: [PATCH] Opportunistically resolve regions when processing region
 outlives obligations

---
 .../src/infer/outlives/obligations.rs         | 10 +++++++++-
 .../impl-implied-bounds-compatibility.rs      |  2 +-
 .../impl-implied-bounds-compatibility.stderr  | 20 ++++---------------
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index c0a99e5cc4177..8dd3a1f40cc10 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -62,6 +62,7 @@
 use crate::infer::outlives::components::{push_outlives_components, Component};
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::outlives::verify::VerifyBoundCx;
+use crate::infer::resolve::OpportunisticRegionResolver;
 use crate::infer::{
     self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, UndoLog, VerifyBound,
 };
@@ -69,7 +70,9 @@ use crate::traits::{ObligationCause, ObligationCauseCode};
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::traits::query::NoSolution;
-use rustc_middle::ty::{self, GenericArgsRef, Region, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{
+    self, GenericArgsRef, Region, Ty, TyCtxt, TypeFoldable as _, TypeVisitableExt,
+};
 use rustc_middle::ty::{GenericArgKind, PolyTypeOutlivesPredicate};
 use rustc_span::DUMMY_SP;
 use smallvec::smallvec;
@@ -176,6 +179,11 @@ impl<'tcx> InferCtxt<'tcx> {
                         .map_err(|NoSolution| (outlives, origin.clone()))?
                         .no_bound_vars()
                         .expect("started with no bound vars, should end with no bound vars");
+                // `TypeOutlives` is structural, so we should try to opportunistically resolve all
+                // region vids before processing regions, so we have a better chance to match clauses
+                // in our param-env.
+                let (sup_type, sub_region) =
+                    (sup_type, sub_region).fold_with(&mut OpportunisticRegionResolver::new(self));
 
                 debug!(?sup_type, ?sub_region, ?origin);
 
diff --git a/tests/ui/implied-bounds/impl-implied-bounds-compatibility.rs b/tests/ui/implied-bounds/impl-implied-bounds-compatibility.rs
index 7f3817b326adc..5e3d21eb26ad7 100644
--- a/tests/ui/implied-bounds/impl-implied-bounds-compatibility.rs
+++ b/tests/ui/implied-bounds/impl-implied-bounds-compatibility.rs
@@ -10,7 +10,7 @@ pub trait MessageListenersInterface {
 
 impl<'a> MessageListenersInterface for MessageListeners<'a> {
     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
-        //~^ ERROR cannot infer an appropriate lifetime for lifetime parameter 'b in generic type due to conflicting requirements
+        //~^ ERROR in type `&'a MessageListeners<'_>`, reference has a longer lifetime than the data it references
         self
     }
 }
diff --git a/tests/ui/implied-bounds/impl-implied-bounds-compatibility.stderr b/tests/ui/implied-bounds/impl-implied-bounds-compatibility.stderr
index 6a412eb5e4bee..e0cf15d0d1f33 100644
--- a/tests/ui/implied-bounds/impl-implied-bounds-compatibility.stderr
+++ b/tests/ui/implied-bounds/impl-implied-bounds-compatibility.stderr
@@ -1,27 +1,15 @@
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'b in generic type due to conflicting requirements
+error[E0491]: in type `&'a MessageListeners<'_>`, reference has a longer lifetime than the data it references
   --> $DIR/impl-implied-bounds-compatibility.rs:12:5
    |
 LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: first, the lifetime cannot outlive the lifetime `'c` as defined here...
-  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
-   |
-LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: ...so that the method type is compatible with trait
-  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
-   |
-LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected `fn(&'c MessageListeners<'_>) -> &'c MessageListeners<'c>`
-              found `fn(&MessageListeners<'_>) -> &'a MessageListeners<'_>`
-note: but, the lifetime must be valid for the lifetime `'a` as defined here...
+note: the pointer is valid for the lifetime `'a` as defined here
   --> $DIR/impl-implied-bounds-compatibility.rs:11:6
    |
 LL | impl<'a> MessageListenersInterface for MessageListeners<'a> {
    |      ^^
-note: ...so that the reference type `&'a MessageListeners<'_>` does not outlive the data it points at
+note: but the referenced data is only valid for the lifetime `'c` as defined here
   --> $DIR/impl-implied-bounds-compatibility.rs:12:5
    |
 LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
@@ -29,4 +17,4 @@ LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0495`.
+For more information about this error, try `rustc --explain E0491`.