From ffb42065774b5d500ef5649d4caa91e9e344ff46 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 17 Apr 2024 21:58:27 -0400
Subject: [PATCH] Don't repeatedly duplicate TAIT lifetimes for each
 subsequently nested TAIT

---
 .../src/collect/generics_of.rs                | 15 +++--
 tests/ui/type-alias-impl-trait/variance.rs    | 24 ++++++++
 .../ui/type-alias-impl-trait/variance.stderr  | 56 ++++++++++++++++++-
 3 files changed, 88 insertions(+), 7 deletions(-)

diff --git a/compiler/rustc_hir_analysis/src/collect/generics_of.rs b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
index 4d6a02f50bf..f83ddc51c76 100644
--- a/compiler/rustc_hir_analysis/src/collect/generics_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
@@ -195,16 +195,19 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 }
                 Some(fn_def_id.to_def_id())
             }
-            ItemKind::OpaqueTy(hir::OpaqueTy {
-                origin: hir::OpaqueTyOrigin::TyAlias { .. },
+            ItemKind::OpaqueTy(&hir::OpaqueTy {
+                origin: hir::OpaqueTyOrigin::TyAlias { parent, in_assoc_ty },
                 ..
             }) => {
-                let parent_id = tcx.hir().get_parent_item(hir_id);
-                assert_ne!(parent_id, hir::CRATE_OWNER_ID);
-                debug!("generics_of: parent of opaque ty {:?} is {:?}", def_id, parent_id);
+                if in_assoc_ty {
+                    assert!(matches!(tcx.def_kind(parent), DefKind::AssocTy));
+                } else {
+                    assert!(matches!(tcx.def_kind(parent), DefKind::TyAlias));
+                }
+                debug!("generics_of: parent of opaque ty {:?} is {:?}", def_id, parent);
                 // Opaque types are always nested within another item, and
                 // inherit the generics of the item.
-                Some(parent_id.to_def_id())
+                Some(parent.to_def_id())
             }
             _ => None,
         },
diff --git a/tests/ui/type-alias-impl-trait/variance.rs b/tests/ui/type-alias-impl-trait/variance.rs
index 4b9fa67fd64..ba52eaa0359 100644
--- a/tests/ui/type-alias-impl-trait/variance.rs
+++ b/tests/ui/type-alias-impl-trait/variance.rs
@@ -52,4 +52,28 @@ impl<'i> Foo<'i> for () {
     //~^ ERROR: unconstrained opaque type
 }
 
+trait Nesting<'a> {
+    type Output;
+}
+impl<'a> Nesting<'a> for &'a () {
+    type Output = &'a ();
+}
+type NestedDeeply<'a> =
+    impl Nesting< //~ [*, o]
+        'a,
+        Output = impl Nesting< //~ [*, o]
+            'a,
+            Output = impl Nesting< //~ [*, o]
+                'a,
+                Output = impl Nesting< //~ [*, o]
+                    'a,
+                    Output = impl Nesting<'a> //~ [*, o]
+                >
+            >,
+        >,
+    >;
+fn test<'a>() -> NestedDeeply<'a> {
+    &()
+}
+
 fn main() {}
diff --git a/tests/ui/type-alias-impl-trait/variance.stderr b/tests/ui/type-alias-impl-trait/variance.stderr
index 1aaf36223b7..e5ced7a4981 100644
--- a/tests/ui/type-alias-impl-trait/variance.stderr
+++ b/tests/ui/type-alias-impl-trait/variance.stderr
@@ -176,6 +176,60 @@ error: [*, *, o, o]
 LL |     type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>;
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 24 previous errors
+error: [*, o]
+  --> $DIR/variance.rs:62:5
+   |
+LL | /     impl Nesting<
+LL | |         'a,
+LL | |         Output = impl Nesting<
+LL | |             'a,
+...  |
+LL | |         >,
+LL | |     >;
+   | |_____^
+
+error: [*, o]
+  --> $DIR/variance.rs:64:18
+   |
+LL |           Output = impl Nesting<
+   |  __________________^
+LL | |             'a,
+LL | |             Output = impl Nesting<
+LL | |                 'a,
+...  |
+LL | |             >,
+LL | |         >,
+   | |_________^
+
+error: [*, o]
+  --> $DIR/variance.rs:66:22
+   |
+LL |               Output = impl Nesting<
+   |  ______________________^
+LL | |                 'a,
+LL | |                 Output = impl Nesting<
+LL | |                     'a,
+LL | |                     Output = impl Nesting<'a>
+LL | |                 >
+LL | |             >,
+   | |_____________^
+
+error: [*, o]
+  --> $DIR/variance.rs:68:26
+   |
+LL |                   Output = impl Nesting<
+   |  __________________________^
+LL | |                     'a,
+LL | |                     Output = impl Nesting<'a>
+LL | |                 >
+   | |_________________^
+
+error: [*, o]
+  --> $DIR/variance.rs:70:30
+   |
+LL |                     Output = impl Nesting<'a>
+   |                              ^^^^^^^^^^^^^^^^
+
+error: aborting due to 29 previous errors
 
 For more information about this error, try `rustc --explain E0657`.