From 8bfcd86acbecdac23e6236f6ab0e1732448d4f8f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Le=C3=B3n=20Orell=20Valerian=20Liehr?= <me@fmease.dev>
Date: Sun, 25 Aug 2024 04:44:24 +0200
Subject: [PATCH] Properly deduce the object lifetime default in GAT paths

---
 .../src/collect/resolve_bound_vars.rs         | 31 ++++++++++---------
 src/librustdoc/clean/mod.rs                   | 11 +++----
 .../object-lifetime-default-gat.rs            | 16 ++++++++++
 3 files changed, 37 insertions(+), 21 deletions(-)
 create mode 100644 tests/ui/object-lifetime/object-lifetime-default-gat.rs

diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index e11d3c9c48b4e..fa9d1142ae64f 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -1482,17 +1482,21 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
 
         // Figure out if this is a type/trait segment,
         // which requires object lifetime defaults.
-        let type_def_id = match res {
-            Res::Def(DefKind::AssocTy, def_id) if depth == 1 => Some(self.tcx.parent(def_id)),
-            Res::Def(DefKind::Variant, def_id) if depth == 0 => Some(self.tcx.parent(def_id)),
-            Res::Def(
-                DefKind::Struct
-                | DefKind::Union
-                | DefKind::Enum
-                | DefKind::TyAlias
-                | DefKind::Trait,
-                def_id,
-            ) if depth == 0 => Some(def_id),
+        let type_def_id = match (res, depth) {
+            (Res::Def(DefKind::AssocTy, def_id), 1) => Some(self.tcx.parent(def_id)),
+            (Res::Def(DefKind::Variant, def_id), 0) => Some(self.tcx.parent(def_id)),
+            (
+                Res::Def(
+                    DefKind::Struct
+                    | DefKind::Union
+                    | DefKind::Enum
+                    | DefKind::TyAlias
+                    | DefKind::Trait
+                    | DefKind::AssocTy,
+                    def_id,
+                ),
+                0,
+            ) => Some(def_id),
             _ => None,
         };
 
@@ -1536,9 +1540,6 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             let map = &self.map;
             let generics = self.tcx.generics_of(def_id);
 
-            // `type_def_id` points to an item, so there is nothing to inherit generics from.
-            debug_assert_eq!(generics.parent_count, 0);
-
             let set_to_region = |set: ObjectLifetimeDefault| match set {
                 ObjectLifetimeDefault::Empty => {
                     if in_body {
@@ -1549,8 +1550,8 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                 }
                 ObjectLifetimeDefault::Static => Some(ResolvedArg::StaticLifetime),
                 ObjectLifetimeDefault::Param(param_def_id) => {
-                    // This index can be used with `generic_args` since `parent_count == 0`.
                     let index = generics.param_def_id_to_index[&param_def_id] as usize;
+                    let index = index - generics.parent_count;
                     generic_args.args.get(index).and_then(|arg| match arg {
                         GenericArg::Lifetime(lt) => map.defs.get(&lt.hir_id).copied(),
                         _ => None,
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 53757349a9b2c..781753fed9949 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1995,22 +1995,21 @@ impl<'tcx> ContainerTy<'_, 'tcx> {
                 | DefKind::Union
                 | DefKind::Enum
                 | DefKind::TyAlias
-                | DefKind::Trait) = tcx.def_kind(container)
+                | DefKind::Trait
+                | DefKind::AssocTy) = tcx.def_kind(container)
                 else {
                     return ObjectLifetimeDefault::Empty;
                 };
 
                 let generics = tcx.generics_of(container);
-                debug_assert_eq!(generics.parent_count, 0);
 
                 let param = generics.own_params[index].def_id;
                 let default = tcx.object_lifetime_default(param);
                 match default {
                     rbv::ObjectLifetimeDefault::Param(lifetime) => {
-                        // The index is relative to the parent generics but since we don't have any,
-                        // we don't need to translate it.
-                        let index = generics.param_def_id_to_index[&lifetime];
-                        let arg = args.skip_binder()[index as usize].expect_region();
+                        let index = generics.param_def_id_to_index[&lifetime] as usize;
+                        let index = index - generics.parent_count;
+                        let arg = args.skip_binder()[index].expect_region();
                         ObjectLifetimeDefault::Arg(arg)
                     }
                     rbv::ObjectLifetimeDefault::Empty => ObjectLifetimeDefault::Empty,
diff --git a/tests/ui/object-lifetime/object-lifetime-default-gat.rs b/tests/ui/object-lifetime/object-lifetime-default-gat.rs
new file mode 100644
index 0000000000000..8f28c23da736a
--- /dev/null
+++ b/tests/ui/object-lifetime/object-lifetime-default-gat.rs
@@ -0,0 +1,16 @@
+// Properly deduce the object lifetime default in generic associated type (GAT) *paths*.
+// issue: rust-lang/rust#115379
+//@ check-pass
+
+trait Outer {
+    type Ty<'a, T: ?Sized + 'a>;
+}
+impl Outer for () {
+    type Ty<'a, T: ?Sized + 'a> = &'a T;
+}
+trait Inner {}
+
+fn f<'r>(x: <() as Outer>::Ty<'r, dyn Inner + 'r>) { g(x) }
+fn g<'r>(_: <() as Outer>::Ty<'r, dyn Inner>) {}
+
+fn main() {}