Account for nested impl Trait in TAIT

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -1,7 +1,7 @@
 use rustc_errors::StashKey;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit::{self, Visitor};
-use rustc_hir::{self as hir, Expr, ImplItem, Item, Node, TraitItem};
+use rustc_hir::{self as hir, def, Expr, ImplItem, Item, Node, TraitItem};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::DUMMY_SP;
@@ -74,9 +74,14 @@ pub(super) fn find_opaque_ty_constraints_for_tait(tcx: TyCtxt<'_>, def_id: Local
 
         hidden.ty
     } else {
+        let mut parent_def_id = def_id;
+        while tcx.def_kind(parent_def_id) == def::DefKind::OpaqueTy {
+            // Account for `type Alias = impl Trait<Foo = impl Trait>;` (#116031)
+            parent_def_id = tcx.local_parent(parent_def_id);
+        }
         let reported = tcx.sess.emit_err(UnconstrainedOpaqueType {
             span: tcx.def_span(def_id),
-            name: tcx.item_name(tcx.local_parent(def_id).to_def_id()),
+            name: tcx.item_name(parent_def_id.to_def_id()),
             what: match tcx.hir().get(scope) {
                 _ if scope == hir::CRATE_HIR_ID => "module",
                 Node::Item(hir::Item { kind: hir::ItemKind::Mod(_), .. }) => "module",

tests/ui/type-alias-impl-trait/nested-impl-trait-in-tait.rs

@@ -0,0 +1,9 @@
+#![feature(type_alias_impl_trait)]
+
+pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
+//~^ ERROR use of undeclared lifetime name `'db`
+//~| ERROR cannot find type `Key` in this scope
+//~| ERROR unconstrained opaque type
+//~| ERROR unconstrained opaque type
+
+pub fn main() {}

tests/ui/type-alias-impl-trait/nested-impl-trait-in-tait.stderr

@@ -0,0 +1,47 @@
+error[E0261]: use of undeclared lifetime name `'db`
+  --> $DIR/nested-impl-trait-in-tait.rs:3:40
+   |
+LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
+   |                                        ^^^ undeclared lifetime
+   |
+   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
+help: consider making the bound lifetime-generic with a new `'db` lifetime
+   |
+LL | pub type Tait = impl for<'db> Iterator<Item = (&'db Key, impl Iterator)>;
+   |                      ++++++++
+help: consider introducing lifetime `'db` here
+   |
+LL | pub type Tait<'db> = impl Iterator<Item = (&'db Key, impl Iterator)>;
+   |              +++++
+
+error[E0412]: cannot find type `Key` in this scope
+  --> $DIR/nested-impl-trait-in-tait.rs:3:44
+   |
+LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
+   |                                            ^^^ not found in this scope
+   |
+help: consider importing this struct
+   |
+LL + use std::thread::local_impl::Key;
+   |
+
+error: unconstrained opaque type
+  --> $DIR/nested-impl-trait-in-tait.rs:3:17
+   |
+LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Tait` must be used in combination with a concrete type within the same module
+
+error: unconstrained opaque type
+  --> $DIR/nested-impl-trait-in-tait.rs:3:49
+   |
+LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
+   |                                                 ^^^^^^^^^^^^^
+   |
+   = note: `Tait` must be used in combination with a concrete type within the same module
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0261, E0412.
+For more information about an error, try `rustc --explain E0261`.