update recursion depth in confirm_candidate

(cherry picked from commit 1708ad65a4)

compiler/rustc_middle/src/traits/mod.rs

@@ -697,9 +697,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
     }
 
     pub fn borrow_nested_obligations(&self) -> &[N] {
-        match &self {
+        match self {
-            ImplSource::UserDefined(i) => &i.nested[..],
+            ImplSource::UserDefined(i) => &i.nested,
-            ImplSource::Param(n, _) => &n,
+            ImplSource::Param(n, _) => n,
             ImplSource::Builtin(i) => &i.nested,
             ImplSource::AutoImpl(d) => &d.nested,
             ImplSource::Closure(c) => &c.nested,
@@ -713,6 +713,23 @@ impl<'tcx, N> ImplSource<'tcx, N> {
         }
     }
 
+    pub fn borrow_nested_obligations_mut(&mut self) -> &mut [N] {
+        match self {
+            ImplSource::UserDefined(i) => &mut i.nested,
+            ImplSource::Param(n, _) => n,
+            ImplSource::Builtin(i) => &mut i.nested,
+            ImplSource::AutoImpl(d) => &mut d.nested,
+            ImplSource::Closure(c) => &mut c.nested,
+            ImplSource::Generator(c) => &mut c.nested,
+            ImplSource::Future(c) => &mut c.nested,
+            ImplSource::Object(d) => &mut d.nested,
+            ImplSource::FnPointer(d) => &mut d.nested,
+            ImplSource::TraitAlias(d) => &mut d.nested,
+            ImplSource::TraitUpcasting(d) => &mut d.nested,
+            ImplSource::ConstDestruct(i) => &mut i.nested,
+        }
+    }
+
     pub fn map<M, F>(self, f: F) -> ImplSource<'tcx, M>
     where
         F: FnMut(N) -> M,

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -131,6 +131,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         };
 
+        // The obligations returned by confirmation are recursively evaluated
+        // so we need to make sure they have the correct depth.
+        for subobligation in impl_src.borrow_nested_obligations_mut() {
+            subobligation.set_depth_from_parent(obligation.recursion_depth);
+        }
+
         if !obligation.predicate.is_const_if_const() {
             // normalize nested predicates according to parent predicate's constness.
             impl_src = impl_src.map(|mut o| {