Delegation: fix ICE on bound_vars divergence

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -793,12 +793,20 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         fd: &'tcx hir::FnDecl<'tcx>,
         body_id: hir::BodyId,
         _: Span,
-        _: LocalDefId,
+        def_id: LocalDefId,
     ) {
         let output = match fd.output {
             hir::FnRetTy::DefaultReturn(_) => None,
             hir::FnRetTy::Return(ty) => Some(ty),
         };
+        if let Some(ty) = output
+            && let hir::TyKind::InferDelegation(sig_id, _) = ty.kind
+        {
+            let bound_vars: Vec<_> =
+                self.tcx.fn_sig(sig_id).skip_binder().bound_vars().iter().collect();
+            let hir_id = self.tcx.local_def_id_to_hir_id(def_id);
+            self.map.late_bound_vars.insert(hir_id, bound_vars);
+        }
         self.visit_fn_like_elision(fd.inputs, output, matches!(fk, intravisit::FnKind::Closure));
         intravisit::walk_fn_kind(self, fk);
         self.visit_nested_body(body_id)

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2494,13 +2494,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         hir_ty: Option<&hir::Ty<'_>>,
     ) -> ty::PolyFnSig<'tcx> {
         let tcx = self.tcx();
-        let bound_vars = if let hir::FnRetTy::Return(ret_ty) = decl.output
-            && let hir::TyKind::InferDelegation(sig_id, _) = ret_ty.kind
-        {
-            tcx.fn_sig(sig_id).skip_binder().bound_vars()
-        } else {
-            tcx.late_bound_vars(hir_id)
-        };
+        let bound_vars = tcx.late_bound_vars(hir_id);
         debug!(?bound_vars);
 
         // We proactively collect all the inferred type params to emit a single error per fn def.

tests/ui/delegation/ice-issue-122550.rs

@@ -0,0 +1,18 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+trait Trait {
+    fn description(&self) -> &str {}
+    //~^ ERROR mismatched types
+}
+
+struct F;
+struct S(F);
+
+impl S {
+    reuse <S as Trait>::description { &self.0 }
+    //~^ ERROR mismatched types
+    //~| ERROR the trait bound `S: Trait` is not satisfied
+}
+
+fn main() {}

tests/ui/delegation/ice-issue-122550.stderr

@@ -0,0 +1,31 @@
+error[E0308]: mismatched types
+  --> $DIR/ice-issue-122550.rs:5:35
+   |
+LL |     fn description(&self) -> &str {}
+   |                                   ^^ expected `&str`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/ice-issue-122550.rs:13:39
+   |
+LL |     reuse <S as Trait>::description { &self.0 }
+   |                                       ^^^^^^^ expected `&S`, found `&F`
+   |
+   = note: expected reference `&S`
+              found reference `&F`
+
+error[E0277]: the trait bound `S: Trait` is not satisfied
+  --> $DIR/ice-issue-122550.rs:13:12
+   |
+LL |     reuse <S as Trait>::description { &self.0 }
+   |            ^ the trait `Trait` is not implemented for `S`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/ice-issue-122550.rs:4:1
+   |
+LL | trait Trait {
+   | ^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.