Auto merge of #135272 - BoxyUwU:generic_arg_infer_reliability_2, r=compiler-errors

Forbid usage of `hir` `Infer` const/ty variants in ambiguous contexts

The feature `generic_arg_infer` allows providing `_` as an argument to const generics in order to infer them. This introduces a syntactic ambiguity as to whether generic arguments are type or const arguments. In order to get around this we introduced a fourth `GenericArg` variant, `Infer` used to represent `_` as an argument to generic parameters when we don't know if its a type or a const argument.

This made hir visitors that care about `TyKind::Infer` or `ConstArgKind::Infer` very error prone as checking for `TyKind::Infer`s in  `visit_ty` would find *some* type infer arguments but not *all* of them as they would sometimes be lowered to `GenericArg::Infer` instead.

Additionally the `visit_infer` method would previously only visit `GenericArg::Infer` not *all* infers (e.g. `TyKind::Infer`), this made it very easy to override `visit_infer` and expect it to visit all infers when in reality it would only visit *some* infers.

---

This PR aims to fix those issues by making the `TyKind` and `ConstArgKind` types generic over whether the infer types/consts are represented by `Ty/ConstArgKind::Infer` or out of line (e.g. by a `GenericArg::Infer` or accessible by overiding `visit_infer`). We then make HIR Visitors convert all const args and types to the versions where infer vars are stored out of line and call `visit_infer` in cases where a `Ty`/`Const` would previously have had a `Ty/ConstArgKind::Infer` variant:

API Summary
```rust
enum AmbigArg {}

enum Ty/ConstArgKind<Unambig = ()> {
   ...
   Infer(Unambig),
}

impl Ty/ConstArg {
  fn try_as_ambig_ty/ct(self) -> Option<Ty/ConstArg<AmbigArg>>;
}
impl Ty/ConstArg<AmbigArg> {
  fn as_unambig_ty/ct(self) -> Ty/ConstArg;
}

enum InferKind {
  Ty(Ty),
  Const(ConstArg),
  Ambig(InferArg),
}

trait Visitor {
  ...
  fn visit_ty/const_arg(&mut self, Ty/ConstArg<AmbigArg>) -> Self::Result;
  fn visit_infer(&mut self, id: HirId, sp: Span, kind: InferKind) -> Self::Result;
}

// blanket impl'd, not meant to be overriden
trait VisitorExt {
  fn visit_ty/const_arg_unambig(&mut self, Ty/ConstArg) -> Self::Result;
}

fn walk_unambig_ty/const_arg(&mut V, Ty/ConstArg) -> Self::Result;
fn walk_ty/const_arg(&mut V, Ty/ConstArg<AmbigArg>) -> Self::Result;
```

The end result is that `visit_infer` visits *all* infer args and is also the *only* way to visit an infer arg, `visit_ty` and `visit_const_arg` can now no longer encounter a `Ty/ConstArgKind::Infer`. Representing this in the type system means that it is now very difficult to mess things up, either accessing `TyKind::Infer` "just works" and you won't miss *some* type infers- or it doesn't work and you have to look at `visit_infer` or some `GenericArg::Infer` which forces you to think about the full complexity involved.

Unfortunately there is no lint right now about explicitly matching on uninhabited variants, I can't find the context for why this is the case :woman_shrugging:

I'm not convinced the framing of un/ambig ty/consts is necessarily the right one but I'm not sure what would be better. I somewhat like calling them full/partial types based on the fact that `Ty<Partial>`/`Ty<Full>` directly specifies how many of the type kinds are actually represented compared to `Ty<Ambig>` which which leaves that to the reader to figure out based on the logical consequences of it the type being in an ambiguous position.

---

tool changes have been modified in their own commits for easier reviewing by anyone getting cc'd from subtree changes. I also attempted to split out "bug fixes arising from the refactoring" into their own commit so they arent lumped in with a big general refactor commit

Fixes #112110

9 files changed, 47 insertions, 49 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs
index 2a487a48a8e..b9f3abc2534 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs
@@ -1,8 +1,8 @@
 use core::ops::ControlFlow;
 
-use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::intravisit::{self, Visitor, VisitorExt};
+use rustc_hir::{self as hir, AmbigArg};
 use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::resolve_bound_vars as rbv;
@@ -48,7 +48,7 @@ fn find_component_for_bound_region<'tcx>(
     region_def_id: DefId,
 ) -> Option<&'tcx hir::Ty<'tcx>> {
     FindNestedTypeVisitor { tcx, region_def_id, current_index: ty::INNERMOST }
-        .visit_ty(arg)
+        .visit_ty_unambig(arg)
         .break_value()
 }
 
@@ -74,7 +74,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
         self.tcx.hir()
     }
 
-    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) -> Self::Result {
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx, AmbigArg>) -> Self::Result {
         match arg.kind {
             hir::TyKind::BareFn(_) => {
                 self.current_index.shift_in(1);
@@ -101,7 +101,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                     Some(rbv::ResolvedArg::EarlyBound(id)) => {
                         debug!("EarlyBound id={:?}", id);
                         if id.to_def_id() == self.region_def_id {
-                            return ControlFlow::Break(arg);
+                            return ControlFlow::Break(arg.as_unambig_ty());
                         }
                     }
 
@@ -117,7 +117,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                         if debruijn_index == self.current_index
                             && id.to_def_id() == self.region_def_id
                         {
-                            return ControlFlow::Break(arg);
+                            return ControlFlow::Break(arg.as_unambig_ty());
                         }
                     }
 
@@ -147,7 +147,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                 )
                 .is_break()
                 {
-                    ControlFlow::Break(arg)
+                    ControlFlow::Break(arg.as_unambig_ty())
                 } else {
                     ControlFlow::Continue(())
                 };
@@ -210,7 +210,7 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
         ControlFlow::Continue(())
     }
 
-    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) -> Self::Result {
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx, AmbigArg>) -> Self::Result {
         // ignore nested types
         //
         // If you have a type like `Foo<'a, &Ty>` we
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/mismatched_static_lifetime.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/mismatched_static_lifetime.rs
index 54d8a9e25ca..886581bc35f 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/mismatched_static_lifetime.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/mismatched_static_lifetime.rs
@@ -4,7 +4,7 @@
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{ErrorGuaranteed, MultiSpan};
 use rustc_hir as hir;
-use rustc_hir::intravisit::Visitor;
+use rustc_hir::intravisit::VisitorExt;
 use rustc_middle::bug;
 use rustc_middle::ty::TypeVisitor;
 use tracing::debug;
@@ -87,7 +87,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             for matching_def_id in v.0 {
                 let mut hir_v =
                     super::static_impl_trait::HirTraitObjectVisitor(&mut traits, matching_def_id);
-                hir_v.visit_ty(impl_self_ty);
+                hir_v.visit_ty_unambig(impl_self_ty);
             }
 
             if traits.is_empty() {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
index 503090b5797..dfbef39e9e1 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs
@@ -3,9 +3,9 @@
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Applicability, Diag, ErrorGuaranteed, MultiSpan, Subdiagnostic};
 use rustc_hir::def_id::DefId;
-use rustc_hir::intravisit::{Visitor, walk_ty};
+use rustc_hir::intravisit::{Visitor, VisitorExt, walk_ty};
 use rustc_hir::{
-    self as hir, GenericBound, GenericParam, GenericParamKind, Item, ItemKind, Lifetime,
+    self as hir, AmbigArg, GenericBound, GenericParam, GenericParamKind, Item, ItemKind, Lifetime,
     LifetimeName, LifetimeParamKind, MissingLifetimeKind, Node, TyKind,
 };
 use rustc_middle::ty::{
@@ -153,7 +153,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                     let mut add_label = true;
                     if let hir::FnRetTy::Return(ty) = fn_decl.output {
                         let mut v = StaticLifetimeVisitor(vec![], tcx.hir());
-                        v.visit_ty(ty);
+                        v.visit_ty_unambig(ty);
                         if !v.0.is_empty() {
                             span = v.0.clone().into();
                             spans = v.0;
@@ -374,7 +374,7 @@ pub fn suggest_new_region_bound(
                     }
                 }
             }
-            TyKind::TraitObject(_, lt, _) => {
+            TyKind::TraitObject(_, lt) => {
                 if let LifetimeName::ImplicitObjectLifetimeDefault = lt.res {
                     err.span_suggestion_verbose(
                         fn_return.span.shrink_to_hi(),
@@ -500,7 +500,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                             // In that case, only the first one will get suggestions.
                             let mut traits = vec![];
                             let mut hir_v = HirTraitObjectVisitor(&mut traits, *did);
-                            hir_v.visit_ty(self_ty);
+                            hir_v.visit_ty_unambig(self_ty);
                             !traits.is_empty()
                         })
                     {
@@ -560,7 +560,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         for found_did in found_dids {
             let mut traits = vec![];
             let mut hir_v = HirTraitObjectVisitor(&mut traits, *found_did);
-            hir_v.visit_ty(self_ty);
+            hir_v.visit_ty_unambig(self_ty);
             for &span in &traits {
                 let subdiag = DynTraitConstraintSuggestion { span, ident };
                 subdiag.add_to_diag(err);
@@ -591,12 +591,10 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for TraitObjectVisitor {
 pub struct HirTraitObjectVisitor<'a>(pub &'a mut Vec<Span>, pub DefId);
 
 impl<'a, 'tcx> Visitor<'tcx> for HirTraitObjectVisitor<'a> {
-    fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx>) {
-        if let TyKind::TraitObject(
-            poly_trait_refs,
-            Lifetime { res: LifetimeName::ImplicitObjectLifetimeDefault, .. },
-            _,
-        ) = t.kind
+    fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx, AmbigArg>) {
+        if let TyKind::TraitObject(poly_trait_refs, lifetime_ptr) = t.kind
+            && let Lifetime { res: LifetimeName::ImplicitObjectLifetimeDefault, .. } =
+                lifetime_ptr.pointer()
         {
             for ptr in poly_trait_refs {
                 if Some(self.1) == ptr.trait_ref.trait_def_id() {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
index 95dd1b28a39..bbcd28c0835 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs
@@ -1,10 +1,10 @@
 //! Error Reporting for `impl` items that do not match the obligations from their `trait`.
 
 use rustc_errors::ErrorGuaranteed;
-use rustc_hir as hir;
 use rustc_hir::def::{Namespace, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::intravisit::Visitor;
+use rustc_hir::intravisit::{Visitor, walk_ty};
+use rustc_hir::{self as hir, AmbigArg};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::error::ExpectedFound;
@@ -137,11 +137,13 @@ impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
         self.tcx.hir()
     }
 
-    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx, AmbigArg>) {
         match arg.kind {
             hir::TyKind::Ref(_, ref mut_ty) => {
                 // We don't want to suggest looking into borrowing `&T` or `&Self`.
-                hir::intravisit::walk_ty(self, mut_ty.ty);
+                if let Some(ambig_ty) = mut_ty.ty.try_as_ambig_ty() {
+                    walk_ty(self, ambig_ty);
+                }
                 return;
             }
             hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs
index 21124cf20e5..af7e56961b7 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs
@@ -655,7 +655,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     && let ty::Ref(found_region, _, _) = found.kind()
                     && expected_region.is_bound()
                     && !found_region.is_bound()
-                    && let hir::TyKind::Infer = arg_hir.kind
+                    && let hir::TyKind::Infer(()) = arg_hir.kind
                 {
                     // If the expected region is late bound, the found region is not, and users are asking compiler
                     // to infer the type, we can suggest adding `: &_`.
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index bcd3b0109b7..961719f263c 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -580,8 +580,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             self.tcx.hir_node_by_def_id(obligation.cause.body_id)
                             && let hir::ItemKind::Impl(impl_) = item.kind
                             && let None = impl_.of_trait
-                            && let hir::TyKind::TraitObject(_, _, syntax) = impl_.self_ty.kind
-                            && let TraitObjectSyntax::None = syntax
+                            && let hir::TyKind::TraitObject(_, tagged_ptr) = impl_.self_ty.kind
+                            && let TraitObjectSyntax::None = tagged_ptr.tag()
                             && impl_.self_ty.span.edition().at_least_rust_2021()
                         {
                             // Silence the dyn-compatibility error in favor of the missing dyn on
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
index f2bcc51e687..8111983c539 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
@@ -11,7 +11,7 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{Applicability, Diag, E0038, E0276, MultiSpan, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
-use rustc_hir::{self as hir, LangItem};
+use rustc_hir::{self as hir, AmbigArg, LangItem};
 use rustc_infer::traits::{
     DynCompatibilityViolation, Obligation, ObligationCause, ObligationCauseCode,
     PredicateObligation, SelectionError,
@@ -87,9 +87,9 @@ impl<'v> Visitor<'v> for FindExprBySpan<'v> {
         }
     }
 
-    fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
+    fn visit_ty(&mut self, ty: &'v hir::Ty<'v, AmbigArg>) {
         if self.span == ty.span {
-            self.ty_result = Some(ty);
+            self.ty_result = Some(ty.as_unambig_ty());
         } else {
             hir::intravisit::walk_ty(self, ty);
         }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 4669d286665..471105773e2 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -14,14 +14,13 @@ use rustc_errors::{
     Applicability, Diag, EmissionGuarantee, MultiSpan, Style, SuggestionStyle, pluralize,
     struct_span_code_err,
 };
-use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::intravisit::Visitor;
+use rustc_hir::intravisit::{Visitor, VisitorExt};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{
-    CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, HirId, Node, expr_needs_parens,
-    is_range_literal,
+    self as hir, AmbigArg, CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, HirId, Node,
+    expr_needs_parens, is_range_literal,
 };
 use rustc_infer::infer::{BoundRegionConversionTime, DefineOpaqueTypes, InferCtxt, InferOk};
 use rustc_middle::hir::map;
@@ -179,7 +178,7 @@ pub fn suggest_restriction<'tcx, G: EmissionGuarantee>(
         let mut ty_spans = vec![];
         for input in fn_sig.decl.inputs {
             ReplaceImplTraitVisitor { ty_spans: &mut ty_spans, param_did: param.def_id }
-                .visit_ty(input);
+                .visit_ty_unambig(input);
         }
         // The type param `T: Trait` we will suggest to introduce.
         let type_param = format!("{type_param_name}: {bound_str}");
@@ -3074,7 +3073,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 }
                 if let Some(ty) = ty {
                     match ty.kind {
-                        hir::TyKind::TraitObject(traits, _, _) => {
+                        hir::TyKind::TraitObject(traits, _) => {
                             let (span, kw) = match traits {
                                 [first, ..] if first.span.lo() == ty.span.lo() => {
                                     // Missing `dyn` in front of trait object.
@@ -5065,7 +5064,7 @@ pub struct SelfVisitor<'v> {
 }
 
 impl<'v> Visitor<'v> for SelfVisitor<'v> {
-    fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
+    fn visit_ty(&mut self, ty: &'v hir::Ty<'v, AmbigArg>) {
         if let hir::TyKind::Path(path) = ty.kind
             && let hir::QPath::TypeRelative(inner_ty, segment) = path
             && (Some(segment.ident.name) == self.name || self.name.is_none())
@@ -5073,7 +5072,7 @@ impl<'v> Visitor<'v> for SelfVisitor<'v> {
             && let hir::QPath::Resolved(None, inner_path) = inner_path
             && let Res::SelfTyAlias { .. } = inner_path.res
         {
-            self.paths.push(ty);
+            self.paths.push(ty.as_unambig_ty());
         }
         hir::intravisit::walk_ty(self, ty);
     }
@@ -5187,7 +5186,7 @@ struct ReplaceImplTraitVisitor<'a> {
 }
 
 impl<'a, 'hir> hir::intravisit::Visitor<'hir> for ReplaceImplTraitVisitor<'a> {
-    fn visit_ty(&mut self, t: &'hir hir::Ty<'hir>) {
+    fn visit_ty(&mut self, t: &'hir hir::Ty<'hir, AmbigArg>) {
         if let hir::TyKind::Path(hir::QPath::Resolved(
             None,
             hir::Path { res: Res::Def(_, segment_did), .. },
@@ -5480,7 +5479,7 @@ impl<'v> Visitor<'v> for FindTypeParam {
         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
     }
 
-    fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
+    fn visit_ty(&mut self, ty: &hir::Ty<'_, AmbigArg>) {
         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
index 0bf91ad35c1..2dfa72972ba 100644
--- a/compiler/rustc_trait_selection/src/errors.rs
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -6,11 +6,10 @@ use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, DiagMessage, DiagStyledString, Diagnostic,
     EmissionGuarantee, IntoDiagArg, Level, MultiSpan, SubdiagMessageOp, Subdiagnostic,
 };
-use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::intravisit::{Visitor, walk_ty};
-use rustc_hir::{FnRetTy, GenericParamKind, Node};
+use rustc_hir::intravisit::{Visitor, VisitorExt, walk_ty};
+use rustc_hir::{self as hir, AmbigArg, FnRetTy, GenericParamKind, Node};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_middle::ty::print::{PrintTraitRefExt as _, TraitRefPrintOnlyTraitPath};
 use rustc_middle::ty::{self, Binder, ClosureKind, FnSig, PolyTraitRef, Region, Ty, TyCtxt};
@@ -579,7 +578,7 @@ impl Subdiagnostic for AddLifetimeParamsSuggestion<'_> {
             }
 
             impl<'v> Visitor<'v> for ImplicitLifetimeFinder {
-                fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
+                fn visit_ty(&mut self, ty: &'v hir::Ty<'v, AmbigArg>) {
                     let make_suggestion = |ident: Ident| {
                         if ident.name == kw::Empty && ident.span.is_empty() {
                             format!("{}, ", self.suggestion_param_name)
@@ -642,16 +641,16 @@ impl Subdiagnostic for AddLifetimeParamsSuggestion<'_> {
             if let Some(fn_decl) = node.fn_decl()
                 && let hir::FnRetTy::Return(ty) = fn_decl.output
             {
-                visitor.visit_ty(ty);
+                visitor.visit_ty_unambig(ty);
             }
             if visitor.suggestions.is_empty() {
                 // Do not suggest constraining the `&self` param, but rather the return type.
                 // If that is wrong (because it is not sufficient), a follow up error will tell the
                 // user to fix it. This way we lower the chances of *over* constraining, but still
                 // get the cake of "correctly" contrained in two steps.
-                visitor.visit_ty(self.ty_sup);
+                visitor.visit_ty_unambig(self.ty_sup);
             }
-            visitor.visit_ty(self.ty_sub);
+            visitor.visit_ty_unambig(self.ty_sub);
             if visitor.suggestions.is_empty() {
                 return false;
             }