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 🤷‍♀️

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

Author: bors

clippy_lints/src/box_default.rs

@@ -4,12 +4,12 @@ use clippy_utils::ty::expr_sig;
 use clippy_utils::{is_default_equivalent, path_def_id};
 use rustc_errors::Applicability;
 use rustc_hir::def::Res;
-use rustc_hir::intravisit::{Visitor, walk_ty};
-use rustc_hir::{Block, Expr, ExprKind, LetStmt, Node, QPath, Ty, TyKind};
+use rustc_hir::intravisit::{InferKind, Visitor, VisitorExt, walk_ty};
+use rustc_hir::{AmbigArg, Block, Expr, ExprKind, HirId, LetStmt, Node, QPath, Ty, TyKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_session::declare_lint_pass;
-use rustc_span::sym;
+use rustc_span::{Span, sym};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -92,8 +92,13 @@ fn is_local_vec_expn(cx: &LateContext<'_>, expr: &Expr<'_>, ref_expr: &Expr<'_>)
 struct InferVisitor(bool);
 
 impl Visitor<'_> for InferVisitor {
-    fn visit_ty(&mut self, t: &Ty<'_>) {
-        self.0 |= matches!(t.kind, TyKind::Infer | TyKind::OpaqueDef(..) | TyKind::TraitObject(..));
+    fn visit_infer(&mut self, inf_id: HirId, _inf_span: Span, _kind: InferKind<'_>) -> Self::Result {
+        self.0 = true;
+        self.visit_id(inf_id);
+    }
+
+    fn visit_ty(&mut self, t: &Ty<'_, AmbigArg>) {
+        self.0 |= matches!(t.kind, TyKind::OpaqueDef(..) | TyKind::TraitObject(..));
         if !self.0 {
             walk_ty(self, t);
         }
@@ -104,7 +109,7 @@ fn given_type(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
     match cx.tcx.parent_hir_node(expr.hir_id) {
         Node::LetStmt(LetStmt { ty: Some(ty), .. }) => {
             let mut v = InferVisitor::default();
-            v.visit_ty(ty);
+            v.visit_ty_unambig(ty);
             !v.0
         },
         Node::Expr(Expr {

clippy_lints/src/casts/as_pointer_underscore.rs

@@ -4,7 +4,7 @@ use rustc_middle::ty::Ty;
 
 pub fn check<'tcx>(cx: &LateContext<'tcx>, ty_into: Ty<'_>, cast_to_hir: &'tcx rustc_hir::Ty<'tcx>) {
     if let rustc_hir::TyKind::Ptr(rustc_hir::MutTy { ty, .. }) = cast_to_hir.kind
-        && matches!(ty.kind, rustc_hir::TyKind::Infer)
+        && matches!(ty.kind, rustc_hir::TyKind::Infer(()))
     {
         clippy_utils::diagnostics::span_lint_and_sugg(
             cx,

clippy_lints/src/casts/as_underscore.rs

@@ -7,7 +7,7 @@ use rustc_middle::ty;
 use super::AS_UNDERSCORE;
 
 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, ty: &'tcx Ty<'_>) {
-    if matches!(ty.kind, TyKind::Infer) {
+    if matches!(ty.kind, TyKind::Infer(())) {
         span_lint_and_then(cx, AS_UNDERSCORE, expr.span, "using `as _` conversion", |diag| {
             let ty_resolved = cx.typeck_results().expr_ty(expr);
             if let ty::Error(_) = ty_resolved.kind() {

clippy_lints/src/casts/cast_lossless.rs

@@ -38,7 +38,7 @@ pub(super) fn check(
                 return;
             };
             match cast_to_hir.kind {
-                TyKind::Infer => {
+                TyKind::Infer(()) => {
                     diag.span_suggestion_verbose(
                         expr.span,
                         "use `Into::into` instead",

clippy_lints/src/casts/cast_ptr_alignment.rs

@@ -24,7 +24,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
             && let Some(generic_args) = method_path.args
             && let [GenericArg::Type(cast_to)] = generic_args.args
             // There probably is no obvious reason to do this, just to be consistent with `as` cases.
-            && !is_hir_ty_cfg_dependant(cx, cast_to)
+            && !is_hir_ty_cfg_dependant(cx, cast_to.as_unambig_ty())
         {
             let (cast_from, cast_to) = (cx.typeck_results().expr_ty(self_arg), cx.typeck_results().expr_ty(expr));
             lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);

clippy_lints/src/casts/ptr_as_ptr.rs

@@ -43,9 +43,9 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, msrv: &Msrv) {
     {
         let mut app = Applicability::MachineApplicable;
         let turbofish = match &cast_to_hir_ty.kind {
-            TyKind::Infer => String::new(),
+            TyKind::Infer(()) => String::new(),
             TyKind::Ptr(mut_ty) => {
-                if matches!(mut_ty.ty.kind, TyKind::Infer) {
+                if matches!(mut_ty.ty.kind, TyKind::Infer(())) {
                     String::new()
                 } else {
                     format!(

clippy_lints/src/casts/ref_as_ptr.rs

@@ -34,9 +34,9 @@ pub(super) fn check<'tcx>(
 
         let mut app = Applicability::MachineApplicable;
         let turbofish = match &cast_to_hir_ty.kind {
-            TyKind::Infer => String::new(),
+            TyKind::Infer(()) => String::new(),
             TyKind::Ptr(mut_ty) => {
-                if matches!(mut_ty.ty.kind, TyKind::Infer) {
+                if matches!(mut_ty.ty.kind, TyKind::Infer(())) {
                     String::new()
                 } else {
                     format!(

clippy_lints/src/casts/unnecessary_cast.rs

@@ -43,7 +43,7 @@ pub(super) fn check<'tcx>(
                 }
             },
             // Ignore `p as *const _`
-            TyKind::Infer => return false,
+            TyKind::Infer(()) => return false,
             _ => {},
         }
 

clippy_lints/src/casts/zero_ptr.rs

@@ -18,7 +18,7 @@ pub fn check(cx: &LateContext<'_>, expr: &Expr<'_>, from: &Expr<'_>, to: &Ty<'_>
             Mutability::Not => ("`0 as *const _` detected", "ptr::null"),
         };
 
-        let sugg = if let TyKind::Infer = mut_ty.ty.kind {
+        let sugg = if let TyKind::Infer(()) = mut_ty.ty.kind {
             format!("{std_or_core}::{sugg_fn}()")
         } else if let Some(mut_ty_snip) = mut_ty.ty.span.get_source_text(cx) {
             format!("{std_or_core}::{sugg_fn}::<{mut_ty_snip}>()")

clippy_lints/src/dereference.rs

@@ -11,10 +11,10 @@ use rustc_ast::util::parser::ExprPrecedence;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::Applicability;
 use rustc_hir::def_id::DefId;
-use rustc_hir::intravisit::{Visitor, walk_ty};
+use rustc_hir::intravisit::{InferKind, Visitor, VisitorExt, walk_ty};
 use rustc_hir::{
-    self as hir, BindingMode, Body, BodyId, BorrowKind, Expr, ExprKind, HirId, MatchSource, Mutability, Node, Pat,
-    PatKind, Path, QPath, TyKind, UnOp,
+    self as hir, AmbigArg, BindingMode, Body, BodyId, BorrowKind, Expr, ExprKind, HirId, MatchSource, Mutability, Node,
+    Pat, PatKind, Path, QPath, TyKind, UnOp,
 };
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
@@ -796,7 +796,7 @@ impl TyCoercionStability {
                     if let Some(args) = path.args
                         && args.args.iter().any(|arg| match arg {
                             hir::GenericArg::Infer(_) => true,
-                            hir::GenericArg::Type(ty) => ty_contains_infer(ty),
+                            hir::GenericArg::Type(ty) => ty_contains_infer(ty.as_unambig_ty()),
                             _ => false,
                         })
                     {
@@ -815,7 +815,7 @@ impl TyCoercionStability {
                 | TyKind::Path(_) => Self::Deref,
                 TyKind::OpaqueDef(..)
                 | TyKind::TraitAscription(..)
-                | TyKind::Infer
+                | TyKind::Infer(())
                 | TyKind::Typeof(..)
                 | TyKind::TraitObject(..)
                 | TyKind::InferDelegation(..)
@@ -889,29 +889,23 @@ impl TyCoercionStability {
 fn ty_contains_infer(ty: &hir::Ty<'_>) -> bool {
     struct V(bool);
     impl Visitor<'_> for V {
-        fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
-            if self.0
-                || matches!(
-                    ty.kind,
-                    TyKind::OpaqueDef(..) | TyKind::Infer | TyKind::Typeof(_) | TyKind::Err(_)
-                )
-            {
+        fn visit_infer(&mut self, inf_id: HirId, _inf_span: Span, kind: InferKind<'_>) -> Self::Result {
+            if let InferKind::Ty(_) | InferKind::Ambig(_) = kind {
                 self.0 = true;
-            } else {
-                walk_ty(self, ty);
             }
+            self.visit_id(inf_id);
         }
 
-        fn visit_generic_arg(&mut self, arg: &hir::GenericArg<'_>) {
-            if self.0 || matches!(arg, hir::GenericArg::Infer(_)) {
+        fn visit_ty(&mut self, ty: &hir::Ty<'_, AmbigArg>) {
+            if self.0 || matches!(ty.kind, TyKind::OpaqueDef(..) | TyKind::Typeof(_) | TyKind::Err(_)) {
                 self.0 = true;
-            } else if let hir::GenericArg::Type(ty) = arg {
-                self.visit_ty(ty);
+            } else {
+                walk_ty(self, ty);
             }
         }
     }
     let mut v = V(false);
-    v.visit_ty(ty);
+    v.visit_ty_unambig(ty);
     v.0
 }
 

clippy_lints/src/disallowed_macros.rs

@@ -6,7 +6,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Diag;
 use rustc_hir::def_id::DefIdMap;
 use rustc_hir::{
-    Expr, ExprKind, ForeignItem, HirId, ImplItem, Item, ItemKind, OwnerId, Pat, Path, Stmt, TraitItem, Ty,
+    AmbigArg, Expr, ExprKind, ForeignItem, HirId, ImplItem, Item, ItemKind, OwnerId, Pat, Path, Stmt, TraitItem, Ty,
 };
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::TyCtxt;
@@ -140,7 +140,7 @@ impl LateLintPass<'_> for DisallowedMacros {
         self.check(cx, stmt.span, None);
     }
 
-    fn check_ty(&mut self, cx: &LateContext<'_>, ty: &Ty<'_>) {
+    fn check_ty(&mut self, cx: &LateContext<'_>, ty: &Ty<'_, AmbigArg>) {
         self.check(cx, ty.span, None);
     }
 

clippy_lints/src/disallowed_types.rs

@@ -3,7 +3,7 @@ use clippy_utils::diagnostics::span_lint_and_then;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::Res;
 use rustc_hir::def_id::DefIdMap;
-use rustc_hir::{Item, ItemKind, PolyTraitRef, PrimTy, Ty, TyKind, UseKind};
+use rustc_hir::{AmbigArg, Item, ItemKind, PolyTraitRef, PrimTy, Ty, TyKind, UseKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::impl_lint_pass;
@@ -108,7 +108,7 @@ impl<'tcx> LateLintPass<'tcx> for DisallowedTypes {
         }
     }
 
-    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx>) {
+    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &'tcx Ty<'tcx, AmbigArg>) {
         if let TyKind::Path(path) = &ty.kind {
             self.check_res_emit(cx, &cx.qpath_res(path, ty.hir_id), ty.span);
         }

clippy_lints/src/eta_reduction.rs

@@ -76,22 +76,22 @@ impl<'tcx> LateLintPass<'tcx> for EtaReduction {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
         if let ExprKind::MethodCall(_method, receiver, args, _) = expr.kind {
             for arg in args {
-                check_clousure(cx, Some(receiver), arg);
+                check_closure(cx, Some(receiver), arg);
             }
         }
         if let ExprKind::Call(func, args) = expr.kind {
-            check_clousure(cx, None, func);
+            check_closure(cx, None, func);
             for arg in args {
-                check_clousure(cx, None, arg);
+                check_closure(cx, None, arg);
             }
         }
     }
 }
 
 #[allow(clippy::too_many_lines)]
-fn check_clousure<'tcx>(cx: &LateContext<'tcx>, outer_receiver: Option<&Expr<'tcx>>, expr: &Expr<'tcx>) {
+fn check_closure<'tcx>(cx: &LateContext<'tcx>, outer_receiver: Option<&Expr<'tcx>>, expr: &Expr<'tcx>) {
     let body = if let ExprKind::Closure(c) = expr.kind
-        && c.fn_decl.inputs.iter().all(|ty| matches!(ty.kind, TyKind::Infer))
+        && c.fn_decl.inputs.iter().all(|ty| matches!(ty.kind, TyKind::Infer(())))
         && matches!(c.fn_decl.output, FnRetTy::DefaultReturn(_))
         && !expr.span.from_expansion()
     {

clippy_lints/src/extra_unused_type_parameters.rs

@@ -3,10 +3,10 @@ use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_then};
 use clippy_utils::{is_from_proc_macro, trait_ref_of_method};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::Applicability;
-use rustc_hir::intravisit::{Visitor, walk_impl_item, walk_item, walk_param_bound, walk_ty};
+use rustc_hir::intravisit::{Visitor, walk_impl_item, walk_item, walk_param_bound, walk_ty, walk_unambig_ty};
 use rustc_hir::{
-    BodyId, ExprKind, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem, ImplItemKind, Item, ItemKind,
-    PredicateOrigin, Ty, WherePredicate, WherePredicateKind,
+    AmbigArg, BodyId, ExprKind, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem, ImplItemKind, Item,
+    ItemKind, PredicateOrigin, Ty, WherePredicate, WherePredicateKind,
 };
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::hir::nested_filter;
@@ -196,7 +196,7 @@ fn bound_to_trait_def_id(bound: &GenericBound<'_>) -> Option<LocalDefId> {
 impl<'tcx> Visitor<'tcx> for TypeWalker<'_, 'tcx> {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn visit_ty(&mut self, t: &'tcx Ty<'tcx>) {
+    fn visit_ty(&mut self, t: &'tcx Ty<'tcx, AmbigArg>) {
         if let Some((def_id, _)) = t.peel_refs().as_generic_param() {
             self.ty_params.remove(&def_id);
         } else {
@@ -234,7 +234,7 @@ impl<'tcx> Visitor<'tcx> for TypeWalker<'_, 'tcx> {
                 // type, any params we find nested inside of it are being used as concrete types,
                 // and can therefore can be considered used. So, we're fine to walk the left-hand
                 // side of the where bound.
-                walk_ty(self, predicate.bounded_ty);
+                walk_unambig_ty(self, predicate.bounded_ty);
             }
             for bound in predicate.bounds {
                 walk_param_bound(self, bound);

clippy_lints/src/from_over_into.rs

@@ -103,7 +103,9 @@ impl<'tcx> LateLintPass<'tcx> for FromOverInto {
                         "replace the `Into` implementation with `From<{}>`",
                         middle_trait_ref.self_ty()
                     );
-                    if let Some(suggestions) = convert_to_from(cx, into_trait_seg, target_ty, self_ty, impl_item_ref) {
+                    if let Some(suggestions) =
+                        convert_to_from(cx, into_trait_seg, target_ty.as_unambig_ty(), self_ty, impl_item_ref)
+                    {
                         diag.multipart_suggestion(message, suggestions, Applicability::MachineApplicable);
                     } else {
                         diag.help(message);

clippy_lints/src/implicit_hasher.rs

@@ -2,9 +2,8 @@ use std::borrow::Cow;
 use std::collections::BTreeMap;
 
 use rustc_errors::{Applicability, Diag};
-use rustc_hir as hir;
-use rustc_hir::intravisit::{Visitor, walk_body, walk_expr, walk_inf, walk_ty};
-use rustc_hir::{Body, Expr, ExprKind, GenericArg, Item, ItemKind, QPath, TyKind};
+use rustc_hir::intravisit::{Visitor, VisitorExt, walk_body, walk_expr, walk_ty};
+use rustc_hir::{self as hir, AmbigArg, Body, Expr, ExprKind, GenericArg, Item, ItemKind, QPath, TyKind};
 use rustc_hir_analysis::lower_ty;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
@@ -112,7 +111,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
         match item.kind {
             ItemKind::Impl(impl_) => {
                 let mut vis = ImplicitHasherTypeVisitor::new(cx);
-                vis.visit_ty(impl_.self_ty);
+                vis.visit_ty_unambig(impl_.self_ty);
 
                 for target in &vis.found {
                     if !item.span.eq_ctxt(target.span()) {
@@ -159,7 +158,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
 
                 for ty in sig.decl.inputs {
                     let mut vis = ImplicitHasherTypeVisitor::new(cx);
-                    vis.visit_ty(ty);
+                    vis.visit_ty_unambig(ty);
 
                     for target in &vis.found {
                         if generics.span.from_expansion() {
@@ -287,21 +286,13 @@ impl<'a, 'tcx> ImplicitHasherTypeVisitor<'a, 'tcx> {
 }
 
 impl<'tcx> Visitor<'tcx> for ImplicitHasherTypeVisitor<'_, 'tcx> {
-    fn visit_ty(&mut self, t: &'tcx hir::Ty<'_>) {
-        if let Some(target) = ImplicitHasherType::new(self.cx, t) {
+    fn visit_ty(&mut self, t: &'tcx hir::Ty<'_, AmbigArg>) {
+        if let Some(target) = ImplicitHasherType::new(self.cx, t.as_unambig_ty()) {
             self.found.push(target);
         }
 
         walk_ty(self, t);
     }
-
-    fn visit_infer(&mut self, inf: &'tcx hir::InferArg) {
-        if let Some(target) = ImplicitHasherType::new(self.cx, &inf.to_ty()) {
-            self.found.push(target);
-        }
-
-        walk_inf(self, inf);
-    }
 }
 
 /// Looks for default-hasher-dependent constructors like `HashMap::new`.

clippy_lints/src/implied_bounds_in_impls.rs

@@ -3,8 +3,8 @@ use clippy_utils::source::snippet;
 use rustc_errors::{Applicability, SuggestionStyle};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{
-    AssocItemConstraint, GenericArg, GenericBound, GenericBounds, PredicateOrigin, TraitBoundModifiers, TyKind,
-    WherePredicateKind,
+    AmbigArg, AssocItemConstraint, GenericArg, GenericBound, GenericBounds, PredicateOrigin, TraitBoundModifiers,
+    TyKind, WherePredicateKind,
 };
 use rustc_hir_analysis::lower_ty;
 use rustc_lint::{LateContext, LateLintPass};
@@ -146,7 +146,9 @@ fn try_resolve_type<'tcx>(
     index: usize,
 ) -> Option<Ty<'tcx>> {
     match args.get(index - 1) {
-        Some(GenericArg::Type(ty)) => Some(lower_ty(tcx, ty)),
+        // I don't think we care about `GenericArg::Infer` since this is all for stuff in type signatures
+        // which do not permit inference variables.
+        Some(GenericArg::Type(ty)) => Some(lower_ty(tcx, ty.as_unambig_ty())),
         Some(_) => None,
         None => Some(tcx.type_of(generics.own_params[index].def_id).skip_binder()),
     }
@@ -335,7 +337,7 @@ impl<'tcx> LateLintPass<'tcx> for ImpliedBoundsInImpls {
         }
     }
 
-    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &rustc_hir::Ty<'tcx>) {
+    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &rustc_hir::Ty<'tcx, AmbigArg>) {
         if let TyKind::OpaqueDef(opaque_ty, ..) = ty.kind {
             check(cx, opaque_ty.bounds);
         }

clippy_lints/src/let_with_type_underscore.rs

@@ -28,7 +28,7 @@ declare_lint_pass!(UnderscoreTyped => [LET_WITH_TYPE_UNDERSCORE]);
 impl<'tcx> LateLintPass<'tcx> for UnderscoreTyped {
     fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx LetStmt<'_>) {
         if let Some(ty) = local.ty // Ensure that it has a type defined
-            && let TyKind::Infer = &ty.kind // that type is '_'
+            && let TyKind::Infer(()) = &ty.kind // that type is '_'
             && local.span.eq_ctxt(ty.span)
             && !in_external_macro(cx.tcx.sess, local.span)
             && !is_from_proc_macro(cx, ty)