Auto merge of rust-lang#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 rust-lang#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
 }