Overhaul TyS and Ty.

Specifically, change `Ty` from this:
```
pub type Ty<'tcx> = &'tcx TyS<'tcx>;
```
to this
```
pub struct Ty<'tcx>(Interned<'tcx, TyS<'tcx>>);
```
There are two benefits to this.
- It's now a first class type, so we can define methods on it. This
  means we can move a lot of methods away from `TyS`, leaving `TyS` as a
  barely-used type, which is appropriate given that it's not meant to
  be used directly.
- The uniqueness requirement is now explicit, via the `Interned` type.
  E.g. the pointer-based `Eq` and `Hash` comes from `Interned`, rather
  than via `TyS`, which wasn't obvious at all.

Much of this commit is boring churn. The interesting changes are in
these files:
- compiler/rustc_middle/src/arena.rs
- compiler/rustc_middle/src/mir/visit.rs
- compiler/rustc_middle/src/ty/context.rs
- compiler/rustc_middle/src/ty/mod.rs

Specifically:
- Most mentions of `TyS` are removed. It's very much a dumb struct now;
  `Ty` has all the smarts.
- `TyS` now has `crate` visibility instead of `pub`.
- `TyS::make_for_test` is removed in favour of the static `BOOL_TY`,
  which just works better with the new structure.
- The `Eq`/`Ord`/`Hash` impls are removed from `TyS`. `Interned`s impls
  of `Eq`/`Hash` now suffice. `Ord` is now partly on `Interned`
  (pointer-based, for the `Equal` case) and partly on `TyS`
  (contents-based, for the other cases).
- There are many tedious sigil adjustments, i.e. adding or removing `*`
  or `&`. They seem to be unavoidable.

Author: nnethercote

clippy_lints/src/case_sensitive_file_extension_comparisons.rs

@@ -47,7 +47,7 @@ fn check_case_sensitive_file_extension_comparison(ctx: &LateContext<'_>, expr: &
         then {
             let mut ty = ctx.typeck_results().expr_ty(obj);
             ty = match ty.kind() {
-                ty::Ref(_, ty, ..) => ty,
+                ty::Ref(_, ty, ..) => *ty,
                 _ => ty
             };
 

clippy_lints/src/default_numeric_fallback.rs

@@ -123,7 +123,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                 if let Some(fn_sig) = fn_sig_opt(self.cx, func.hir_id) {
                     for (expr, bound) in iter::zip(*args, fn_sig.skip_binder().inputs()) {
                         // Push found arg type, then visit arg.
-                        self.ty_bounds.push(TyBound::Ty(bound));
+                        self.ty_bounds.push(TyBound::Ty(*bound));
                         self.visit_expr(expr);
                         self.ty_bounds.pop();
                     }
@@ -135,7 +135,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                 if let Some(def_id) = self.cx.typeck_results().type_dependent_def_id(expr.hir_id) {
                     let fn_sig = self.cx.tcx.fn_sig(def_id).skip_binder();
                     for (expr, bound) in iter::zip(*args, fn_sig.inputs()) {
-                        self.ty_bounds.push(TyBound::Ty(bound));
+                        self.ty_bounds.push(TyBound::Ty(*bound));
                         self.visit_expr(expr);
                         self.ty_bounds.pop();
                     }
@@ -210,7 +210,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
 
 fn fn_sig_opt<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId) -> Option<PolyFnSig<'tcx>> {
     let node_ty = cx.typeck_results().node_type_opt(hir_id)?;
-    // We can't use `TyS::fn_sig` because it automatically performs substs, this may result in FNs.
+    // We can't use `Ty::fn_sig` because it automatically performs substs, this may result in FNs.
     match node_ty.kind() {
         ty::FnDef(def_id, _) => Some(cx.tcx.fn_sig(*def_id)),
         ty::FnPtr(fn_sig) => Some(*fn_sig),

clippy_lints/src/eq_op.rs

@@ -7,10 +7,10 @@ use clippy_utils::{ast_utils::is_useless_with_eq_exprs, eq_expr_value, is_in_tes
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir::{
-    def::Res, def_id::DefId, BinOpKind, BorrowKind, Expr, ExprKind, GenericArg, ItemKind, QPath, Ty, TyKind,
+    def::Res, def_id::DefId, BinOpKind, BorrowKind, Expr, ExprKind, GenericArg, ItemKind, QPath, TyKind,
 };
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty::{self, TyS};
+use rustc_middle::ty::{self, Ty};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
 declare_clippy_lint! {
@@ -279,7 +279,7 @@ impl<'tcx> LateLintPass<'tcx> for EqOp {
     }
 }
 
-fn in_impl<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, bin_op: DefId) -> Option<(&'tcx Ty<'tcx>, &'tcx Ty<'tcx>)> {
+fn in_impl<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, bin_op: DefId) -> Option<(&'tcx rustc_hir::Ty<'tcx>, &'tcx rustc_hir::Ty<'tcx>)> {
     if_chain! {
         if let Some(block) = get_enclosing_block(cx, e.hir_id);
         if let Some(impl_def_id) = cx.tcx.impl_of_method(block.hir_id.owner.to_def_id());
@@ -301,7 +301,7 @@ fn in_impl<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, bin_op: DefId) -> Op
     }
 }
 
-fn are_equal<'tcx>(cx: &LateContext<'tcx>, middle_ty: &TyS<'_>, hir_ty: &Ty<'_>) -> bool {
+fn are_equal<'tcx>(cx: &LateContext<'tcx>, middle_ty: Ty<'_>, hir_ty: &rustc_hir::Ty<'_>) -> bool {
     if_chain! {
         if let ty::Adt(adt_def, _) = middle_ty.kind();
         if let Some(local_did) = adt_def.did.as_local();

clippy_lints/src/format_args.rs

@@ -211,7 +211,7 @@ where
             if overloaded_deref.is_some() {
                 n_needed = n_total;
             }
-            ty = target;
+            ty = *target;
         } else {
             return (n_needed, ty);
         }

clippy_lints/src/index_refutable_slice.rs

@@ -118,7 +118,7 @@ fn find_slice_values(cx: &LateContext<'_>, pat: &hir::Pat<'_>) -> FxIndexMap<hir
                 // The values need to use the `ref` keyword if they can't be copied.
                 // This will need to be adjusted if the lint want to support multable access in the future
                 let src_is_ref = bound_ty.is_ref() && binding != hir::BindingAnnotation::Ref;
-                let needs_ref = !(src_is_ref || is_copy(cx, inner_ty));
+                let needs_ref = !(src_is_ref || is_copy(cx, *inner_ty));
 
                 let slice_info = slices
                     .entry(value_hir_id)

clippy_lints/src/large_const_arrays.rs

@@ -55,7 +55,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeConstArrays {
             if let ty::Array(element_type, cst) = ty.kind();
             if let ConstKind::Value(ConstValue::Scalar(element_count)) = cst.val;
             if let Ok(element_count) = element_count.to_machine_usize(&cx.tcx);
-            if let Ok(element_size) = cx.layout_of(element_type).map(|l| l.size.bytes());
+            if let Ok(element_size) = cx.layout_of(*element_type).map(|l| l.size.bytes());
             if self.maximum_allowed_size < element_count * element_size;
 
             then {

clippy_lints/src/large_stack_arrays.rs

@@ -45,7 +45,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeStackArrays {
             if let ty::Array(element_type, cst) = cx.typeck_results().expr_ty(expr).kind();
             if let ConstKind::Value(ConstValue::Scalar(element_count)) = cst.val;
             if let Ok(element_count) = element_count.to_machine_usize(&cx.tcx);
-            if let Ok(element_size) = cx.layout_of(element_type).map(|l| l.size.bytes());
+            if let Ok(element_size) = cx.layout_of(*element_type).map(|l| l.size.bytes());
             if self.maximum_allowed_size < element_count * element_size;
             then {
                 span_lint_and_help(

clippy_lints/src/len_zero.rs

@@ -294,7 +294,7 @@ impl LenOutput<'_> {
 /// Checks if the given signature matches the expectations for `is_empty`
 fn check_is_empty_sig(sig: FnSig<'_>, self_kind: ImplicitSelfKind, len_output: LenOutput<'_>) -> bool {
     match &**sig.inputs_and_output {
-        [arg, res] if len_output.matches_is_empty_output(res) => {
+        [arg, res] if len_output.matches_is_empty_output(*res) => {
             matches!(
                 (arg.kind(), self_kind),
                 (ty::Ref(_, _, Mutability::Not), ImplicitSelfKind::ImmRef)

clippy_lints/src/loops/explicit_counter_loop.rs

@@ -7,7 +7,7 @@ use rustc_errors::Applicability;
 use rustc_hir::intravisit::{walk_block, walk_expr};
 use rustc_hir::{Expr, Pat};
 use rustc_lint::LateContext;
-use rustc_middle::ty::{self, UintTy};
+use rustc_middle::ty::{self, Ty, UintTy};
 
 // To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
 // incremented exactly once in the loop body, and initialized to zero
@@ -36,7 +36,7 @@ pub(super) fn check<'tcx>(
                 then {
                     let mut applicability = Applicability::MachineApplicable;
 
-                    let int_name = match ty.map(ty::TyS::kind) {
+                    let int_name = match ty.map(Ty::kind) {
                         // usize or inferred
                         Some(ty::Uint(UintTy::Usize)) | None => {
                             span_lint_and_sugg(

clippy_lints/src/loops/manual_memcpy.rs

@@ -335,8 +335,8 @@ struct Start<'hir> {
 fn get_slice_like_element_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
     match ty.kind() {
         ty::Adt(adt, subs) if cx.tcx.is_diagnostic_item(sym::Vec, adt.did) => Some(subs.type_at(0)),
-        ty::Ref(_, subty, _) => get_slice_like_element_ty(cx, subty),
-        ty::Slice(ty) | ty::Array(ty, _) => Some(ty),
+        ty::Ref(_, subty, _) => get_slice_like_element_ty(cx, *subty),
+        ty::Slice(ty) | ty::Array(ty, _) => Some(*ty),
         _ => None,
     }
 }