Overhaul Const.

Specifically, rename the `Const` struct as `ConstS` and re-introduce `Const` as
this:
```
pub struct Const<'tcx>(&'tcx Interned<ConstS>);
```
This now matches `Ty` and `Predicate` more closely, including using
pointer-based `eq` and `hash`.

Notable changes:
- `mk_const` now takes a `ConstS`.
- `Const` was copy, despite being 48 bytes. Now `ConstS` is not, so need a
  we need separate arena for it, because we can't use the `Dropless` one any
  more.
- Many `&'tcx Const<'tcx>`/`&Const<'tcx>` to `Const<'tcx>` changes
- Many `ct.ty` to `ct.ty()` and `ct.val` to `ct.val()` changes.
- Lots of tedious sigil fiddling.

Author: nnethercote

clippy_lints/src/large_const_arrays.rs

@@ -53,7 +53,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeConstArrays {
             if let ItemKind::Const(hir_ty, _) = &item.kind;
             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
             if let ty::Array(element_type, cst) = ty.kind();
-            if let ConstKind::Value(ConstValue::Scalar(element_count)) = cst.val;
+            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 self.maximum_allowed_size < element_count * element_size;

clippy_lints/src/large_stack_arrays.rs

@@ -43,7 +43,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeStackArrays {
         if_chain! {
             if let ExprKind::Repeat(_, _) = expr.kind;
             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 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 self.maximum_allowed_size < element_count * element_size;

clippy_lints/src/non_copy_const.rs

@@ -136,14 +136,14 @@ fn is_value_unfrozen_raw<'tcx>(
     result: Result<ConstValue<'tcx>, ErrorHandled>,
     ty: Ty<'tcx>,
 ) -> bool {
-    fn inner<'tcx>(cx: &LateContext<'tcx>, val: &'tcx Const<'tcx>) -> bool {
-        match val.ty.kind() {
+    fn inner<'tcx>(cx: &LateContext<'tcx>, val: Const<'tcx>) -> bool {
+        match val.ty().kind() {
             // the fact that we have to dig into every structs to search enums
             // leads us to the point checking `UnsafeCell` directly is the only option.
             ty::Adt(ty_def, ..) if Some(ty_def.did) == cx.tcx.lang_items().unsafe_cell_type() => true,
             ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
                 let val = cx.tcx.destructure_const(cx.param_env.and(val));
-                val.fields.iter().any(|field| inner(cx, field))
+                val.fields.iter().any(|field| inner(cx, *field))
             },
             _ => false,
         }

clippy_utils/src/consts.rs

@@ -567,11 +567,11 @@ impl<'a, 'tcx> ConstEvalLateContext<'a, 'tcx> {
     }
 }
 
-pub fn miri_to_const(result: &ty::Const<'_>) -> Option<Constant> {
+pub fn miri_to_const(result: ty::Const<'_>) -> Option<Constant> {
     use rustc_middle::mir::interpret::ConstValue;
-    match result.val {
+    match result.val() {
         ty::ConstKind::Value(ConstValue::Scalar(Scalar::Int(int))) => {
-            match result.ty.kind() {
+            match result.ty().kind() {
                 ty::Bool => Some(Constant::Bool(int == ScalarInt::TRUE)),
                 ty::Uint(_) | ty::Int(_) => Some(Constant::Int(int.assert_bits(int.size()))),
                 ty::Float(FloatTy::F32) => Some(Constant::F32(f32::from_bits(
@@ -590,7 +590,7 @@ pub fn miri_to_const(result: &ty::Const<'_>) -> Option<Constant> {
                 _ => None,
             }
         },
-        ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => match result.ty.kind() {
+        ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => match result.ty().kind() {
             ty::Ref(_, tam, _) => match tam.kind() {
                 ty::Str => String::from_utf8(
                     data.inspect_with_uninit_and_ptr_outside_interpreter(start..end)
@@ -602,9 +602,9 @@ pub fn miri_to_const(result: &ty::Const<'_>) -> Option<Constant> {
             },
             _ => None,
         },
-        ty::ConstKind::Value(ConstValue::ByRef { alloc, offset: _ }) => match result.ty.kind() {
+        ty::ConstKind::Value(ConstValue::ByRef { alloc, offset: _ }) => match result.ty().kind() {
             ty::Array(sub_type, len) => match sub_type.kind() {
-                ty::Float(FloatTy::F32) => match miri_to_const(len) {
+                ty::Float(FloatTy::F32) => match miri_to_const(*len) {
                     Some(Constant::Int(len)) => alloc
                         .inspect_with_uninit_and_ptr_outside_interpreter(0..(4 * len as usize))
                         .to_owned()
@@ -618,7 +618,7 @@ pub fn miri_to_const(result: &ty::Const<'_>) -> Option<Constant> {
                         .map(Constant::Vec),
                     _ => None,
                 },
-                ty::Float(FloatTy::F64) => match miri_to_const(len) {
+                ty::Float(FloatTy::F64) => match miri_to_const(*len) {
                     Some(Constant::Int(len)) => alloc
                         .inspect_with_uninit_and_ptr_outside_interpreter(0..(8 * len as usize))
                         .to_owned()