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.

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -146,7 +146,7 @@ fn push_debuginfo_type_name<'tcx>(
             if cpp_like_debuginfo {
                 output.push_str("array$<");
                 push_debuginfo_type_name(tcx, inner_type, true, output, visited);
-                match len.val {
+                match len.val() {
                     ty::ConstKind::Param(param) => write!(output, ",{}>", param.name).unwrap(),
                     _ => write!(output, ",{}>", len.eval_usize(tcx, ty::ParamEnv::reveal_all()))
                         .unwrap(),
@@ -154,7 +154,7 @@ fn push_debuginfo_type_name<'tcx>(
             } else {
                 output.push('[');
                 push_debuginfo_type_name(tcx, inner_type, true, output, visited);
-                match len.val {
+                match len.val() {
                     ty::ConstKind::Param(param) => write!(output, "; {}]", param.name).unwrap(),
                     _ => write!(output, "; {}]", len.eval_usize(tcx, ty::ParamEnv::reveal_all()))
                         .unwrap(),
@@ -645,19 +645,19 @@ fn push_generic_params_internal<'tcx>(
     true
 }
 
-fn push_const_param<'tcx>(tcx: TyCtxt<'tcx>, ct: &'tcx ty::Const<'tcx>, output: &mut String) {
-    match ct.val {
+fn push_const_param<'tcx>(tcx: TyCtxt<'tcx>, ct: ty::Const<'tcx>, output: &mut String) {
+    match ct.val() {
         ty::ConstKind::Param(param) => {
             write!(output, "{}", param.name)
         }
-        _ => match ct.ty.kind() {
+        _ => match ct.ty().kind() {
             ty::Int(ity) => {
-                let bits = ct.eval_bits(tcx, ty::ParamEnv::reveal_all(), ct.ty);
+                let bits = ct.eval_bits(tcx, ty::ParamEnv::reveal_all(), ct.ty());
                 let val = Integer::from_int_ty(&tcx, *ity).size().sign_extend(bits) as i128;
                 write!(output, "{}", val)
             }
             ty::Uint(_) => {
-                let val = ct.eval_bits(tcx, ty::ParamEnv::reveal_all(), ct.ty);
+                let val = ct.eval_bits(tcx, ty::ParamEnv::reveal_all(), ct.ty());
                 write!(output, "{}", val)
             }
             ty::Bool => {
@@ -672,7 +672,7 @@ fn push_const_param<'tcx>(tcx: TyCtxt<'tcx>, ct: &'tcx ty::Const<'tcx>, output:
                 let mut hasher = StableHasher::new();
                 hcx.while_hashing_spans(false, |hcx| {
                     hcx.with_node_id_hashing_mode(NodeIdHashingMode::HashDefPath, |hcx| {
-                        ct.val.hash_stable(hcx, &mut hasher);
+                        ct.val().hash_stable(hcx, &mut hasher);
                     });
                 });
                 // Let's only emit 64 bits of the hash value. That should be plenty for

compiler/rustc_codegen_ssa/src/mir/constant.rs

@@ -29,7 +29,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::ConstantKind::Ty(ct) => ct,
             mir::ConstantKind::Val(val, _) => return Ok(val),
         };
-        match ct.val {
+        match ct.val() {
             ty::ConstKind::Unevaluated(ct) => self
                 .cx
                 .tcx()
@@ -61,11 +61,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let c = ty::Const::from_value(bx.tcx(), val, ty);
                 let values: Vec<_> = bx
                     .tcx()
-                    .destructure_const(ty::ParamEnv::reveal_all().and(&c))
+                    .destructure_const(ty::ParamEnv::reveal_all().and(c))
                     .fields
                     .iter()
                     .map(|field| {
-                        if let Some(prim) = field.val.try_to_scalar() {
+                        if let Some(prim) = field.val().try_to_scalar() {
                             let layout = bx.layout_of(field_ty);
                             let scalar = match layout.abi {
                                 Abi::Scalar(x) => x,
@@ -78,7 +78,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     })
                     .collect();
                 let llval = bx.const_struct(&values, false);
-                (llval, c.ty)
+                (llval, c.ty())
             })
             .unwrap_or_else(|_| {
                 bx.tcx().sess.span_err(span, "could not evaluate shuffle_indices at compile time");