Auto merge of #52712 - oli-obk:const_eval_cleanups, r=RalfJung

Reintroduce `Undef` and properly check constant value sizes

r? @RalfJung

cc @eddyb

basically all kinds of silent failures that never occurred are assertions now

diff --combined src/librustc_mir/monomorphize/collector.rs
index 6283ee9cfe648ed9f60ac54bad1df188722ba6c8,bd90b308fabe80e2ac9fc5263e2bf86d2d7194ca..97f1f2a76ca2b71ef2b346aafa812604ecdd5ed5
--- 1/src/librustc_mir/monomorphize/collector.rs
--- 2/src/librustc_mir/monomorphize/collector.rs
+++ b/src/librustc_mir/monomorphize/collector.rs
@@@ -193,7 -193,7 +193,7 @@@ use rustc::hir::itemlikevisit::ItemLike
  
  use rustc::hir::map as hir_map;
  use rustc::hir::def_id::DefId;
- use rustc::mir::interpret::{AllocId, ConstValue};
+ use rustc::mir::interpret::{AllocId, ConstValue, ScalarMaybeUndef};
  use rustc::middle::lang_items::{ExchangeMallocFnLangItem, StartFnLangItem};
  use rustc::ty::subst::Substs;
  use rustc::ty::{self, TypeFoldable, Ty, TyCtxt, GenericParamDefKind};
@@@ -240,7 -240,7 +240,7 @@@ impl<'tcx> InliningMap<'tcx> 
          InliningMap {
              index: FxHashMap(),
              targets: Vec::new(),
 -            inlines: BitVector::new(1024),
 +            inlines: BitVector::with_capacity(1024),
          }
      }
  
@@@ -1264,11 -1264,11 +1264,11 @@@ fn collect_const<'a, 'tcx>
      };
      match val {
          ConstValue::Unevaluated(..) => bug!("const eval yielded unevaluated const"),
-         ConstValue::ScalarPair(Scalar::Ptr(a), Scalar::Ptr(b)) => {
+         ConstValue::ScalarPair(Scalar::Ptr(a), ScalarMaybeUndef::Scalar(Scalar::Ptr(b))) => {
              collect_miri(tcx, a.alloc_id, output);
              collect_miri(tcx, b.alloc_id, output);
          }
-         ConstValue::ScalarPair(_, Scalar::Ptr(ptr)) |
+         ConstValue::ScalarPair(_, ScalarMaybeUndef::Scalar(Scalar::Ptr(ptr))) |
          ConstValue::ScalarPair(Scalar::Ptr(ptr), _) |
          ConstValue::Scalar(Scalar::Ptr(ptr)) =>
              collect_miri(tcx, ptr.alloc_id, output),