Auto merge of #54693 - RalfJung:ctfe-scalar-pair-undef, r=oli-obk

do not normalize all non-scalar constants to a ConstValue::ScalarPair

We still need `ConstValue::ScalarPair` for match handling (matching slices and strings), but that will never see anything `Undef`. For non-fat-ptr `ScalarPair`, just point to the allocation like larger data structures do.

Fixes https://github.com/rust-lang/rust/issues/54387

r? @eddyb

19 files changed, 166 insertions, 150 deletions

diff --git a/src/librustc/ich/impls_ty.rs b/src/librustc/ich/impls_ty.rs
index 78caa45187c..9c4e995e24d 100644
--- a/src/librustc/ich/impls_ty.rs
+++ b/src/librustc/ich/impls_ty.rs
@@ -391,11 +391,6 @@ for ::mir::interpret::ConstValue<'gcx> {
     }
 }
 
-impl_stable_hash_for!(enum mir::interpret::ScalarMaybeUndef {
-    Scalar(v),
-    Undef
-});
-
 impl_stable_hash_for!(struct mir::interpret::Pointer {
     alloc_id,
     offset
diff --git a/src/librustc/mir/interpret/mod.rs b/src/librustc/mir/interpret/mod.rs
index be0639f2420..e9d0b041339 100644
--- a/src/librustc/mir/interpret/mod.rs
+++ b/src/librustc/mir/interpret/mod.rs
@@ -23,7 +23,7 @@ pub use self::error::{
     FrameInfo, ConstEvalResult,
 };
 
-pub use self::value::{Scalar, ConstValue, ScalarMaybeUndef};
+pub use self::value::{Scalar, ConstValue};
 
 use std::fmt;
 use mir;
diff --git a/src/librustc/mir/interpret/value.rs b/src/librustc/mir/interpret/value.rs
index be7d9b06fb4..d695ad88e47 100644
--- a/src/librustc/mir/interpret/value.rs
+++ b/src/librustc/mir/interpret/value.rs
@@ -24,15 +24,18 @@ pub enum ConstValue<'tcx> {
     /// to allow HIR creation to happen for everything before needing to be able to run constant
     /// evaluation
     Unevaluated(DefId, &'tcx Substs<'tcx>),
+
     /// Used only for types with layout::abi::Scalar ABI and ZSTs
     ///
     /// Not using the enum `Value` to encode that this must not be `Undef`
     Scalar(Scalar),
-    /// Used only for types with layout::abi::ScalarPair
+
+    /// Used only for *fat pointers* with layout::abi::ScalarPair
     ///
-    /// The second field may be undef in case of `Option<usize>::None`
-    ScalarPair(Scalar, ScalarMaybeUndef),
-    /// Used only for the remaining cases. An allocation + offset into the allocation.
+    /// Needed for pattern matching code related to slices and strings.
+    ScalarPair(Scalar, Scalar),
+
+    /// An allocation + offset into the allocation.
     /// Invariant: The AllocId matches the allocation.
     ByRef(AllocId, &'tcx Allocation, Size),
 }
@@ -67,12 +70,12 @@ impl<'tcx> ConstValue<'tcx> {
         ConstValue::ScalarPair(val, Scalar::Bits {
             bits: len as u128,
             size: cx.data_layout().pointer_size.bytes() as u8,
-        }.into())
+        })
     }
 
     #[inline]
     pub fn new_dyn_trait(val: Scalar, vtable: Pointer) -> Self {
-        ConstValue::ScalarPair(val, Scalar::Ptr(vtable).into())
+        ConstValue::ScalarPair(val, Scalar::Ptr(vtable))
     }
 }
 
@@ -340,96 +343,3 @@ pub enum Scalar<Id=AllocId> {
     /// relocation and its associated offset together as a `Pointer` here.
     Ptr(Pointer<Id>),
 }
-
-#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, RustcEncodable, RustcDecodable, Hash)]
-pub enum ScalarMaybeUndef<Id=AllocId> {
-    Scalar(Scalar<Id>),
-    Undef,
-}
-
-impl From<Scalar> for ScalarMaybeUndef {
-    #[inline(always)]
-    fn from(s: Scalar) -> Self {
-        ScalarMaybeUndef::Scalar(s)
-    }
-}
-
-impl<'tcx> ScalarMaybeUndef {
-    #[inline]
-    pub fn not_undef(self) -> EvalResult<'static, Scalar> {
-        match self {
-            ScalarMaybeUndef::Scalar(scalar) => Ok(scalar),
-            ScalarMaybeUndef::Undef => err!(ReadUndefBytes(Size::from_bytes(0))),
-        }
-    }
-
-    #[inline(always)]
-    pub fn to_ptr(self) -> EvalResult<'tcx, Pointer> {
-        self.not_undef()?.to_ptr()
-    }
-
-    #[inline(always)]
-    pub fn to_bits(self, target_size: Size) -> EvalResult<'tcx, u128> {
-        self.not_undef()?.to_bits(target_size)
-    }
-
-    #[inline(always)]
-    pub fn to_bool(self) -> EvalResult<'tcx, bool> {
-        self.not_undef()?.to_bool()
-    }
-
-    #[inline(always)]
-    pub fn to_char(self) -> EvalResult<'tcx, char> {
-        self.not_undef()?.to_char()
-    }
-
-    #[inline(always)]
-    pub fn to_f32(self) -> EvalResult<'tcx, f32> {
-        self.not_undef()?.to_f32()
-    }
-
-    #[inline(always)]
-    pub fn to_f64(self) -> EvalResult<'tcx, f64> {
-        self.not_undef()?.to_f64()
-    }
-
-    #[inline(always)]
-    pub fn to_u8(self) -> EvalResult<'tcx, u8> {
-        self.not_undef()?.to_u8()
-    }
-
-    #[inline(always)]
-    pub fn to_u32(self) -> EvalResult<'tcx, u32> {
-        self.not_undef()?.to_u32()
-    }
-
-    #[inline(always)]
-    pub fn to_u64(self) -> EvalResult<'tcx, u64> {
-        self.not_undef()?.to_u64()
-    }
-
-    #[inline(always)]
-    pub fn to_usize(self, cx: impl HasDataLayout) -> EvalResult<'tcx, u64> {
-        self.not_undef()?.to_usize(cx)
-    }
-
-    #[inline(always)]
-    pub fn to_i8(self) -> EvalResult<'tcx, i8> {
-        self.not_undef()?.to_i8()
-    }
-
-    #[inline(always)]
-    pub fn to_i32(self) -> EvalResult<'tcx, i32> {
-        self.not_undef()?.to_i32()
-    }
-
-    #[inline(always)]
-    pub fn to_i64(self) -> EvalResult<'tcx, i64> {
-        self.not_undef()?.to_i64()
-    }
-
-    #[inline(always)]
-    pub fn to_isize(self, cx: impl HasDataLayout) -> EvalResult<'tcx, i64> {
-        self.not_undef()?.to_isize(cx)
-    }
-}
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 3e6246f2ea8..f36d9553d31 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -17,7 +17,7 @@ use hir::def::CtorKind;
 use hir::def_id::DefId;
 use hir::{self, HirId, InlineAsm};
 use middle::region;
-use mir::interpret::{ConstValue, EvalErrorKind, Scalar, ScalarMaybeUndef};
+use mir::interpret::{ConstValue, EvalErrorKind, Scalar};
 use mir::visit::MirVisitable;
 use rustc_apfloat::ieee::{Double, Single};
 use rustc_apfloat::Float;
@@ -2397,7 +2397,7 @@ pub fn fmt_const_val(f: &mut impl Write, const_val: &ty::Const<'_>) -> fmt::Resu
     // print string literals
     if let ConstValue::ScalarPair(ptr, len) = value {
         if let Scalar::Ptr(ptr) = ptr {
-            if let ScalarMaybeUndef::Scalar(Scalar::Bits { bits: len, .. }) = len {
+            if let Scalar::Bits { bits: len, .. } = len {
                 if let Ref(_, &ty::TyS { sty: Str, .. }, _) = ty.sty {
                     return ty::tls::with(|tcx| {
                         let alloc = tcx.alloc_map.lock().get(ptr.alloc_id);
diff --git a/src/librustc_codegen_llvm/mir/operand.rs b/src/librustc_codegen_llvm/mir/operand.rs
index bfa0e0a451e..ab43531240f 100644
--- a/src/librustc_codegen_llvm/mir/operand.rs
+++ b/src/librustc_codegen_llvm/mir/operand.rs
@@ -8,9 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use rustc::mir::interpret::ConstEvalErr;
+use rustc::mir::interpret::{ConstValue, ConstEvalErr};
 use rustc::mir;
-use rustc::mir::interpret::{ConstValue, ScalarMaybeUndef};
 use rustc::ty;
 use rustc::ty::layout::{self, Align, LayoutOf, TyLayout};
 use rustc_data_structures::sync::Lrc;
@@ -114,15 +113,12 @@ impl OperandRef<'ll, 'tcx> {
                     layout.scalar_pair_element_llvm_type(bx.cx, 0, true),
                 );
                 let b_layout = layout.scalar_pair_element_llvm_type(bx.cx, 1, true);
-                let b_llval = match b {
-                    ScalarMaybeUndef::Scalar(b) => scalar_to_llvm(
-                        bx.cx,
-                        b,
-                        b_scalar,
-                        b_layout,
-                    ),
-                    ScalarMaybeUndef::Undef => C_undef(b_layout),
-                };
+                let b_llval = scalar_to_llvm(
+                    bx.cx,
+                    b,
+                    b_scalar,
+                    b_layout,
+                );
                 OperandValue::Pair(a_llval, b_llval)
             },
             ConstValue::ByRef(_, alloc, offset) => {
diff --git a/src/librustc_mir/const_eval.rs b/src/librustc_mir/const_eval.rs
index 99a49122ef7..07e2cfe1e80 100644
--- a/src/librustc_mir/const_eval.rs
+++ b/src/librustc_mir/const_eval.rs
@@ -17,7 +17,7 @@ use rustc::hir::{self, def_id::DefId};
 use rustc::mir::interpret::ConstEvalErr;
 use rustc::mir;
 use rustc::ty::{self, TyCtxt, Instance, query::TyCtxtAt};
-use rustc::ty::layout::{LayoutOf, TyLayout};
+use rustc::ty::layout::{self, LayoutOf, TyLayout};
 use rustc::ty::subst::Subst;
 use rustc_data_structures::indexed_vec::IndexVec;
 
@@ -97,8 +97,18 @@ pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(
 pub fn op_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
     op: OpTy<'tcx>,
-    normalize: bool,
+    may_normalize: bool,
 ) -> EvalResult<'tcx, &'tcx ty::Const<'tcx>> {
+    // We do not normalize just any data.  Only scalar layout and fat pointers.
+    let normalize = may_normalize
+        && match op.layout.abi {
+            layout::Abi::Scalar(..) => true,
+            layout::Abi::ScalarPair(..) => {
+                // Must be a fat pointer
+                op.layout.ty.builtin_deref(true).is_some()
+            },
+            _ => false,
+        };
     let normalized_op = if normalize {
         ecx.try_read_value(op)?
     } else {
@@ -125,7 +135,7 @@ pub fn op_to_const<'tcx>(
         Ok(Value::Scalar(x)) =>
             ConstValue::Scalar(x.not_undef()?),
         Ok(Value::ScalarPair(a, b)) =>
-            ConstValue::ScalarPair(a.not_undef()?, b),
+            ConstValue::ScalarPair(a.not_undef()?, b.not_undef()?),
     };
     Ok(ty::Const::from_const_value(ecx.tcx.tcx, val, op.layout.ty))
 }
diff --git a/src/librustc_mir/hair/pattern/mod.rs b/src/librustc_mir/hair/pattern/mod.rs
index c72f8783685..b22cc4a1a42 100644
--- a/src/librustc_mir/hair/pattern/mod.rs
+++ b/src/librustc_mir/hair/pattern/mod.rs
@@ -1124,13 +1124,6 @@ pub fn compare_const_vals<'a, 'tcx>(
                         len_b,
                     ),
                 ) if ptr_a.offset.bytes() == 0 && ptr_b.offset.bytes() == 0 => {
-                    let len_a = len_a.not_undef().ok();
-                    let len_b = len_b.not_undef().ok();
-                    if len_a.is_none() || len_b.is_none() {
-                        tcx.sess.struct_err("str slice len is undef").delay_as_bug();
-                    }
-                    let len_a = len_a?;
-                    let len_b = len_b?;
                     if let Ok(len_a) = len_a.to_bits(tcx.data_layout.pointer_size) {
                         if let Ok(len_b) = len_b.to_bits(tcx.data_layout.pointer_size) {
                             if len_a == len_b {
diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 6cbe8065c7e..ff059e7d185 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -25,14 +25,13 @@ use rustc_data_structures::indexed_vec::IndexVec;
 use rustc::mir::interpret::{
     GlobalId, Scalar, FrameInfo, AllocId,
     EvalResult, EvalErrorKind,
-    ScalarMaybeUndef,
     truncate, sign_extend,
 };
 
 use syntax::source_map::{self, Span};
 
 use super::{
-    Value, Operand, MemPlace, MPlaceTy, Place,
+    Value, Operand, MemPlace, MPlaceTy, Place, ScalarMaybeUndef,
     Memory, Machine
 };
 
diff --git a/src/librustc_mir/interpret/memory.rs b/src/librustc_mir/interpret/memory.rs
index b5eb06b82ff..11e27a597fb 100644
--- a/src/librustc_mir/interpret/memory.rs
+++ b/src/librustc_mir/interpret/memory.rs
@@ -21,7 +21,7 @@ use std::ptr;
 
 use rustc::ty::{self, Instance, query::TyCtxtAt};
 use rustc::ty::layout::{self, Align, TargetDataLayout, Size, HasDataLayout};
-use rustc::mir::interpret::{Pointer, AllocId, Allocation, ConstValue, ScalarMaybeUndef, GlobalId,
+use rustc::mir::interpret::{Pointer, AllocId, Allocation, ConstValue, GlobalId,
                             EvalResult, Scalar, EvalErrorKind, AllocType, PointerArithmetic,
                             truncate};
 pub use rustc::mir::interpret::{write_target_uint, read_target_uint};
@@ -29,7 +29,7 @@ use rustc_data_structures::fx::{FxHashSet, FxHashMap};
 
 use syntax::ast::Mutability;
 
-use super::Machine;
+use super::{Machine, ScalarMaybeUndef};
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)]
 pub enum MemoryKind<T> {
diff --git a/src/librustc_mir/interpret/mod.rs b/src/librustc_mir/interpret/mod.rs
index 6f07c2996b9..b840af193b6 100644
--- a/src/librustc_mir/interpret/mod.rs
+++ b/src/librustc_mir/interpret/mod.rs
@@ -34,4 +34,4 @@ pub use self::memory::{Memory, MemoryKind};
 
 pub use self::machine::Machine;
 
-pub use self::operand::{Value, ValTy, Operand, OpTy};
+pub use self::operand::{ScalarMaybeUndef, Value, ValTy, Operand, OpTy};
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index a11150c4783..b53bcfa993d 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -19,11 +19,105 @@ use rustc::ty::layout::{self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerEx
 
 use rustc::mir::interpret::{
     GlobalId, AllocId,
-    ConstValue, Pointer, Scalar, ScalarMaybeUndef,
+    ConstValue, Pointer, Scalar,
     EvalResult, EvalErrorKind
 };
 use super::{EvalContext, Machine, MemPlace, MPlaceTy, MemoryKind};
 
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, RustcEncodable, RustcDecodable, Hash)]
+pub enum ScalarMaybeUndef<Id=AllocId> {
+    Scalar(Scalar<Id>),
+    Undef,
+}
+
+impl From<Scalar> for ScalarMaybeUndef {
+    #[inline(always)]
+    fn from(s: Scalar) -> Self {
+        ScalarMaybeUndef::Scalar(s)
+    }
+}
+
+impl<'tcx> ScalarMaybeUndef {
+    #[inline]
+    pub fn not_undef(self) -> EvalResult<'static, Scalar> {
+        match self {
+            ScalarMaybeUndef::Scalar(scalar) => Ok(scalar),
+            ScalarMaybeUndef::Undef => err!(ReadUndefBytes(Size::from_bytes(0))),
+        }
+    }
+
+    #[inline(always)]
+    pub fn to_ptr(self) -> EvalResult<'tcx, Pointer> {
+        self.not_undef()?.to_ptr()
+    }
+
+    #[inline(always)]
+    pub fn to_bits(self, target_size: Size) -> EvalResult<'tcx, u128> {
+        self.not_undef()?.to_bits(target_size)
+    }
+
+    #[inline(always)]
+    pub fn to_bool(self) -> EvalResult<'tcx, bool> {
+        self.not_undef()?.to_bool()
+    }
+
+    #[inline(always)]
+    pub fn to_char(self) -> EvalResult<'tcx, char> {
+        self.not_undef()?.to_char()
+    }
+
+    #[inline(always)]
+    pub fn to_f32(self) -> EvalResult<'tcx, f32> {
+        self.not_undef()?.to_f32()
+    }
+
+    #[inline(always)]
+    pub fn to_f64(self) -> EvalResult<'tcx, f64> {
+        self.not_undef()?.to_f64()
+    }
+
+    #[inline(always)]
+    pub fn to_u8(self) -> EvalResult<'tcx, u8> {
+        self.not_undef()?.to_u8()
+    }
+
+    #[inline(always)]
+    pub fn to_u32(self) -> EvalResult<'tcx, u32> {
+        self.not_undef()?.to_u32()
+    }
+
+    #[inline(always)]
+    pub fn to_u64(self) -> EvalResult<'tcx, u64> {
+        self.not_undef()?.to_u64()
+    }
+
+    #[inline(always)]
+    pub fn to_usize(self, cx: impl HasDataLayout) -> EvalResult<'tcx, u64> {
+        self.not_undef()?.to_usize(cx)
+    }
+
+    #[inline(always)]
+    pub fn to_i8(self) -> EvalResult<'tcx, i8> {
+        self.not_undef()?.to_i8()
+    }
+
+    #[inline(always)]
+    pub fn to_i32(self) -> EvalResult<'tcx, i32> {
+        self.not_undef()?.to_i32()
+    }
+
+    #[inline(always)]
+    pub fn to_i64(self) -> EvalResult<'tcx, i64> {
+        self.not_undef()?.to_i64()
+    }
+
+    #[inline(always)]
+    pub fn to_isize(self, cx: impl HasDataLayout) -> EvalResult<'tcx, i64> {
+        self.not_undef()?.to_isize(cx)
+    }
+}
+
+
 /// A `Value` represents a single immediate self-contained Rust value.
 ///
 /// For optimization of a few very common cases, there is also a representation for a pair of
@@ -490,7 +584,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                 Ok(Operand::Indirect(MemPlace::from_ptr(Pointer::new(id, offset), alloc.align)))
             },
             ConstValue::ScalarPair(a, b) =>
-                Ok(Operand::Immediate(Value::ScalarPair(a.into(), b))),
+                Ok(Operand::Immediate(Value::ScalarPair(a.into(), b.into()))),
             ConstValue::Scalar(x) =>
                 Ok(Operand::Immediate(Value::Scalar(x.into()))),
         }
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 27765558258..8878e5ca83f 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -19,9 +19,9 @@ use rustc::ty::{self, Ty};
 use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout};
 
 use rustc::mir::interpret::{
-    GlobalId, AllocId, Scalar, EvalResult, Pointer, ScalarMaybeUndef, PointerArithmetic
+    GlobalId, AllocId, Scalar, EvalResult, Pointer, PointerArithmetic
 };
-use super::{EvalContext, Machine, Value, ValTy, Operand, OpTy, MemoryKind};
+use super::{EvalContext, Machine, Value, ValTy, ScalarMaybeUndef, Operand, OpTy, MemoryKind};
 
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
 pub struct MemPlace<Id=AllocId> {
diff --git a/src/librustc_mir/interpret/snapshot.rs b/src/librustc_mir/interpret/snapshot.rs
index 18ca2848110..9b507eca363 100644
--- a/src/librustc_mir/interpret/snapshot.rs
+++ b/src/librustc_mir/interpret/snapshot.rs
@@ -11,7 +11,7 @@ use std::mem;
 use rustc::ich::{StableHashingContext, StableHashingContextProvider};
 use rustc::mir;
 use rustc::mir::interpret::{
-    AllocId, Pointer, Scalar, ScalarMaybeUndef,
+    AllocId, Pointer, Scalar,
     Relocations, Allocation, UndefMask,
     EvalResult, EvalErrorKind,
 };
@@ -25,7 +25,7 @@ use syntax::ast::Mutability;
 use syntax::source_map::Span;
 
 use super::eval_context::{LocalValue, StackPopCleanup};
-use super::{Frame, Memory, Operand, MemPlace, Place, Value};
+use super::{Frame, Memory, Operand, MemPlace, Place, Value, ScalarMaybeUndef};
 use const_eval::CompileTimeInterpreter;
 
 #[derive(Default)]
@@ -193,6 +193,11 @@ impl<'a, Ctx> Snapshot<'a, Ctx> for Scalar
     }
 }
 
+impl_stable_hash_for!(enum ::interpret::ScalarMaybeUndef {
+    Scalar(v),
+    Undef
+});
+
 impl_snapshot_for!(enum ScalarMaybeUndef {
     Scalar(s),
     Undef,
diff --git a/src/librustc_mir/interpret/validity.rs b/src/librustc_mir/interpret/validity.rs
index f816983ab47..c5238d24cf7 100644
--- a/src/librustc_mir/interpret/validity.rs
+++ b/src/librustc_mir/interpret/validity.rs
@@ -15,11 +15,11 @@ use rustc::ty::layout::{self, Size, Primitive};
 use rustc::ty::{self, Ty};
 use rustc_data_structures::fx::FxHashSet;
 use rustc::mir::interpret::{
-    Scalar, AllocType, EvalResult, ScalarMaybeUndef, EvalErrorKind, PointerArithmetic
+    Scalar, AllocType, EvalResult, EvalErrorKind, PointerArithmetic
 };
 
 use super::{
-    OpTy, Machine, EvalContext
+    OpTy, Machine, EvalContext, ScalarMaybeUndef
 };
 
 macro_rules! validation_failure{
diff --git a/src/librustc_mir/monomorphize/collector.rs b/src/librustc_mir/monomorphize/collector.rs
index ea01d17ac13..b2fa8349384 100644
--- a/src/librustc_mir/monomorphize/collector.rs
+++ b/src/librustc_mir/monomorphize/collector.rs
@@ -193,7 +193,7 @@ use rustc::hir::itemlikevisit::ItemLikeVisitor;
 
 use rustc::hir::Node;
 use rustc::hir::def_id::DefId;
-use rustc::mir::interpret::{AllocId, ConstValue, ScalarMaybeUndef};
+use rustc::mir::interpret::{AllocId, ConstValue};
 use rustc::middle::lang_items::{ExchangeMallocFnLangItem, StartFnLangItem};
 use rustc::ty::subst::Substs;
 use rustc::ty::{self, TypeFoldable, Ty, TyCtxt, GenericParamDefKind};
@@ -1263,11 +1263,11 @@ fn collect_const<'a, 'tcx>(
     };
     match val {
         ConstValue::Unevaluated(..) => bug!("const eval yielded unevaluated const"),
-        ConstValue::ScalarPair(Scalar::Ptr(a), ScalarMaybeUndef::Scalar(Scalar::Ptr(b))) => {
+        ConstValue::ScalarPair(Scalar::Ptr(a), Scalar::Ptr(b)) => {
             collect_miri(tcx, a.alloc_id, output);
             collect_miri(tcx, b.alloc_id, output);
         }
-        ConstValue::ScalarPair(_, ScalarMaybeUndef::Scalar(Scalar::Ptr(ptr))) |
+        ConstValue::ScalarPair(_, Scalar::Ptr(ptr)) |
         ConstValue::ScalarPair(Scalar::Ptr(ptr), _) |
         ConstValue::Scalar(Scalar::Ptr(ptr)) =>
             collect_miri(tcx, ptr.alloc_id, output),
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index e2a6cee0549..70d50d589d1 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -18,10 +18,10 @@ use rustc::mir::{NullOp, UnOp, StatementKind, Statement, BasicBlock, LocalKind};
 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
 use rustc::mir::visit::{Visitor, PlaceContext};
 use rustc::mir::interpret::{
-    ConstEvalErr, EvalErrorKind, ScalarMaybeUndef, Scalar, GlobalId, EvalResult
+    ConstEvalErr, EvalErrorKind, Scalar, GlobalId, EvalResult
 };
 use rustc::ty::{TyCtxt, self, Instance};
-use interpret::{self, EvalContext, Value, OpTy, MemoryKind};
+use interpret::{self, EvalContext, Value, OpTy, MemoryKind, ScalarMaybeUndef};
 use const_eval::{CompileTimeInterpreter, eval_promoted, mk_borrowck_eval_cx};
 use transform::{MirPass, MirSource};
 use syntax::source_map::{Span, DUMMY_SP};
diff --git a/src/test/ui/consts/const-eval/union-ice.rs b/src/test/ui/consts/const-eval/union-ice.rs
index 0cdb78c9780..5d50004e554 100644
--- a/src/test/ui/consts/const-eval/union-ice.rs
+++ b/src/test/ui/consts/const-eval/union-ice.rs
@@ -22,7 +22,7 @@ const UNION: DummyUnion = DummyUnion { field1: 1065353216 };
 
 const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant cannot be used
 
-const FIELD_PATH: Struct = Struct { //~ ERROR this constant cannot be used
+const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibits undefined behavior
     a: 42,
     b: unsafe { UNION.field3 },
 };
diff --git a/src/test/ui/consts/const-eval/union-ice.stderr b/src/test/ui/consts/const-eval/union-ice.stderr
index e8a7b2f5005..ec51802681e 100644
--- a/src/test/ui/consts/const-eval/union-ice.stderr
+++ b/src/test/ui/consts/const-eval/union-ice.stderr
@@ -6,14 +6,16 @@ LL | const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant can
    |
    = note: #[deny(const_err)] on by default
 
-error: this constant cannot be used
+error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ice.rs:25:1
    |
-LL | / const FIELD_PATH: Struct = Struct { //~ ERROR this constant cannot be used
+LL | / const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibits undefined behavior
 LL | |     a: 42,
 LL | |     b: unsafe { UNION.field3 },
 LL | | };
-   | |__^ attempted to read undefined bytes
+   | |__^ type validation failed: encountered undefined bytes at .b
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ice.rs:35:1
diff --git a/src/test/ui/issues/issue-54387.rs b/src/test/ui/issues/issue-54387.rs
new file mode 100644
index 00000000000..ac1033add0e
--- /dev/null
+++ b/src/test/ui/issues/issue-54387.rs
@@ -0,0 +1,12 @@
+// compile-pass
+
+pub struct GstRc {
+    _obj: *const (),
+    _borrowed: bool,
+}
+
+const FOO: Option<GstRc> = None;
+
+fn main() {
+    let _meh = FOO;
+}