Add a query for dereferencing constants of reference type

compiler/rustc_middle/src/query/mod.rs

@@ -749,6 +749,14 @@ rustc_queries! {
             desc { "destructure constant" }
         }
 
+        /// Dereference a constant reference or raw pointer and turn the result into a constant
+        /// again.
+        query deref_const(
+            key: ty::ParamEnvAnd<'tcx, &'tcx ty::Const<'tcx>>
+        ) -> &'tcx ty::Const<'tcx> {
+            desc { "deref constant" }
+        }
+
         query const_caller_location(key: (rustc_span::Symbol, u32, u32)) -> ConstValue<'tcx> {
             desc { "get a &core::panic::Location referring to a span" }
         }

compiler/rustc_mir/src/const_eval/mod.rs

@@ -2,11 +2,14 @@
 
 use std::convert::TryFrom;
 
+use rustc_hir::Mutability;
 use rustc_middle::mir;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::{source_map::DUMMY_SP, symbol::Symbol};
 
-use crate::interpret::{intern_const_alloc_recursive, ConstValue, InternKind, InterpCx};
+use crate::interpret::{
+    intern_const_alloc_recursive, ConstValue, InternKind, InterpCx, MemPlaceMeta, Scalar,
+};
 
 mod error;
 mod eval_queries;
@@ -67,3 +70,40 @@ pub(crate) fn destructure_const<'tcx>(
 
     mir::DestructuredConst { variant, fields }
 }
+
+pub(crate) fn deref_const<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    val: &'tcx ty::Const<'tcx>,
+) -> &'tcx ty::Const<'tcx> {
+    trace!("deref_const: {:?}", val);
+    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, false);
+    let op = ecx.const_to_op(val, None).unwrap();
+    let mplace = ecx.deref_operand(op).unwrap();
+    if let Scalar::Ptr(ptr) = mplace.ptr {
+        assert_eq!(
+            ecx.memory.get_raw(ptr.alloc_id).unwrap().mutability,
+            Mutability::Not,
+            "deref_const cannot be used with mutable allocations as \
+            that could allow pattern matching to observe mutable statics",
+        );
+    }
+
+    let ty = match mplace.meta {
+        MemPlaceMeta::None => mplace.layout.ty,
+        MemPlaceMeta::Poison => bug!("poison metadata in `deref_const`: {:#?}", mplace),
+        // In case of unsized types, figure out the real type behind.
+        MemPlaceMeta::Meta(scalar) => match mplace.layout.ty.kind {
+            ty::Dynamic(..) => ecx.read_drop_type_from_vtable(scalar).unwrap().1,
+            ty::Str => bug!("there's no sized equivalent of a `str`"),
+            ty::Slice(elem_ty) => tcx.mk_array(elem_ty, scalar.to_machine_usize(&tcx).unwrap()),
+            _ => bug!(
+                "type {} should not have metadata, but had {:?}",
+                mplace.layout.ty,
+                mplace.meta
+            ),
+        },
+    };
+
+    tcx.mk_const(ty::Const { val: ty::ConstKind::Value(op_to_const(&ecx, mplace.into())), ty })
+}

compiler/rustc_mir/src/lib.rs

@@ -60,4 +60,8 @@ pub fn provide(providers: &mut Providers) {
         let (param_env, value) = param_env_and_value.into_parts();
         const_eval::destructure_const(tcx, param_env, value)
     };
+    providers.deref_const = |tcx, param_env_and_value| {
+        let (param_env, value) = param_env_and_value.into_parts();
+        const_eval::deref_const(tcx, param_env, value)
+    };
 }