implement (as of now still unused) query for valtree -> constvalue conversion

compiler/rustc_const_eval/src/lib.rs

@@ -35,6 +35,7 @@ pub mod transform;
 pub mod util;
 
 use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::ParamEnv;
 
 pub fn provide(providers: &mut Providers) {
     const_eval::provide(providers);
@@ -49,6 +50,9 @@ pub fn provide(providers: &mut Providers) {
         let (param_env, raw) = param_env_and_value.into_parts();
         const_eval::const_to_valtree(tcx, param_env, raw)
     };
+    providers.valtree_to_const_val = |tcx, (ty, valtree)| {
+        const_eval::valtree_to_const_value(tcx, ParamEnv::empty().and(ty), valtree)
+    };
     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)

compiler/rustc_middle/src/query/mod.rs

@@ -936,6 +936,11 @@ rustc_queries! {
         remap_env_constness
     }
 
+    /// Converts a type level constant value into `ConstValue`
+    query valtree_to_const_val(key: (Ty<'tcx>, ty::ValTree<'tcx>)) -> ConstValue<'tcx> {
+        desc { "convert type-level constant value to mir constant value"}
+    }
+
     /// Destructure a constant ADT or array into its variant index and its
     /// field values or return `None` if constant is invalid.
     ///

compiler/rustc_query_impl/src/keys.rs

@@ -502,3 +502,14 @@ impl<'tcx> Key for (ty::Instance<'tcx>, &'tcx ty::List<Ty<'tcx>>) {
         self.0.default_span(tcx)
     }
 }
+
+impl<'tcx> Key for (Ty<'tcx>, ty::ValTree<'tcx>) {
+    #[inline(always)]
+    fn query_crate_is_local(&self) -> bool {
+        true
+    }
+
+    fn default_span(&self, _: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}