Specialize try_destructure_mir_constant for its sole user

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -6,7 +6,7 @@ use crate::interpret::{
 };
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{EvalToValTreeResult, GlobalId};
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::{source_map::DUMMY_SP, symbol::Symbol};
 
 mod error;
@@ -89,14 +89,15 @@ pub(crate) fn eval_to_valtree<'tcx>(
 #[instrument(skip(tcx), level = "debug")]
 pub(crate) fn try_destructure_mir_constant<'tcx>(
     tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    val: mir::ConstantKind<'tcx>,
+    val: ConstValue<'tcx>,
+    ty: Ty<'tcx>,
 ) -> InterpResult<'tcx, mir::DestructuredConstant<'tcx>> {
+    let param_env = ty::ParamEnv::reveal_all();
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessStatics::No);
-    let op = ecx.eval_mir_constant(&val, None, None)?;
+    let op = ecx.const_val_to_op(val, ty, None)?;
 
     // We go to `usize` as we cannot allocate anything bigger anyway.
-    let (field_count, variant, down) = match val.ty().kind() {
+    let (field_count, variant, down) = match ty.kind() {
         ty::Array(_, len) => (len.eval_target_usize(tcx, param_env) as usize, None, op),
         ty::Adt(def, _) if def.variants().is_empty() => {
             throw_ub!(Unreachable)

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -633,7 +633,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    pub(super) fn const_val_to_op(
+    pub(crate) fn const_val_to_op(
         &self,
         val_val: ConstValue<'tcx>,
         ty: Ty<'tcx>,

compiler/rustc_const_eval/src/lib.rs

@@ -52,10 +52,8 @@ pub fn provide(providers: &mut Providers) {
         let (param_env, raw) = param_env_and_value.into_parts();
         const_eval::eval_to_valtree(tcx, param_env, raw)
     };
-    providers.try_destructure_mir_constant = |tcx, param_env_and_value| {
-        let (param_env, value) = param_env_and_value.into_parts();
-        const_eval::try_destructure_mir_constant(tcx, param_env, value).ok()
-    };
+    providers.try_destructure_mir_constant =
+        |tcx, (cv, ty)| const_eval::try_destructure_mir_constant(tcx, cv, ty).ok();
     providers.valtree_to_const_val = |tcx, (ty, valtree)| {
         const_eval::valtree_to_const_value(tcx, ty::ParamEnv::empty().and(ty), valtree)
     };

compiler/rustc_middle/src/mir/mod.rs

@@ -2882,15 +2882,10 @@ fn pretty_print_const_value<'tcx>(
             // introducing ICEs (e.g. via `layout_of`) from missing bounds.
             // E.g. `transmute([0usize; 2]): (u8, *mut T)` needs to know `T: Sized`
             // to be able to destructure the tuple into `(0u8, *mut T)`
-            //
-            // FIXME(eddyb) for `--emit=mir`/`-Z dump-mir`, we should provide the
-            // correct `ty::ParamEnv` to allow printing *all* constant values.
             (_, ty::Array(..) | ty::Tuple(..) | ty::Adt(..)) if !ty.has_non_region_param() => {
                 let ct = tcx.lift(ct).unwrap();
                 let ty = tcx.lift(ty).unwrap();
-                if let Some(contents) = tcx.try_destructure_mir_constant(
-                    ty::ParamEnv::reveal_all().and(ConstantKind::Val(ct, ty)),
-                ) {
+                if let Some(contents) = tcx.try_destructure_mir_constant((ct, ty)) {
                     let fields = contents.fields.to_vec();
                     match *ty.kind() {
                         ty::Array(..) => {

compiler/rustc_middle/src/query/keys.rs

@@ -2,6 +2,7 @@
 
 use crate::infer::canonical::Canonical;
 use crate::mir;
+use crate::mir::interpret::ConstValue;
 use crate::traits;
 use crate::ty::fast_reject::SimplifiedType;
 use crate::ty::layout::{TyAndLayout, ValidityRequirement};
@@ -333,6 +334,14 @@ impl<'tcx> Key for (ty::Const<'tcx>, FieldIdx) {
     }
 }
 
+impl<'tcx> Key for (ConstValue<'tcx>, Ty<'tcx>) {
+    type CacheSelector = DefaultCacheSelector<Self>;
+
+    fn default_span(&self, _: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}
+
 impl<'tcx> Key for mir::interpret::ConstAlloc<'tcx> {
     type CacheSelector = DefaultCacheSelector<Self>;
 

compiler/rustc_middle/src/query/mod.rs

@@ -1087,11 +1087,13 @@ rustc_queries! {
     }
 
     /// Tries to destructure an `mir::ConstantKind` ADT or array into its variant index
-    /// and its field values.
+    /// and its field values. This should only be used for pretty printing.
     query try_destructure_mir_constant(
-        key: ty::ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
+        key: (ConstValue<'tcx>, Ty<'tcx>)
     ) -> Option<mir::DestructuredConstant<'tcx>> {
         desc { "destructuring MIR constant"}
+        no_hash
+        eval_always
     }
 
     query const_caller_location(key: (rustc_span::Symbol, u32, u32)) -> ConstValue<'tcx> {