add ZST fast path to valtree_to_const_value

also factor some repeated code into a helper function

Author: RalfJung

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -189,12 +189,11 @@ fn reconstruct_place_meta<'tcx>(
 }
 
 #[instrument(skip(ecx), level = "debug", ret)]
-fn create_pointee_place<'tcx>(
+fn create_valtree_place<'tcx>(
     ecx: &mut CompileTimeEvalContext<'tcx, 'tcx>,
-    ty: Ty<'tcx>,
+    layout: TyAndLayout<'tcx>,
     valtree: ty::ValTree<'tcx>,
 ) -> MPlaceTy<'tcx> {
-    let layout = ecx.layout_of(ty).unwrap();
     let meta = reconstruct_place_meta(layout, valtree, ecx.tcx.tcx);
     ecx.allocate_dyn(layout, MemoryKind::Stack, meta).unwrap()
 }
@@ -216,11 +215,6 @@ pub fn valtree_to_const_value<'tcx>(
     // FIXME Does this need an example?
 
     let (param_env, ty) = param_env_ty.into_parts();
-    let mut ecx: crate::interpret::InterpCx<
-        '_,
-        '_,
-        crate::const_eval::CompileTimeInterpreter<'_, '_>,
-    > = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessStatics::No);
 
     match ty.kind() {
         ty::FnDef(..) => {
@@ -233,33 +227,29 @@ pub fn valtree_to_const_value<'tcx>(
                 "ValTrees for Bool, Int, Uint, Float or Char should have the form ValTree::Leaf"
             ),
         },
-        ty::Ref(_, _, _) | ty::Tuple(_) | ty::Array(_, _) | ty::Adt(..) => {
-            let place = match ty.kind() {
-                ty::Ref(_, inner_ty, _) => {
-                    // Need to create a place for the pointee (the reference itself will be an immediate)
-                    create_pointee_place(&mut ecx, *inner_ty, valtree)
-                }
-                _ => {
-                    // Need to create a place for this valtree.
-                    create_pointee_place(&mut ecx, ty, valtree)
-                }
-            };
-            debug!(?place);
+        ty::Ref(_, inner_ty, _) => {
+            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessStatics::No);
+            let imm = valtree_to_ref(&mut ecx, valtree, *inner_ty);
+            let imm = ImmTy::from_immediate(imm, tcx.layout_of(param_env_ty).unwrap());
+            op_to_const(&ecx, &imm.into())
+        }
+        ty::Tuple(_) | ty::Array(_, _) | ty::Adt(..) => {
+            let layout = tcx.layout_of(param_env_ty).unwrap();
+            if layout.is_zst() {
+                // Fast path to avoid some allocations.
+                return ConstValue::ZeroSized;
+            }
+
+            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessStatics::No);
+
+            // Need to create a place for this valtree.
+            let place = create_valtree_place(&mut ecx, layout, valtree);
 
             valtree_into_mplace(&mut ecx, &place, valtree);
             dump_place(&ecx, &place);
             intern_const_alloc_recursive(&mut ecx, InternKind::Constant, &place).unwrap();
 
-            match ty.kind() {
-                ty::Ref(_, _, _) => {
-                    let ref_place = place.to_ref(&tcx);
-                    let imm =
-                        ImmTy::from_immediate(ref_place, tcx.layout_of(param_env_ty).unwrap());
-
-                    op_to_const(&ecx, &imm.into())
-                }
-                _ => op_to_const(&ecx, &place.into()),
-            }
+            op_to_const(&ecx, &place.into())
         }
         ty::Never
         | ty::Error(_)
@@ -283,6 +273,22 @@ pub fn valtree_to_const_value<'tcx>(
     }
 }
 
+/// Put a valtree into memory and return a reference to that.
+fn valtree_to_ref<'tcx>(
+    ecx: &mut CompileTimeEvalContext<'tcx, 'tcx>,
+    valtree: ty::ValTree<'tcx>,
+    pointee_ty: Ty<'tcx>,
+) -> Immediate {
+    let pointee_place = create_valtree_place(ecx, ecx.layout_of(pointee_ty).unwrap(), valtree);
+    debug!(?pointee_place);
+
+    valtree_into_mplace(ecx, &pointee_place, valtree);
+    dump_place(ecx, &pointee_place);
+    intern_const_alloc_recursive(ecx, InternKind::Constant, &pointee_place).unwrap();
+
+    pointee_place.to_ref(&ecx.tcx)
+}
+
 #[instrument(skip(ecx), level = "debug")]
 fn valtree_into_mplace<'tcx>(
     ecx: &mut CompileTimeEvalContext<'tcx, 'tcx>,
@@ -292,7 +298,6 @@ fn valtree_into_mplace<'tcx>(
     // This will match on valtree and write the value(s) corresponding to the ValTree
     // inside the place recursively.
 
-    let tcx = ecx.tcx.tcx;
     let ty = place.layout.ty;
 
     match ty.kind() {
@@ -305,27 +310,8 @@ fn valtree_into_mplace<'tcx>(
             ecx.write_immediate(Immediate::Scalar(scalar_int.into()), place).unwrap();
         }
         ty::Ref(_, inner_ty, _) => {
-            let pointee_place = create_pointee_place(ecx, *inner_ty, valtree);
-            debug!(?pointee_place);
-
-            valtree_into_mplace(ecx, &pointee_place, valtree);
-            dump_place(ecx, &pointee_place);
-            intern_const_alloc_recursive(ecx, InternKind::Constant, &pointee_place).unwrap();
-
-            let imm = match inner_ty.kind() {
-                ty::Slice(_) | ty::Str => {
-                    let len = valtree.unwrap_branch().len();
-                    let len_scalar = Scalar::from_target_usize(len as u64, &tcx);
-
-                    Immediate::ScalarPair(
-                        Scalar::from_maybe_pointer(pointee_place.ptr(), &tcx),
-                        len_scalar,
-                    )
-                }
-                _ => pointee_place.to_ref(&tcx),
-            };
+            let imm = valtree_to_ref(ecx, valtree, *inner_ty);
             debug!(?imm);
-
             ecx.write_immediate(imm, place).unwrap();
         }
         ty::Adt(_, _) | ty::Tuple(_) | ty::Array(_, _) | ty::Str | ty::Slice(_) => {