UnpackAndDiscard

Summary: Utility for micro-optimization in D59611468.

Reviewed By: JakobDegen

Differential Revision: D59611470

fbshipit-source-id: bf1e09fd8f626fd0c2ab8ebcb5b30928a6c6fd80

Author: stepancheg

starlark/src/values.rs

@@ -96,6 +96,7 @@ pub use crate::values::types::tuple;
 pub use crate::values::unpack::UnpackValue;
 pub use crate::values::unpack::UnpackValueError;
 pub use crate::values::unpack::UnpackValueErrorInfallible;
+pub use crate::values::unpack_and_discard::UnpackAndDiscard;
 pub use crate::values::value_of::ValueOf;
 pub use crate::values::value_of_unchecked::FrozenValueOfUnchecked;
 pub use crate::values::value_of_unchecked::ValueOfUnchecked;
@@ -120,5 +121,6 @@ pub mod type_repr;
 pub(crate) mod types;
 pub mod typing;
 mod unpack;
+mod unpack_and_discard;
 pub(crate) mod value_of;
 pub(crate) mod value_of_unchecked;

starlark/src/values/unpack_and_discard.rs

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2018 The Starlark in Rust Authors.
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+use std::marker::PhantomData;
+
+use allocative::Allocative;
+use dupe::Clone_;
+use dupe::Copy_;
+use dupe::Dupe_;
+
+use crate::values::type_repr::StarlarkTypeRepr;
+use crate::values::UnpackValue;
+use crate::values::Value;
+
+/// Unpack the value of type `T`, but do not store result.
+///
+/// This can be used when type needs to be checked, but the unpacked value is not needed.
+#[derive(Clone_, Copy_, Dupe_, Allocative)]
+#[allocative(bound = "")]
+pub struct UnpackAndDiscard<T>(PhantomData<fn() -> T>);
+
+impl<T: StarlarkTypeRepr> StarlarkTypeRepr for UnpackAndDiscard<T> {
+    type Canonical = T::Canonical;
+
+    fn starlark_type_repr() -> crate::typing::Ty {
+        <Self::Canonical as StarlarkTypeRepr>::starlark_type_repr()
+    }
+}
+
+impl<'v, T: UnpackValue<'v>> UnpackValue<'v> for UnpackAndDiscard<T> {
+    type Error = T::Error;
+
+    fn unpack_value_impl(value: Value<'v>) -> Result<Option<Self>, Self::Error> {
+        match T::unpack_value_impl(value) {
+            Ok(None) => Ok(None),
+            Ok(Some(_)) => Ok(Some(UnpackAndDiscard(PhantomData))),
+            Err(e) => Err(e),
+        }
+    }
+}