rust/proxied.rs - third_party/protobuf - Git at Google

 // Protocol Buffers - Google's data interchange format
 // Copyright 2023 Google LLC.  All rights reserved.
 //
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd

 //! Operating on borrowed data owned by a message is a central concept in
 //! Protobuf (and Rust in general). The way this is normally accomplished in
 //! Rust is to pass around references and operate on those. Unfortunately,
 //! references come with two major drawbacks:
 //!
 //! * We must store the value somewhere in the memory to create a reference to
 //!   it. The value must be readable by a single load. However for Protobuf
 //!   fields it happens that the actual memory representation of a value differs
 //!   from what users expect and it is an implementation detail that can change
 //!   as more optimizations are implemented. For example, rarely accessed
 //!   `int64` fields can be represented in a packed format with 32 bits for the
 //!   value in the common case. Or, a single logical value can be spread across
 //!   multiple memory locations. For example, presence information for all the
 //!   fields in a protobuf message is centralized in a bitset.
 //! * We cannot store extra data on the reference that might be necessary for
 //!   correctly manipulating it (and custom-metadata DSTs do not exist yet in
 //!   Rust). Concretely, messages, string, bytes, and repeated fields in UPB
 //!   need to carry around an arena parameter separate from the data pointer to
 //!   enable mutation (for example adding an element to a repeated field) or
 //!   potentially to enable optimizations (for example referencing a string
 //!   value using a Cord-like type instead of copying it if the source and
 //!   target messages are on the same arena already). Mutable references to
 //!   messages have one additional drawback: Rust allows users to
 //!   indiscriminately run a bytewise swap() on mutable references, which could
 //!   result in pointers to the wrong arena winding up on a message. For
 //!   example, imagine swapping a submessage across two root messages allocated
 //!   on distinct arenas A and B; after the swap, the message allocated in A may
 //!   contain pointers from B by way of the submessage, because the swap does
 //!   not know to fix up those pointers as needed. The C++ API uses
 //!   message-owned arenas, and this ends up resembling self-referential types,
 //!   which need `Pin` in order to be sound. However, `Pin` has much stronger
 //!   guarantees than we need to uphold.
 //!
 //! These drawbacks put the "idiomatic Rust" goal in conflict with the
 //! "performance", "evolvability", and "safety" goals. Given the project design
 //! priorities we decided to not use plain Rust references. Instead, we
 //! implemented the concept of "proxy" types. Proxy types are a reference-like
 //! indirection between the user and the internal memory representation.

 use crate::__internal::Private;
 use std::fmt::Debug;

 /// A type that can be accessed through a reference-like proxy.
 ///
 /// An instance of a `Proxied` can be accessed immutably via `Proxied::View`.
 ///
 /// All Protobuf field types implement `Proxied`.
 pub trait Proxied: Sized {
     /// The proxy type that provides shared access to a `T`, like a `&'msg T`.
     ///
     /// Most code should use the type alias [`View`].
     type View<'msg>: ViewProxy<'msg, Proxied = Self> + Copy + Send
     where
         Self: 'msg;
 }

 /// A type that can be be accessed through a reference-like proxy.
 ///
 /// An instance of a `MutProxied` can be accessed mutably via `MutProxied::Mut`
 /// and immutably via `MutProxied::View`.
 ///
 /// `MutProxied` is implemented by message, map and repeated field types.
 pub trait MutProxied: Proxied {
     /// The proxy type that provides exclusive mutable access to a `T`, like a
     /// `&'msg mut T`.
     ///
     /// Most code should use the type alias [`Mut`].
     type Mut<'msg>: MutProxy<'msg, Proxied = Self>
     where
         Self: 'msg;
 }

 /// A proxy type that provides shared access to a `T`, like a `&'msg T`.
 ///
 /// This is more concise than fully spelling the associated type.
 #[allow(dead_code)]
 pub type View<'msg, T> = <T as Proxied>::View<'msg>;

 /// A proxy type that provides exclusive mutable access to a `T`, like a
 /// `&'msg mut T`.
 ///
 /// This is more concise than fully spelling the associated type.
 #[allow(dead_code)]
 pub type Mut<'msg, T> = <T as MutProxied>::Mut<'msg>;

 /// Declares conversion operations common to all views.
 ///
 /// This trait is intentionally made non-object-safe to prevent a potential
 /// future incompatible change.
 pub trait ViewProxy<'msg>: 'msg + Sync + Unpin + Sized + Debug {
     type Proxied: 'msg + Proxied + ?Sized;

     /// Converts a borrow into a `View` with the lifetime of that borrow.
     ///
     /// In non-generic code we don't need to use `as_view` because the proxy
     /// types are covariant over `'msg`. However, generic code conservatively
     /// treats `'msg` as [invariant], therefore we need to call
     /// `as_view` to explicitly perform the operation that in concrete code
     /// coercion would perform implicitly.
     ///
     /// For example, the call to `.as_view()` in the following snippet
     /// wouldn't be necessary in concrete code:
     /// ```
     /// fn reborrow<'a, 'b, T>(x: &'b View<'a, T>) -> View<'b, T>
     /// where 'a: 'b, T: Proxied
     /// {
     ///   x.as_view()
     /// }
     /// ```
     ///
     /// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
     fn as_view(&self) -> View<'_, Self::Proxied>;

     /// Converts into a `View` with a potentially shorter lifetime.
     ///
     /// In non-generic code we don't need to use `into_view` because the proxy
     /// types are covariant over `'msg`. However, generic code conservatively
     /// treats `'msg` as [invariant], therefore we need to call
     /// `into_view` to explicitly perform the operation that in concrete
     /// code coercion would perform implicitly.
     ///
     /// ```
     /// fn reborrow_generic_view_into_view<'a, 'b, T>(
     ///     x: View<'a, T>,
     ///     y: View<'b, T>,
     /// ) -> [View<'b, T>; 2]
     /// where
     ///     T: MutProxied,
     ///     'a: 'b,
     /// {
     ///     // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
     ///     // lifetime parameter is (conservatively) invariant.
     ///     // `[x.as_view(), y]` fails because that borrow cannot outlive `'b`.
     ///     [x.into_view(), y]
     /// }
     /// ```
     ///
     /// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
     fn into_view<'shorter>(self) -> View<'shorter, Self::Proxied>
     where
         'msg: 'shorter;
 }

 /// Declares operations common to all mutators.
 ///
 /// This trait is intentionally made non-object-safe to prevent a potential
 /// future incompatible change.
 pub trait MutProxy<'msg>: ViewProxy<'msg>
 where
     Self::Proxied: MutProxied,
 {
     /// Gets an immutable view of this field. This is shorthand for `as_view`.
     ///
     /// This provides a shorter lifetime than `into_view` but can also be called
     /// multiple times - if the result of `get` is not living long enough
     /// for your use, use that instead.
     fn get(&self) -> View<'_, Self::Proxied> {
         self.as_view()
     }

     /// Converts a borrow into a `Mut` with the lifetime of that borrow.
     ///
     /// This function enables calling multiple methods consuming `self`, for
     /// example:
     ///
     /// ```ignore
     ///   let mut sub: Mut<SubMsg> = msg.submsg_mut();
     ///   sub.as_mut().field_x_mut().set(10);  // field_x_mut is fn(self)
     ///   sub.field_y_mut().set(20);  // `sub` is now consumed
     /// ```
     ///
     /// `as_mut` is also useful in generic code to explicitly perform the
     /// operation that in concrete code coercion would perform implicitly.
     fn as_mut(&mut self) -> Mut<'_, Self::Proxied>;

     /// Converts into a `Mut` with a potentially shorter lifetime.
     ///
     /// In non-generic code we don't need to use `into_mut` because the proxy
     /// types are covariant over `'msg`. However, generic code conservatively
     /// treats `'msg` as [invariant], therefore we need to call
     /// `into_mut` to explicitly perform the operation that in concrete code
     /// coercion would perform implicitly.
     ///
     /// ```
     /// fn reborrow_generic_mut_into_mut<'a, 'b, T>(x: Mut<'a, T>, y: Mut<'b, T>) -> [Mut<'b, T>; 2]
     /// where
     ///     T: Proxied,
     ///     'a: 'b,
     /// {
     ///     // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `Mut`
     ///     // lifetime parameter is (conservatively) invariant.
     ///     // `[x.as_mut(), y]` fails because that borrow cannot outlive `'b`.
     ///     [x.into_mut(), y]
     /// }
     /// ```
     ///
     /// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
     fn into_mut<'shorter>(self) -> Mut<'shorter, Self::Proxied>
     where
         'msg: 'shorter;
 }

 /// A value to `Proxied`-value conversion that consumes the input value.
 ///
 /// All setter functions accept types that implement `IntoProxied`. The purpose
 /// of `IntoProxied` is to allow setting arbitrary values on Protobuf fields
 /// with the minimal number of copies.
 ///
 /// This trait must not be implemented on types outside the Protobuf codegen and
 /// runtime. We expect it to change in backwards incompatible ways in the
 /// future.
 pub trait IntoProxied<T: Proxied> {
     fn into_proxied(self, _private: Private) -> T;
 }

 #[cfg(test)]
 mod tests {
     use super::*;
     use googletest::prelude::*;

     #[derive(Debug, Default, PartialEq)]
     struct MyProxied {
         val: String,
     }

     impl MyProxied {
         fn as_view(&self) -> View<'_, Self> {
             MyProxiedView { my_proxied_ref: self }
         }

         fn as_mut(&mut self) -> Mut<'_, Self> {
             MyProxiedMut { my_proxied_ref: self }
         }
     }

     impl Proxied for MyProxied {
         type View<'msg> = MyProxiedView<'msg>;
     }

     impl MutProxied for MyProxied {
         type Mut<'msg> = MyProxiedMut<'msg>;
     }

     #[derive(Debug, Clone, Copy)]
     struct MyProxiedView<'msg> {
         my_proxied_ref: &'msg MyProxied,
     }

     impl MyProxiedView<'_> {
         fn val(&self) -> &str {
             &self.my_proxied_ref.val
         }
     }

     impl<'msg> ViewProxy<'msg> for MyProxiedView<'msg> {
         type Proxied = MyProxied;

         fn as_view(&self) -> View<'msg, MyProxied> {
             *self
         }

         fn into_view<'shorter>(self) -> View<'shorter, MyProxied>
         where
             'msg: 'shorter,
         {
             self
         }
     }

     #[derive(Debug)]
     struct MyProxiedMut<'msg> {
         my_proxied_ref: &'msg mut MyProxied,
     }

     impl<'msg> ViewProxy<'msg> for MyProxiedMut<'msg> {
         type Proxied = MyProxied;

         fn as_view(&self) -> View<'_, MyProxied> {
             MyProxiedView { my_proxied_ref: self.my_proxied_ref }
         }
         fn into_view<'shorter>(self) -> View<'shorter, MyProxied>
         where
             'msg: 'shorter,
         {
             MyProxiedView { my_proxied_ref: self.my_proxied_ref }
         }
     }

     impl<'msg> MutProxy<'msg> for MyProxiedMut<'msg> {
         fn as_mut(&mut self) -> Mut<'_, MyProxied> {
             MyProxiedMut { my_proxied_ref: self.my_proxied_ref }
         }

         fn into_mut<'shorter>(self) -> Mut<'shorter, MyProxied>
         where
             'msg: 'shorter,
         {
             self
         }
     }

     #[test]
     fn test_as_view() {
         let my_proxied = MyProxied { val: "Hello World".to_string() };

         let my_view = my_proxied.as_view();

         assert_that!(my_view.val(), eq(&my_proxied.val));
     }

     fn reborrow_mut_into_view<'msg>(x: Mut<'msg, MyProxied>) -> View<'msg, MyProxied> {
         // x.as_view() fails to compile with:
         //   `ERROR: attempt to return function-local borrowed content`
         x.into_view() // OK: we return the same lifetime as we got in.
     }

     #[test]
     fn test_mut_into_view() {
         let mut my_proxied = MyProxied { val: "Hello World".to_string() };
         reborrow_mut_into_view(my_proxied.as_mut());
     }

     fn require_unified_lifetimes<'msg>(_x: Mut<'msg, MyProxied>, _y: View<'msg, MyProxied>) {}

     #[test]
     fn test_require_unified_lifetimes() {
         let mut my_proxied = MyProxied { val: "Hello1".to_string() };
         let my_mut = my_proxied.as_mut();

         {
             let other_proxied = MyProxied { val: "Hello2".to_string() };
             let other_view = other_proxied.as_view();
             require_unified_lifetimes(my_mut, other_view);
         }
     }

     fn reborrow_generic_as_view<'a, 'b, T>(
         x: &'b mut Mut<'a, T>,
         y: &'b View<'a, T>,
     ) -> [View<'b, T>; 2]
     where
         T: MutProxied,
         'a: 'b,
     {
         // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
         // lifetime parameter is (conservatively) invariant.
         [x.as_view(), y.as_view()]
     }

     #[test]
     fn test_reborrow_generic_as_view() {
         let mut my_proxied = MyProxied { val: "Hello1".to_string() };
         let mut my_mut = my_proxied.as_mut();
         let my_ref = &mut my_mut;

         {
             let other_proxied = MyProxied { val: "Hello2".to_string() };
             let other_view = other_proxied.as_view();
             reborrow_generic_as_view::<MyProxied>(my_ref, &other_view);
         }
     }

     fn reborrow_generic_view_into_view<'a, 'b, T>(
         x: View<'a, T>,
         y: View<'b, T>,
     ) -> [View<'b, T>; 2]
     where
         T: Proxied,
         'a: 'b,
     {
         // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
         // lifetime parameter is (conservatively) invariant.
         // `[x.as_view(), y]` fails because that borrow cannot outlive `'b`.
         [x.into_view(), y]
     }

     #[test]
     fn test_reborrow_generic_into_view() {
         let my_proxied = MyProxied { val: "Hello1".to_string() };
         let my_view = my_proxied.as_view();

         {
             let other_proxied = MyProxied { val: "Hello2".to_string() };
             let other_view = other_proxied.as_view();
             reborrow_generic_view_into_view::<MyProxied>(my_view, other_view);
         }
     }

     fn reborrow_generic_mut_into_view<'a, 'b, T>(x: Mut<'a, T>, y: View<'b, T>) -> [View<'b, T>; 2]
     where
         T: MutProxied,
         'a: 'b,
     {
         [x.into_view(), y]
     }

     #[test]
     fn test_reborrow_generic_mut_into_view() {
         let mut my_proxied = MyProxied { val: "Hello1".to_string() };
         let my_mut = my_proxied.as_mut();

         {
             let other_proxied = MyProxied { val: "Hello2".to_string() };
             let other_view = other_proxied.as_view();
             reborrow_generic_mut_into_view::<MyProxied>(my_mut, other_view);
         }
     }

     fn reborrow_generic_mut_into_mut<'a, 'b, T>(x: Mut<'a, T>, y: Mut<'b, T>) -> [Mut<'b, T>; 2]
     where
         T: MutProxied,
         'a: 'b,
     {
         // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `Mut`
         // lifetime parameter is (conservatively) invariant.
         // `[x.as_mut(), y]` fails because that borrow cannot outlive `'b`.
         [x.into_mut(), y]
     }

     #[test]
     fn test_reborrow_generic_mut_into_mut() {
         let mut my_proxied = MyProxied { val: "Hello1".to_string() };
         let my_mut = my_proxied.as_mut();

         {
             let mut other_proxied = MyProxied { val: "Hello2".to_string() };
             let other_mut = other_proxied.as_mut();
             // No need to reborrow, even though lifetime of &other_view is different
             // than the lifetiem of my_ref. Rust references are covariant over their
             // lifetime.
             reborrow_generic_mut_into_mut::<MyProxied>(my_mut, other_mut);
         }
     }
 }
	// Protocol Buffers - Google's data interchange format
	// Copyright 2023 Google LLC. All rights reserved.
	//
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file or at
	// https://developers.google.com/open-source/licenses/bsd

	//! Operating on borrowed data owned by a message is a central concept in
	//! Protobuf (and Rust in general). The way this is normally accomplished in
	//! Rust is to pass around references and operate on those. Unfortunately,
	//! references come with two major drawbacks:
	//!
	//! * We must store the value somewhere in the memory to create a reference to
	//! it. The value must be readable by a single load. However for Protobuf
	//! fields it happens that the actual memory representation of a value differs
	//! from what users expect and it is an implementation detail that can change
	//! as more optimizations are implemented. For example, rarely accessed
	//! `int64` fields can be represented in a packed format with 32 bits for the
	//! value in the common case. Or, a single logical value can be spread across
	//! multiple memory locations. For example, presence information for all the
	//! fields in a protobuf message is centralized in a bitset.
	//! * We cannot store extra data on the reference that might be necessary for
	//! correctly manipulating it (and custom-metadata DSTs do not exist yet in
	//! Rust). Concretely, messages, string, bytes, and repeated fields in UPB
	//! need to carry around an arena parameter separate from the data pointer to
	//! enable mutation (for example adding an element to a repeated field) or
	//! potentially to enable optimizations (for example referencing a string
	//! value using a Cord-like type instead of copying it if the source and
	//! target messages are on the same arena already). Mutable references to
	//! messages have one additional drawback: Rust allows users to
	//! indiscriminately run a bytewise swap() on mutable references, which could
	//! result in pointers to the wrong arena winding up on a message. For
	//! example, imagine swapping a submessage across two root messages allocated
	//! on distinct arenas A and B; after the swap, the message allocated in A may
	//! contain pointers from B by way of the submessage, because the swap does
	//! not know to fix up those pointers as needed. The C++ API uses
	//! message-owned arenas, and this ends up resembling self-referential types,
	//! which need `Pin` in order to be sound. However, `Pin` has much stronger
	//! guarantees than we need to uphold.
	//!
	//! These drawbacks put the "idiomatic Rust" goal in conflict with the
	//! "performance", "evolvability", and "safety" goals. Given the project design
	//! priorities we decided to not use plain Rust references. Instead, we
	//! implemented the concept of "proxy" types. Proxy types are a reference-like
	//! indirection between the user and the internal memory representation.

	use crate::__internal::Private;
	use std::fmt::Debug;

	/// A type that can be accessed through a reference-like proxy.
	///
	/// An instance of a `Proxied` can be accessed immutably via `Proxied::View`.
	///
	/// All Protobuf field types implement `Proxied`.
	pub trait Proxied: Sized {
	/// The proxy type that provides shared access to a `T`, like a `&'msg T`.
	///
	/// Most code should use the type alias [`View`].
	type View<'msg>: ViewProxy<'msg, Proxied = Self> + Copy + Send
	where
	Self: 'msg;
	}

	/// A type that can be be accessed through a reference-like proxy.
	///
	/// An instance of a `MutProxied` can be accessed mutably via `MutProxied::Mut`
	/// and immutably via `MutProxied::View`.
	///
	/// `MutProxied` is implemented by message, map and repeated field types.
	pub trait MutProxied: Proxied {
	/// The proxy type that provides exclusive mutable access to a `T`, like a
	/// `&'msg mut T`.
	///
	/// Most code should use the type alias [`Mut`].
	type Mut<'msg>: MutProxy<'msg, Proxied = Self>
	where
	Self: 'msg;
	}

	/// A proxy type that provides shared access to a `T`, like a `&'msg T`.
	///
	/// This is more concise than fully spelling the associated type.
	#[allow(dead_code)]
	pub type View<'msg, T> = <T as Proxied>::View<'msg>;

	/// A proxy type that provides exclusive mutable access to a `T`, like a
	/// `&'msg mut T`.
	///
	/// This is more concise than fully spelling the associated type.
	#[allow(dead_code)]
	pub type Mut<'msg, T> = <T as MutProxied>::Mut<'msg>;

	/// Declares conversion operations common to all views.
	///
	/// This trait is intentionally made non-object-safe to prevent a potential
	/// future incompatible change.
	pub trait ViewProxy<'msg>: 'msg + Sync + Unpin + Sized + Debug {
	type Proxied: 'msg + Proxied + ?Sized;

	/// Converts a borrow into a `View` with the lifetime of that borrow.
	///
	/// In non-generic code we don't need to use `as_view` because the proxy
	/// types are covariant over `'msg`. However, generic code conservatively
	/// treats `'msg` as [invariant], therefore we need to call
	/// `as_view` to explicitly perform the operation that in concrete code
	/// coercion would perform implicitly.
	///
	/// For example, the call to `.as_view()` in the following snippet
	/// wouldn't be necessary in concrete code:
	/// ```
	/// fn reborrow<'a, 'b, T>(x: &'b View<'a, T>) -> View<'b, T>
	/// where 'a: 'b, T: Proxied
	/// {
	/// x.as_view()
	/// }
	/// ```
	///
	/// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
	fn as_view(&self) -> View<'_, Self::Proxied>;

	/// Converts into a `View` with a potentially shorter lifetime.
	///
	/// In non-generic code we don't need to use `into_view` because the proxy
	/// types are covariant over `'msg`. However, generic code conservatively
	/// treats `'msg` as [invariant], therefore we need to call
	/// `into_view` to explicitly perform the operation that in concrete
	/// code coercion would perform implicitly.
	///
	/// ```
	/// fn reborrow_generic_view_into_view<'a, 'b, T>(
	/// x: View<'a, T>,
	/// y: View<'b, T>,
	/// ) -> [View<'b, T>; 2]
	/// where
	/// T: MutProxied,
	/// 'a: 'b,
	/// {
	/// // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
	/// // lifetime parameter is (conservatively) invariant.
	/// // `[x.as_view(), y]` fails because that borrow cannot outlive `'b`.
	/// [x.into_view(), y]
	/// }
	/// ```
	///
	/// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
	fn into_view<'shorter>(self) -> View<'shorter, Self::Proxied>
	where
	'msg: 'shorter;
	}

	/// Declares operations common to all mutators.
	///
	/// This trait is intentionally made non-object-safe to prevent a potential
	/// future incompatible change.
	pub trait MutProxy<'msg>: ViewProxy<'msg>
	where
	Self::Proxied: MutProxied,
	{
	/// Gets an immutable view of this field. This is shorthand for `as_view`.
	///
	/// This provides a shorter lifetime than `into_view` but can also be called
	/// multiple times - if the result of `get` is not living long enough
	/// for your use, use that instead.
	fn get(&self) -> View<'_, Self::Proxied> {
	self.as_view()
	}

	/// Converts a borrow into a `Mut` with the lifetime of that borrow.
	///
	/// This function enables calling multiple methods consuming `self`, for
	/// example:
	///
	/// ```ignore
	/// let mut sub: Mut<SubMsg> = msg.submsg_mut();
	/// sub.as_mut().field_x_mut().set(10); // field_x_mut is fn(self)
	/// sub.field_y_mut().set(20); // `sub` is now consumed
	/// ```
	///
	/// `as_mut` is also useful in generic code to explicitly perform the
	/// operation that in concrete code coercion would perform implicitly.
	fn as_mut(&mut self) -> Mut<'_, Self::Proxied>;

	/// Converts into a `Mut` with a potentially shorter lifetime.
	///
	/// In non-generic code we don't need to use `into_mut` because the proxy
	/// types are covariant over `'msg`. However, generic code conservatively
	/// treats `'msg` as [invariant], therefore we need to call
	/// `into_mut` to explicitly perform the operation that in concrete code
	/// coercion would perform implicitly.
	///
	/// ```
	/// fn reborrow_generic_mut_into_mut<'a, 'b, T>(x: Mut<'a, T>, y: Mut<'b, T>) -> [Mut<'b, T>; 2]
	/// where
	/// T: Proxied,
	/// 'a: 'b,
	/// {
	/// // `[x, y]` fails to compile because `'a` is not the same as `'b` and the `Mut`
	/// // lifetime parameter is (conservatively) invariant.
	/// // `[x.as_mut(), y]` fails because that borrow cannot outlive `'b`.
	/// [x.into_mut(), y]
	/// }
	/// ```
	///
	/// [invariant]: https://doc.rust-lang.org/nomicon/subtyping.html#variance
	fn into_mut<'shorter>(self) -> Mut<'shorter, Self::Proxied>
	where
	'msg: 'shorter;
	}

	/// A value to `Proxied`-value conversion that consumes the input value.
	///
	/// All setter functions accept types that implement `IntoProxied`. The purpose
	/// of `IntoProxied` is to allow setting arbitrary values on Protobuf fields
	/// with the minimal number of copies.
	///
	/// This trait must not be implemented on types outside the Protobuf codegen and
	/// runtime. We expect it to change in backwards incompatible ways in the
	/// future.
	pub trait IntoProxied<T: Proxied> {
	fn into_proxied(self, _private: Private) -> T;
	}

	#[cfg(test)]
	mod tests {
	use super::*;
	use googletest::prelude::*;

	#[derive(Debug, Default, PartialEq)]
	struct MyProxied {
	val: String,
	}

	impl MyProxied {
	fn as_view(&self) -> View<'_, Self> {
	MyProxiedView { my_proxied_ref: self }
	}

	fn as_mut(&mut self) -> Mut<'_, Self> {
	MyProxiedMut { my_proxied_ref: self }
	}
	}

	impl Proxied for MyProxied {
	type View<'msg> = MyProxiedView<'msg>;
	}

	impl MutProxied for MyProxied {
	type Mut<'msg> = MyProxiedMut<'msg>;
	}

	#[derive(Debug, Clone, Copy)]
	struct MyProxiedView<'msg> {
	my_proxied_ref: &'msg MyProxied,
	}

	impl MyProxiedView<'_> {
	fn val(&self) -> &str {
	&self.my_proxied_ref.val
	}
	}

	impl<'msg> ViewProxy<'msg> for MyProxiedView<'msg> {
	type Proxied = MyProxied;

	fn as_view(&self) -> View<'msg, MyProxied> {
	*self
	}

	fn into_view<'shorter>(self) -> View<'shorter, MyProxied>
	where
	'msg: 'shorter,
	{
	self
	}
	}

	#[derive(Debug)]
	struct MyProxiedMut<'msg> {
	my_proxied_ref: &'msg mut MyProxied,
	}

	impl<'msg> ViewProxy<'msg> for MyProxiedMut<'msg> {
	type Proxied = MyProxied;

	fn as_view(&self) -> View<'_, MyProxied> {
	MyProxiedView { my_proxied_ref: self.my_proxied_ref }
	}
	fn into_view<'shorter>(self) -> View<'shorter, MyProxied>
	where
	'msg: 'shorter,
	{
	MyProxiedView { my_proxied_ref: self.my_proxied_ref }
	}
	}

	impl<'msg> MutProxy<'msg> for MyProxiedMut<'msg> {
	fn as_mut(&mut self) -> Mut<'_, MyProxied> {
	MyProxiedMut { my_proxied_ref: self.my_proxied_ref }
	}

	fn into_mut<'shorter>(self) -> Mut<'shorter, MyProxied>
	where
	'msg: 'shorter,
	{
	self
	}
	}

	#[test]
	fn test_as_view() {
	let my_proxied = MyProxied { val: "Hello World".to_string() };

	let my_view = my_proxied.as_view();

	assert_that!(my_view.val(), eq(&my_proxied.val));
	}

	fn reborrow_mut_into_view<'msg>(x: Mut<'msg, MyProxied>) -> View<'msg, MyProxied> {
	// x.as_view() fails to compile with:
	// `ERROR: attempt to return function-local borrowed content`
	x.into_view() // OK: we return the same lifetime as we got in.
	}

	#[test]
	fn test_mut_into_view() {
	let mut my_proxied = MyProxied { val: "Hello World".to_string() };
	reborrow_mut_into_view(my_proxied.as_mut());
	}

	fn require_unified_lifetimes<'msg>(_x: Mut<'msg, MyProxied>, _y: View<'msg, MyProxied>) {}

	#[test]
	fn test_require_unified_lifetimes() {
	let mut my_proxied = MyProxied { val: "Hello1".to_string() };
	let my_mut = my_proxied.as_mut();

	{
	let other_proxied = MyProxied { val: "Hello2".to_string() };
	let other_view = other_proxied.as_view();
	require_unified_lifetimes(my_mut, other_view);
	}
	}

	fn reborrow_generic_as_view<'a, 'b, T>(
	x: &'b mut Mut<'a, T>,
	y: &'b View<'a, T>,
	) -> [View<'b, T>; 2]
	where
	T: MutProxied,
	'a: 'b,
	{
	// `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
	// lifetime parameter is (conservatively) invariant.
	[x.as_view(), y.as_view()]
	}

	#[test]
	fn test_reborrow_generic_as_view() {
	let mut my_proxied = MyProxied { val: "Hello1".to_string() };
	let mut my_mut = my_proxied.as_mut();
	let my_ref = &mut my_mut;

	{
	let other_proxied = MyProxied { val: "Hello2".to_string() };
	let other_view = other_proxied.as_view();
	reborrow_generic_as_view::<MyProxied>(my_ref, &other_view);
	}
	}

	fn reborrow_generic_view_into_view<'a, 'b, T>(
	x: View<'a, T>,
	y: View<'b, T>,
	) -> [View<'b, T>; 2]
	where
	T: Proxied,
	'a: 'b,
	{
	// `[x, y]` fails to compile because `'a` is not the same as `'b` and the `View`
	// lifetime parameter is (conservatively) invariant.
	// `[x.as_view(), y]` fails because that borrow cannot outlive `'b`.
	[x.into_view(), y]
	}

	#[test]
	fn test_reborrow_generic_into_view() {
	let my_proxied = MyProxied { val: "Hello1".to_string() };
	let my_view = my_proxied.as_view();

	{
	let other_proxied = MyProxied { val: "Hello2".to_string() };
	let other_view = other_proxied.as_view();
	reborrow_generic_view_into_view::<MyProxied>(my_view, other_view);
	}
	}

	fn reborrow_generic_mut_into_view<'a, 'b, T>(x: Mut<'a, T>, y: View<'b, T>) -> [View<'b, T>; 2]
	where
	T: MutProxied,
	'a: 'b,
	{
	[x.into_view(), y]
	}

	#[test]
	fn test_reborrow_generic_mut_into_view() {
	let mut my_proxied = MyProxied { val: "Hello1".to_string() };
	let my_mut = my_proxied.as_mut();

	{
	let other_proxied = MyProxied { val: "Hello2".to_string() };
	let other_view = other_proxied.as_view();
	reborrow_generic_mut_into_view::<MyProxied>(my_mut, other_view);
	}
	}

	fn reborrow_generic_mut_into_mut<'a, 'b, T>(x: Mut<'a, T>, y: Mut<'b, T>) -> [Mut<'b, T>; 2]
	where
	T: MutProxied,
	'a: 'b,
	{
	// `[x, y]` fails to compile because `'a` is not the same as `'b` and the `Mut`
	// lifetime parameter is (conservatively) invariant.
	// `[x.as_mut(), y]` fails because that borrow cannot outlive `'b`.
	[x.into_mut(), y]
	}

	#[test]
	fn test_reborrow_generic_mut_into_mut() {
	let mut my_proxied = MyProxied { val: "Hello1".to_string() };
	let my_mut = my_proxied.as_mut();

	{
	let mut other_proxied = MyProxied { val: "Hello2".to_string() };
	let other_mut = other_proxied.as_mut();
	// No need to reborrow, even though lifetime of &other_view is different
	// than the lifetiem of my_ref. Rust references are covariant over their
	// lifetime.
	reborrow_generic_mut_into_mut::<MyProxied>(my_mut, other_mut);
	}
	}
	}