[PATCH 3/8] rust: pin-init: internal: add `PhantomInvariant` and `PhantomInvariantLifetime`

[Gary Guo <gary@garyguo.net>]

Currently, the `pin_init` library has an `Invariant` type alias, and it is
instantiated using `PhantomData`. Generated code from `pin_data` on the
other hand cannot access the crate-local type alias, so it generates
`PhantomData<fn(T) -> T>` directly. This is all very inconsistent, despite
the exact same use case of ensuring invariance.

Add `PhantomInvariant` and `PhantomInvariantLifetime` and switch all users
that need to express the concept of invariance to use these. They're
polyfills of unstable types in the same names in the Rust standard library.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/internal/src/pin_data.rs | 12 +++-----
 rust/pin-init/src/__internal.rs        | 56 +++++++++++++++++++++++++++++-----
 rust/pin-init/src/lib.rs               | 12 ++++----
 3 files changed, 59 insertions(+), 21 deletions(-)

diff --git a/rust/pin-init/internal/src/pin_data.rs b/rust/pin-init/internal/src/pin_data.rs
index 0199d0143308..44d0bd18e4ae 100644
--- a/rust/pin-init/internal/src/pin_data.rs
+++ b/rust/pin-init/internal/src/pin_data.rs
@@ -180,10 +180,8 @@ struct __Unpin #generics_with_pin_lt
         #where_token
             #predicates
         {
-            __phantom_pin: ::core::marker::PhantomData<fn(&'__pin ()) -> &'__pin ()>,
-            __phantom: ::core::marker::PhantomData<
-                fn(#ident #ty_generics) -> #ident #ty_generics
-            >,
+            __phantom_pin: ::pin_init::__internal::PhantomInvariantLifetime<'__pin>,
+            __phantom: ::pin_init::__internal::PhantomInvariant<#ident #ty_generics>,
             #(#pinned_fields),*
         }
 
@@ -434,9 +432,7 @@ fn generate_the_pin_data(
         #vis struct __ThePinData #generics
             #whr
         {
-            __phantom: ::core::marker::PhantomData<
-                fn(#struct_name #ty_generics) -> #struct_name #ty_generics
-            >,
+            __phantom: ::pin_init::__internal::PhantomInvariant<#struct_name #ty_generics>,
         }
 
         impl #impl_generics ::core::clone::Clone for __ThePinData #ty_generics
@@ -465,7 +461,7 @@ unsafe impl #impl_generics ::pin_init::__internal::HasPinData for #struct_name #
             type PinData = __ThePinData #ty_generics;
 
             unsafe fn __pin_data() -> Self::PinData {
-                __ThePinData { __phantom: ::core::marker::PhantomData }
+                __ThePinData { __phantom: ::pin_init::__internal::PhantomInvariant::new() }
             }
         }
 
diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs
index 5720a621aed7..e54d90a4742e 100644
--- a/rust/pin-init/src/__internal.rs
+++ b/rust/pin-init/src/__internal.rs
@@ -7,20 +7,62 @@
 
 use super::*;
 
-/// See the [nomicon] for what subtyping is. See also [this table].
+/// Zero-sized type used to mark a type as invariant.
+///
+/// This is a polyfill for the [unstable type] in the standard library of the same name.
 ///
-/// The reason for not using `PhantomData<*mut T>` is that that type never implements [`Send`] and
-/// [`Sync`]. Hence `fn(*mut T) -> *mut T` is used, as that type always implements them.
+/// See the [nomicon] for what subtyping is. See also [this table].
 ///
+/// [unstable type]: https://doc.rust-lang.org/nightly/std/marker/struct.PhantomInvariant.html
 /// [nomicon]: https://doc.rust-lang.org/nomicon/subtyping.html
 /// [this table]: https://doc.rust-lang.org/nomicon/phantom-data.html#table-of-phantomdata-patterns
-pub(crate) type Invariant<T> = PhantomData<fn(*mut T) -> *mut T>;
+#[repr(transparent)]
+pub struct PhantomInvariant<T: ?Sized>(PhantomData<fn(T) -> T>);
+
+impl<T: ?Sized> Clone for PhantomInvariant<T> {
+    #[inline(always)]
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: ?Sized> Copy for PhantomInvariant<T> {}
+
+impl<T: ?Sized> Default for PhantomInvariant<T> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T: ?Sized> PhantomInvariant<T> {
+    #[inline(always)]
+    pub const fn new() -> Self {
+        Self(PhantomData)
+    }
+}
+
+/// Zero-sized type used to mark a lifetime as invariant.
+///
+/// This is a polyfill for the [unstable type] in the standard library of the same name.
+///
+/// [unstable type]: https://doc.rust-lang.org/nightly/std/marker/struct.PhantomInvariantLifetime.html
+#[repr(transparent)]
+#[derive(Clone, Copy, Default)]
+pub struct PhantomInvariantLifetime<'a>(PhantomInvariant<&'a ()>);
+
+impl PhantomInvariantLifetime<'_> {
+    #[inline(always)]
+    pub const fn new() -> Self {
+        Self(PhantomInvariant::new())
+    }
+}
 
 /// Module-internal type implementing `PinInit` and `Init`.
 ///
 /// It is unsafe to create this type, since the closure needs to fulfill the same safety
 /// requirement as the `__pinned_init`/`__init` functions.
-pub(crate) struct InitClosure<F, T: ?Sized, E>(pub(crate) F, pub(crate) Invariant<(E, T)>);
+pub(crate) struct InitClosure<F, T: ?Sized, E>(pub(crate) F, pub(crate) PhantomInvariant<(E, T)>);
 
 // SAFETY: While constructing the `InitClosure`, the user promised that it upholds the
 // `__init` invariants.
@@ -126,7 +168,7 @@ fn make_closure<F, E>(self, f: F) -> F
     }
 }
 
-pub struct AllData<T: ?Sized>(Invariant<T>);
+pub struct AllData<T: ?Sized>(PhantomInvariant<T>);
 
 impl<T: ?Sized> Clone for AllData<T> {
     fn clone(&self) -> Self {
@@ -146,7 +188,7 @@ unsafe impl<T: ?Sized> HasInitData for T {
     type InitData = AllData<T>;
 
     unsafe fn __init_data() -> Self::InitData {
-        AllData(PhantomData)
+        AllData(PhantomInvariant::new())
     }
 }
 
diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 80c476e605f7..4098c65d63c3 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -947,12 +947,12 @@ fn pin_chain<F>(self, f: F) -> ChainPinInit<Self, F, T, E>
     where
         F: FnOnce(Pin<&mut T>) -> Result<(), E>,
     {
-        ChainPinInit(self, f, PhantomData)
+        ChainPinInit(self, f, __internal::PhantomInvariant::new())
     }
 }
 
 /// An initializer returned by [`PinInit::pin_chain`].
-pub struct ChainPinInit<I, F, T: ?Sized, E>(I, F, __internal::Invariant<(E, T)>);
+pub struct ChainPinInit<I, F, T: ?Sized, E>(I, F, __internal::PhantomInvariant<(E, T)>);
 
 // SAFETY: The `__pinned_init` function is implemented such that it
 // - returns `Ok(())` on successful initialization,
@@ -1055,12 +1055,12 @@ fn chain<F>(self, f: F) -> ChainInit<Self, F, T, E>
     where
         F: FnOnce(&mut T) -> Result<(), E>,
     {
-        ChainInit(self, f, PhantomData)
+        ChainInit(self, f, __internal::PhantomInvariant::new())
     }
 }
 
 /// An initializer returned by [`Init::chain`].
-pub struct ChainInit<I, F, T: ?Sized, E>(I, F, __internal::Invariant<(E, T)>);
+pub struct ChainInit<I, F, T: ?Sized, E>(I, F, __internal::PhantomInvariant<(E, T)>);
 
 // SAFETY: The `__init` function is implemented such that it
 // - returns `Ok(())` on successful initialization,
@@ -1108,7 +1108,7 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
 pub const unsafe fn pin_init_from_closure<T: ?Sized, E>(
     f: impl FnOnce(*mut T) -> Result<(), E>,
 ) -> impl PinInit<T, E> {
-    __internal::InitClosure(f, PhantomData)
+    __internal::InitClosure(f, __internal::PhantomInvariant::new())
 }
 
 /// Creates a new [`Init<T, E>`] from the given closure.
@@ -1127,7 +1127,7 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
 pub const unsafe fn init_from_closure<T: ?Sized, E>(
     f: impl FnOnce(*mut T) -> Result<(), E>,
 ) -> impl Init<T, E> {
-    __internal::InitClosure(f, PhantomData)
+    __internal::InitClosure(f, __internal::PhantomInvariant::new())
 }
 
 /// Changes the to be initialized type.

-- 
2.51.2