[PATCH 1/2] pin-init: add PinInit::map_err() for error type conversion

[Danilo Krummrich <dakr@kernel.org>]

Add a map_err() method to PinInit that converts the error type of an
initializer. This is useful when combining initializers with different
error types, for example when an infallible initializer (error type
Infallible) needs to be used in a context that expects a different error
type.

The MapErr wrapper stores the original initializer and mapping function,
implementing PinInit and Init with the appropriate trait bounds on the
original initializer.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/pin-init/src/lib.rs | 44 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index fd40c8f244a1..deaf65147e99 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -949,6 +949,18 @@ fn pin_chain<F>(self, f: F) -> ChainPinInit<Self, F, T, E>
     {
         ChainPinInit(self, f, __internal::PhantomInvariant::new())
     }
+
+    /// Convert the error type of this pin-initializer.
+    ///
+    /// This is useful when combining initializers with different error types, for example
+    /// when an infallible initializer (error type [`Infallible`]) needs to be used in a context
+    /// that expects a different error type.
+    fn map_err<E2, F>(self, f: F) -> MapErr<Self, F, T, E>
+    where
+        F: FnOnce(E) -> E2,
+    {
+        MapErr(self, f, __internal::PhantomInvariant::new())
+    }
 }
 
 /// An initializer returned by [`PinInit::pin_chain`].
@@ -975,6 +987,38 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
     }
 }
 
+/// An initializer returned by [`PinInit::map_err`].
+pub struct MapErr<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,
+// - returns `Err(err)` on error and in this case `slot` will be dropped.
+// - considers `slot` pinned.
+unsafe impl<T: ?Sized, E, E2, I, F> PinInit<T, E2> for MapErr<I, F, T, E>
+where
+    I: PinInit<T, E>,
+    F: FnOnce(E) -> E2,
+{
+    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E2> {
+        // SAFETY: All requirements fulfilled since this function is `__pinned_init`.
+        unsafe { self.0.__pinned_init(slot).map_err(self.1) }
+    }
+}
+
+// SAFETY: The `__init` function is implemented such that it
+// - returns `Ok(())` on successful initialization,
+// - returns `Err(err)` on error and in this case `slot` will be dropped.
+unsafe impl<T: ?Sized, E, E2, I, F> Init<T, E2> for MapErr<I, F, T, E>
+where
+    I: Init<T, E>,
+    F: FnOnce(E) -> E2,
+{
+    unsafe fn __init(self, slot: *mut T) -> Result<(), E2> {
+        // SAFETY: All requirements fulfilled since this function is `__init`.
+        unsafe { self.0.__init(slot).map_err(self.1) }
+    }
+}
+
 /// An initializer for `T`.
 ///
 /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
-- 
2.54.0