lib: make InPlaceInit available even without alloc feature

The only dependency of the InPlaceInit trait on the allocator API is the
AllocError type.  Replace it with Infallible instead, i.e. allow any
error as long as it has an "impl From<Infallible> for MyError" - which
can have a trivial implementation as seen in examples/rror.rs.

While admittedly of limited usefulness due to orphan rules, this is
a first step towards allowing usage of pinned_init entirely without
the allocator API, and therefore on stable Rust.

Signed-off-by: Paolo Bonzini <[email protected]>

src/lib.rs

@@ -258,6 +258,11 @@ use core::{
 #[cfg(feature = "alloc")]
 use core::alloc::AllocError;
 
+// Allocations are infallible without the allocator API.  In that case, just
+// require From<Infallible> for the trait that is passed to the try_* macros,
+#[cfg(not(feature = "alloc"))]
+type AllocError = Infallible;
+
 #[doc(hidden)]
 pub mod __internal;
 #[doc(hidden)]
@@ -1091,7 +1096,6 @@ unsafe impl<T, E> PinInit<T, E> for T {
 }
 
 /// Smart pointer that can initialize memory in-place.
-#[cfg(feature = "alloc")]
 pub trait InPlaceInit<T>: Sized {
     /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
     /// type.