rust: alloc: remove the `borrow` module (`ToOwned`, `Cow`)

The `Cow` type [1] requires that its generic parameter type implements
the `ToOwned` trait [2], which provides a method to create owned data
from borrowed data, usually by cloning.

However, it is infallible, and thus in most cases it is not useful for
the kernel. [3]

Therefore, introduce `cfg(no_borrow)` to remove the `borrow` module
(which contains `ToOwned` and `Cow`) from `alloc`.

Link: https://doc.rust-lang.org/alloc/borrow/enum.Cow.html [1]
Link: https://doc.rust-lang.org/alloc/borrow/trait.ToOwned.html [2]
Link: https://lore.kernel.org/rust-for-linux/20221204103153.117675b1@GaryWorkstation/ [3]
Cc: Gary Guo <gary@garyguo.net>
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Finn Behrens <fin@nyantec.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/rust/alloc/vec/mod.rs b/rust/alloc/vec/mod.rs
index 8ac6c1e3b2a8..f77c7368d534 100644
--- a/rust/alloc/vec/mod.rs
+++ b/rust/alloc/vec/mod.rs
@@ -72,6 +72,7 @@ use core::ptr::{self, NonNull};
 use core::slice::{self, SliceIndex};
 
 use crate::alloc::{Allocator, Global};
+#[cfg(not(no_borrow))]
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
 use crate::collections::TryReserveError;
@@ -94,6 +95,7 @@ pub use self::drain::Drain;
 
 mod drain;
 
+#[cfg(not(no_borrow))]
 #[cfg(not(no_global_oom_handling))]
 mod cow;
 
@@ -3103,6 +3105,7 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
     }
 }
 
+#[cfg(not(no_borrow))]
 #[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
 impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
 where