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>

rust/Makefile

@@ -50,6 +50,7 @@ core-cfgs = \
     --cfg no_fp_fmt_parse
 
 alloc-cfgs = \
+    --cfg no_borrow \
     --cfg no_fmt \
     --cfg no_global_oom_handling \
     --cfg no_macros \

rust/alloc/lib.rs

@@ -100,7 +100,7 @@
 #![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
 #![feature(const_box)]
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
-#![feature(const_cow_is_borrowed)]
+#![cfg_attr(not(no_borrow), feature(const_cow_is_borrowed))]
 #![feature(const_convert)]
 #![feature(const_size_of_val)]
 #![feature(const_align_of_val)]
@@ -215,6 +215,7 @@
 mod boxed {
     pub use std::boxed::Box;
 }
+#[cfg(not(no_borrow))]
 pub mod borrow;
 pub mod collections;
 #[cfg(not(no_global_oom_handling))]

rust/alloc/vec/mod.rs

@@ -72,6 +72,7 @@
 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 @@
 
 mod drain;
 
+#[cfg(not(no_borrow))]
 #[cfg(not(no_global_oom_handling))]
 mod cow;
 
@@ -3103,6 +3105,7 @@ fn from(s: &mut [T]) -> 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