rust: adapt `alloc` crate to the kernel

This customizes the subset of the Rust standard library `alloc` that
was just imported as-is, mainly by:

  - Adding SPDX license identifiers.

  - Skipping modules (e.g. `rc` and `sync`) via new `cfg`s.

  - Adding fallible (`try_*`) versions of existing infallible methods
    (i.e. returning a `Result` instead of panicking).

    Since the standard library requires stable/unstable attributes,
    these additions are annotated with:

        #[stable(feature = "kernel", since = "1.0.0")]

    Using "kernel" as the feature allows to have the additions
    clearly marked. The "1.0.0" version is just a placeholder.

    (At the moment, only one is needed, but in the future more
    fallible methods will be added).
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Co-developed-by: Matthew Bakhtiari <dev@mtbk.me>
Signed-off-by: Matthew Bakhtiari <dev@mtbk.me>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/alloc/README.md

+# `alloc`
+
+These source files come from the Rust standard library, hosted in
+the <https://github.com/rust-lang/rust> repository, licensed under
+"Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
+see <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
+
+Please note that these files should be kept as close as possible to
+upstream. In general, only additions should be performed (e.g. new
+methods). Eventually, changes should make it into upstream so that,
+at some point, this fork can be dropped from the kernel tree.
+
+
+## Rationale
+
+On one hand, kernel folks wanted to keep `alloc` in-tree to have more
+freedom in both workflow and actual features if actually needed
+(e.g. receiver types if we ended up using them), which is reasonable.
+
+On the other hand, Rust folks wanted to keep `alloc` as close as
+upstream as possible and avoid as much divergence as possible, which
+is also reasonable.
+
+We agreed on a middle-ground: we would keep a subset of `alloc`
+in-tree that would be as small and as close as possible to upstream.
+Then, upstream can start adding the functions that we add to `alloc`
+etc., until we reach a point where the kernel already knows exactly
+what it needs in `alloc` and all the new methods are merged into
+upstream, so that we can drop `alloc` from the kernel tree and go back
+to using the upstream one.
+
+By doing this, the kernel can go a bit faster now, and Rust can
+slowly incorporate and discuss the changes as needed.

rust/alloc/alloc.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! Memory allocation APIs
 
 #![stable(feature = "alloc_module", since = "1.28.0")]

rust/alloc/borrow.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! A module for working with borrowed data.
 
 #![stable(feature = "rust1", since = "1.0.0")]
@@ -11,7 +13,7 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::borrow::{Borrow, BorrowMut};
 
-use crate::fmt;
+use core::fmt;
 #[cfg(not(no_global_oom_handling))]
 use crate::string::String;
 

rust/alloc/boxed.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! A pointer type for heap allocation.
 //!
 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
@@ -163,9 +165,11 @@
 #[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
 
+#[cfg(not(no_thin))]
 #[unstable(feature = "thin_box", issue = "92791")]
 pub use thin::ThinBox;
 
+#[cfg(not(no_thin))]
 mod thin;
 
 /// A pointer type for heap allocation.

rust/alloc/collections/mod.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! Collection types.
 
 #![stable(feature = "rust1", since = "1.0.0")]

rust/alloc/lib.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! # The Rust core allocation and collections library
 //!
 //! This library provides smart pointers and collections for managing
@@ -192,6 +194,7 @@
 extern crate test;
 
 // Module with internal macros used by other modules (needs to be included before other modules).
+#[cfg(not(no_macros))]
 #[macro_use]
 mod macros;
 
@@ -216,11 +219,16 @@ mod boxed {
 pub mod collections;
 #[cfg(not(no_global_oom_handling))]
 pub mod ffi;
+#[cfg(not(no_fmt))]
 pub mod fmt;
+#[cfg(not(no_rc))]
 pub mod rc;
 pub mod slice;
+#[cfg(not(no_str))]
 pub mod str;
+#[cfg(not(no_string))]
 pub mod string;
+#[cfg(not(no_sync))]
 #[cfg(target_has_atomic = "ptr")]
 pub mod sync;
 #[cfg(all(not(no_global_oom_handling), target_has_atomic = "ptr"))]

rust/alloc/raw_vec.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 #![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
 
 use core::alloc::LayoutError;
@@ -307,6 +309,12 @@ pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryRe
         }
     }
 
+    /// The same as `reserve_for_push`, but returns on errors instead of panicking or aborting.
+    #[inline(never)]
+    pub fn try_reserve_for_push(&mut self, len: usize) -> Result<(), TryReserveError> {
+        self.grow_amortized(len, 1)
+    }
+
     /// Ensures that the buffer contains at least enough space to hold `len +
     /// additional` elements. If it doesn't already, will reallocate the
     /// minimum possible amount of memory necessary. Generally this will be
@@ -421,6 +429,7 @@ fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserve
         Ok(())
     }
 
+    #[allow(dead_code)]
     fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
         assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
 

rust/alloc/slice.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! A dynamically-sized view into a contiguous sequence, `[T]`.
 //!
 //! *[See also the slice primitive type](slice).*

rust/alloc/vec/drain.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::alloc::{Allocator, Global};
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};

rust/alloc/vec/drain_filter.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::alloc::{Allocator, Global};
 use core::ptr::{self};
 use core::slice::{self};

rust/alloc/vec/into_iter.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 #[cfg(not(no_global_oom_handling))]
 use super::AsVecIntoIter;
 use crate::alloc::{Allocator, Global};
@@ -9,6 +11,7 @@
 };
 use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
+#[cfg(not(no_global_oom_handling))]
 use core::ops::Deref;
 use core::ptr::{self, NonNull};
 use core::slice::{self};
@@ -123,6 +126,7 @@ pub(super) fn forget_allocation_drop_remaining(&mut self) {
     }
 
     /// Forgets to Drop the remaining elements while still allowing the backing allocation to be freed.
+    #[allow(dead_code)]
     pub(crate) fn forget_remaining_elements(&mut self) {
         self.ptr = self.end;
     }

rust/alloc/vec/is_zero.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::boxed::Box;
 
 #[rustc_specialization_trait]

rust/alloc/vec/mod.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! A contiguous growable array type with heap-allocated contents, written
 //! `Vec<T>`.
 //!
@@ -1739,6 +1741,29 @@ pub fn push(&mut self, value: T) {
         }
     }
 
+    /// Tries to append an element to the back of a collection.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2];
+    /// vec.try_push(3).unwrap();
+    /// assert_eq!(vec, [1, 2, 3]);
+    /// ```
+    #[inline]
+    #[stable(feature = "kernel", since = "1.0.0")]
+    pub fn try_push(&mut self, value: T) -> Result<(), TryReserveError> {
+        if self.len == self.buf.capacity() {
+            self.buf.try_reserve_for_push(self.len)?;
+        }
+        unsafe {
+            let end = self.as_mut_ptr().add(self.len);
+            ptr::write(end, value);
+            self.len += 1;
+        }
+        Ok(())
+    }
+
     /// Removes the last element from a vector and returns it, or [`None`] if it
     /// is empty.
     ///

rust/alloc/vec/partial_eq.rs

+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use crate::alloc::Allocator;
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;