rust: upgrade to Rust 1.77.1

This is the next upgrade to the Rust toolchain, from 1.76.0 to 1.77.1
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4da06e ("rust: upgrade to Rust 1.68.2").

# Unstable features

The `offset_of` feature (single-field `offset_of!`) that we were using
got stabilized in Rust 1.77.0 [3].

Therefore, now the only unstable features allowed to be used outside the
`kernel` crate is `new_uninit`, though other code to be upstreamed may
increase the list.

Please see [4] for details.

# Required changes

Rust 1.77.0 merged the `unused_tuple_struct_fields` lint into `dead_code`,
thus upgrading it from `allow` to `warn` [5]. In turn, this made `rustc`
complain about the `ThisModule`'s pointer field being never read, but
the previous patch adds the `as_ptr` method to it, needed by Binder [6],
so that we do not need to locally `allow` it.

# Other changes

Rust 1.77.0 introduces the `--check-cfg` feature [7], for which there
is a Call for Testing going on [8]. We were requested to test it and
we found it useful [9] -- we will likely enable it in the future.

# `alloc` upgrade and reviewing

The vast majority of changes are due to our `alloc` fork being upgraded
at once.

There are two kinds of changes to be aware of: the ones coming from
upstream, which we should follow as closely as possible, and the updates
needed in our added fallible APIs to keep them matching the newer
infallible APIs coming from upstream.

Instead of taking a look at the diff of this patch, an alternative
approach is reviewing a diff of the changes between upstream `alloc` and
the kernel's. This allows to easily inspect the kernel additions only,
especially to check if the fallible methods we already have still match
the infallible ones in the new version coming from upstream.

Another approach is reviewing the changes introduced in the additions in
the kernel fork between the two versions. This is useful to spot
potentially unintended changes to our additions.

To apply these approaches, one may follow steps similar to the following
to generate a pair of patches that show the differences between upstream
Rust and the kernel (for the subset of `alloc` we use) before and after
applying this patch:

    # Get the difference with respect to the old version.
    git -C rust checkout $(linux/scripts/min-tool-version.sh rustc)
    git -C linux ls-tree -r --name-only HEAD -- rust/alloc |
        cut -d/ -f3- |
        grep -Fv README.md |
        xargs -IPATH cp rust/library/alloc/src/PATH linux/rust/alloc/PATH
    git -C linux diff --patch-with-stat --summary -R > old.patch
    git -C linux restore rust/alloc

    # Apply this patch.
    git -C linux am rust-upgrade.patch

    # Get the difference with respect to the new version.
    git -C rust checkout $(linux/scripts/min-tool-version.sh rustc)
    git -C linux ls-tree -r --name-only HEAD -- rust/alloc |
        cut -d/ -f3- |
        grep -Fv README.md |
        xargs -IPATH cp rust/library/alloc/src/PATH linux/rust/alloc/PATH
    git -C linux diff --patch-with-stat --summary -R > new.patch
    git -C linux restore rust/alloc

Now one may check the `new.patch` to take a look at the additions (first
approach) or at the difference between those two patches (second
approach). For the latter, a side-by-side tool is recommended.

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1770-2024-03-21 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://github.com/rust-lang/rust/pull/118799 [3]
Link: https://github.com/Rust-for-Linux/linux/issues/2 [4]
Link: https://github.com/rust-lang/rust/pull/118297 [5]
Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-2-08ba9197f637@google.com/#Z31rust:kernel:lib.rs [6]
Link: https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html [7]
Link: https://github.com/rust-lang/rfcs/pull/3013#issuecomment-1936648479 [8]
Link: https://github.com/rust-lang/rust/issues/82450#issuecomment-1947462977 [9]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240217002717.57507-1-ojeda@kernel.org
[ Upgraded to 1.77.1. Removed `allow(dead_code)` thanks to the previous
  patch. Reworded accordingly. No changes to `alloc` during the beta. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 69 insertions, 39 deletions

diff --git a/rust/alloc/vec/into_iter.rs b/rust/alloc/vec/into_iter.rs
index 136bfe94af6c..0f11744c44b3 100644
--- a/rust/alloc/vec/into_iter.rs
+++ b/rust/alloc/vec/into_iter.rs
@@ -20,6 +20,17 @@ use core::ops::Deref;
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
+macro non_null {
+    (mut $place:expr, $t:ident) => {{
+        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
+        unsafe { &mut *(ptr::addr_of_mut!($place) as *mut NonNull<$t>) }
+    }},
+    ($place:expr, $t:ident) => {{
+        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
+        unsafe { *(ptr::addr_of!($place) as *const NonNull<$t>) }
+    }},
+}
+
 /// An iterator that moves out of a vector.
 ///
 /// This `struct` is created by the `into_iter` method on [`Vec`](super::Vec)
@@ -43,10 +54,12 @@ pub struct IntoIter<
     // the drop impl reconstructs a RawVec from buf, cap and alloc
     // to avoid dropping the allocator twice we need to wrap it into ManuallyDrop
     pub(super) alloc: ManuallyDrop<A>,
-    pub(super) ptr: *const T,
-    pub(super) end: *const T, // If T is a ZST, this is actually ptr+len. This encoding is picked so that
-                              // ptr == end is a quick test for the Iterator being empty, that works
-                              // for both ZST and non-ZST.
+    pub(super) ptr: NonNull<T>,
+    /// If T is a ZST, this is actually ptr+len. This encoding is picked so that
+    /// ptr == end is a quick test for the Iterator being empty, that works
+    /// for both ZST and non-ZST.
+    /// For non-ZSTs the pointer is treated as `NonNull<T>`
+    pub(super) end: *const T,
 }
 
 #[stable(feature = "vec_intoiter_debug", since = "1.13.0")]
@@ -70,7 +83,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
     /// ```
     #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
     pub fn as_slice(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.ptr, self.len()) }
+        unsafe { slice::from_raw_parts(self.ptr.as_ptr(), self.len()) }
     }
 
     /// Returns the remaining items of this iterator as a mutable slice.
@@ -99,7 +112,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
     }
 
     fn as_raw_mut_slice(&mut self) -> *mut [T] {
-        ptr::slice_from_raw_parts_mut(self.ptr as *mut T, self.len())
+        ptr::slice_from_raw_parts_mut(self.ptr.as_ptr(), self.len())
     }
 
     /// Drops remaining elements and relinquishes the backing allocation.
@@ -126,7 +139,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
         // this creates less assembly
         self.cap = 0;
         self.buf = unsafe { NonNull::new_unchecked(RawVec::NEW.ptr()) };
-        self.ptr = self.buf.as_ptr();
+        self.ptr = self.buf;
         self.end = self.buf.as_ptr();
 
         // Dropping the remaining elements can panic, so this needs to be
@@ -138,9 +151,9 @@ impl<T, A: Allocator> IntoIter<T, A> {
 
     /// Forgets to Drop the remaining elements while still allowing the backing allocation to be freed.
     pub(crate) fn forget_remaining_elements(&mut self) {
-        // For th ZST case, it is crucial that we mutate `end` here, not `ptr`.
+        // For the ZST case, it is crucial that we mutate `end` here, not `ptr`.
         // `ptr` must stay aligned, while `end` may be unaligned.
-        self.end = self.ptr;
+        self.end = self.ptr.as_ptr();
     }
 
     #[cfg(not(no_global_oom_handling))]
@@ -162,7 +175,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
                 // say that they're all at the beginning of the "allocation".
                 0..this.len()
             } else {
-                this.ptr.sub_ptr(buf)..this.end.sub_ptr(buf)
+                this.ptr.sub_ptr(this.buf)..this.end.sub_ptr(buf)
             };
             let cap = this.cap;
             let alloc = ManuallyDrop::take(&mut this.alloc);
@@ -189,29 +202,35 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn next(&mut self) -> Option<T> {
-        if self.ptr == self.end {
-            None
-        } else if T::IS_ZST {
-            // `ptr` has to stay where it is to remain aligned, so we reduce the length by 1 by
-            // reducing the `end`.
-            self.end = self.end.wrapping_byte_sub(1);
-
-            // Make up a value of this ZST.
-            Some(unsafe { mem::zeroed() })
+        if T::IS_ZST {
+            if self.ptr.as_ptr() == self.end as *mut _ {
+                None
+            } else {
+                // `ptr` has to stay where it is to remain aligned, so we reduce the length by 1 by
+                // reducing the `end`.
+                self.end = self.end.wrapping_byte_sub(1);
+
+                // Make up a value of this ZST.
+                Some(unsafe { mem::zeroed() })
+            }
         } else {
-            let old = self.ptr;
-            self.ptr = unsafe { self.ptr.add(1) };
+            if self.ptr == non_null!(self.end, T) {
+                None
+            } else {
+                let old = self.ptr;
+                self.ptr = unsafe { old.add(1) };
 
-            Some(unsafe { ptr::read(old) })
+                Some(unsafe { ptr::read(old.as_ptr()) })
+            }
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let exact = if T::IS_ZST {
-            self.end.addr().wrapping_sub(self.ptr.addr())
+            self.end.addr().wrapping_sub(self.ptr.as_ptr().addr())
         } else {
-            unsafe { self.end.sub_ptr(self.ptr) }
+            unsafe { non_null!(self.end, T).sub_ptr(self.ptr) }
         };
         (exact, Some(exact))
     }
@@ -219,7 +238,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     #[inline]
     fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let step_size = self.len().min(n);
-        let to_drop = ptr::slice_from_raw_parts_mut(self.ptr as *mut T, step_size);
+        let to_drop = ptr::slice_from_raw_parts_mut(self.ptr.as_ptr(), step_size);
         if T::IS_ZST {
             // See `next` for why we sub `end` here.
             self.end = self.end.wrapping_byte_sub(step_size);
@@ -261,7 +280,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
             // Safety: `len` indicates that this many elements are available and we just checked that
             // it fits into the array.
             unsafe {
-                ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, len);
+                ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, len);
                 self.forget_remaining_elements();
                 return Err(array::IntoIter::new_unchecked(raw_ary, 0..len));
             }
@@ -270,7 +289,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // Safety: `len` is larger than the array size. Copy a fixed amount here to fully initialize
         // the array.
         return unsafe {
-            ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, N);
+            ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, N);
             self.ptr = self.ptr.add(N);
             Ok(raw_ary.transpose().assume_init())
         };
@@ -288,7 +307,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // Also note the implementation of `Self: TrustedRandomAccess` requires
         // that `T: Copy` so reading elements from the buffer doesn't invalidate
         // them for `Drop`.
-        unsafe { if T::IS_ZST { mem::zeroed() } else { ptr::read(self.ptr.add(i)) } }
+        unsafe { if T::IS_ZST { mem::zeroed() } else { self.ptr.add(i).read() } }
     }
 }
 
@@ -296,18 +315,25 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     #[inline]
     fn next_back(&mut self) -> Option<T> {
-        if self.end == self.ptr {
-            None
-        } else if T::IS_ZST {
-            // See above for why 'ptr.offset' isn't used
-            self.end = self.end.wrapping_byte_sub(1);
-
-            // Make up a value of this ZST.
-            Some(unsafe { mem::zeroed() })
+        if T::IS_ZST {
+            if self.end as *mut _ == self.ptr.as_ptr() {
+                None
+            } else {
+                // See above for why 'ptr.offset' isn't used
+                self.end = self.end.wrapping_byte_sub(1);
+
+                // Make up a value of this ZST.
+                Some(unsafe { mem::zeroed() })
+            }
         } else {
-            self.end = unsafe { self.end.sub(1) };
+            if non_null!(self.end, T) == self.ptr {
+                None
+            } else {
+                let new_end = unsafe { non_null!(self.end, T).sub(1) };
+                *non_null!(mut self.end, T) = new_end;
 
-            Some(unsafe { ptr::read(self.end) })
+                Some(unsafe { ptr::read(new_end.as_ptr()) })
+            }
         }
     }
 
@@ -333,7 +359,11 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {
     fn is_empty(&self) -> bool {
-        self.ptr == self.end
+        if T::IS_ZST {
+            self.ptr.as_ptr() == self.end as *mut _
+        } else {
+            self.ptr == non_null!(self.end, T)
+        }
     }
 }