This is the next upgrade to the Rust toolchain, from 1.73.0 to 1.74.1
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4d ("rust: upgrade to Rust 1.68.2").

# Unstable features

No unstable features (that we use) were stabilized.

Therefore, the only unstable features allowed to be used outside the
`kernel` crate are still `new_uninit,offset_of`, though other code to
be upstreamed may increase the list (e.g. `offset_of` was added recently).

Please see [3] for details.

# Other improvements

Rust 1.74.0 allows to use `#[repr(Rust)]` explicitly [4], which can be
useful to be explicit about particular cases that would normally use
e.g. the C representation, such as silencing lints like the upcoming
additions we requested [5] to the `no_mangle_with_rust_abi` Clippy lint
(which in turn triggered the `#[repr(Rust)]` addition).

Rust 1.74.0 includes a fix for one of the false negative cases we reported
in Clippy's `disallowed_macros` lint [6] that we would like to use in
the future.

Rust 1.74.1 fixes an ICE that the Apple AGX GPU driver was hitting [7].

# Required changes

For this upgrade, no changes were required (i.e. on our side).

# `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-1741-2023-12-07 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://github.com/Rust-for-Linux/linux/issues/2 [3]
Link: https://github.com/rust-lang/rust/pull/114201 [4]
Link: https://github.com/rust-lang/rust-clippy/issues/11219 [5]
Link: https://github.com/rust-lang/rust-clippy/issues/11431 [6]
Link: https://github.com/rust-lang/rust/issues/117976#issuecomment-1822225691 [7]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20231214092958.377061-1-ojeda@kernel.orgSigned-off-by: Miguel Ojeda <ojeda@kernel.org>