Re: [PATCH 1/2] rust: upgrade to Rust 1.71.0

[Trevor Gross]

On Sat, Jul 29, 2023 at 6:05 PM Miguel Ojeda <ojeda@xxxxxxxxxx> wrote:
>
> This is the second upgrade to the Rust toolchain, from 1.68.2 to 1.71.0
> (i.e. the latest).
>
> See the upgrade policy [1] and the comments on the first upgrade in
> commit 3ed03f4da06e ("rust: upgrade to Rust 1.68.2").
>
> # Unstable features
>
> No unstable features (that we use) were stabilized.
>
> Therefore, the only unstable feature allowed to be used outside
> the `kernel` crate is still `new_uninit`, though other code to be
> upstreamed may increase the list.
>
> Please see [2] for details.
>
> # Required changes
>
> For the upgrade, this patch requires the following changes:
>
>   - Removal of the `__rust_*` allocator functions, together with
>     the addition of the `__rust_no_alloc_shim_is_unstable` static.
>     See [3] for details.
>
>   - Some more compiler builtins added due to `<f{32,64}>::midpoint()`
>     that got added in Rust 1.71 [4].
>
> # `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://rust-for-linux.com/rust-version-policy [1]
> Link: https://github.com/Rust-for-Linux/linux/issues/2 [2]
> Link: https://github.com/rust-lang/rust/pull/86844 [3]
> Link: https://github.com/rust-lang/rust/pull/92048 [4]
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> [...]

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>

A few highlights since 1.68.2 that may eventually be of use:

- `CStr::is_empty` and `CStr::from_bytes_until_nul` help smooth FFI a bit
- `NonNull::slice_from_raw_parts` for slice pointers
- `AtomicX::as_ptr` makes interop with C more clear (doesn't seem like we
  use these types anywhere yet)
- const `ptr::read` and `ptr::read_unaligned`
- `{Option, Result}::is_some_and`
- `core::cell::OnceCell` one-time initialization
- `repr(C)` enums now use `c_int` rather than always `i32` (no difference for
  currently supported targets)