This is the next upgrade to the Rust toolchain, from 1.77.1 to 1.78.0
(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").

It is much smaller than previous upgrades, since the `alloc` fork was
dropped in commit 9d0441ba ("rust: alloc: remove our fork of the
`alloc` crate") [3].

# Unstable features

There have been no changes to the set of unstable features used in
our own code. Therefore, the only unstable features allowed to be used
outside the `kernel` crate is still `new_uninit`.

However, since we finally dropped our `alloc` fork [3], all the unstable
features used by `alloc` (~30 language ones, ~60 library ones) are not
a concern anymore. This reduces the maintenance burden, increases the
chances of new compiler versions working without changes and gets us
closer to the goal of supporting several compiler versions.

It also means that, ignoring non-language/library features, we are
currently left with just the few language features needed to implement the
kernel `Arc`, the `new_uninit` library feature, the `compiler_builtins`
marker and the few `no_*` `cfg`s we pass when compiling `core`/`alloc`.

Please see [4] for details.

# Required changes

## LLVM's data layout

Rust 1.77.0 (i.e. the previous upgrade) introduced a check for matching
LLVM data layouts [5]. Then, Rust 1.78.0 upgraded LLVM's bundled major
version from 17 to 18 [6], which changed the data layout in x86 [7]. Thus
update the data layout in our custom target specification for x86 so
that the compiler does not complain about the mismatch:

    error: data-layout for target `target-5559158138856098584`,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128`,
    differs from LLVM target's `x86_64-linux-gnu` default layout,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

In the future, the goal is to drop the custom target specifications.
Meanwhile, if we want to support other LLVM versions used in `rustc`
(e.g. for LTO), we will need to add some extra logic (e.g. conditional on
LLVM's version, or extracting the data layout from an existing built-in
target specification).

## `unused_imports`

Rust's `unused_imports` lint covers both unused and redundant imports.
Now, in 1.78.0, the lint detects more cases of redundant imports [8].
Thus one of the previous patches cleaned them up.

## Clippy's `new_without_default`

Clippy now suggests to implement `Default` even when `new()` is `const`,
since `Default::default()` may call `const` functions even if it is not
`const` itself [9]. Thus one of the previous patches implemented it.

# Other changes in Rust

Rust 1.78.0 introduced `feature(asm_goto)` [10] [11]. This feature was
discussed in the past [12].

Rust 1.78.0 introduced `feature(const_refs_to_static)` [13] to allow
referencing statics in constants and extended `feature(const_mut_refs)`
to allow raw mutable pointers in constants. Together, this should cover
the kernel's `VTABLE` use case. In fact, the implementation [14] in
upstream Rust added a test case for it [15].

Rust 1.78.0 with debug assertions enabled (i.e. `-Cdebug-assertions=y`,
kernel's `CONFIG_RUST_DEBUG_ASSERTIONS=y`) now always checks all unsafe
preconditions, though without a way to opt-out for particular cases [16].
It would be ideal to have a way to selectively disable certain checks
per-call site for this one (i.e. not just per check but for particular
instances of a check), even if the vast majority of the checks remain
in place [17].

Rust 1.78.0 also improved a couple issues we reported when giving feedback
for the new `--check-cfg` feature [18] [19].

# `alloc` upgrade and reviewing

As mentioned above, compiler upgrades will not update `alloc` anymore,
since we dropped our `alloc` fork [3].

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1780-2024-05-02 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://lore.kernel.org/rust-for-linux/20240328013603.206764-1-wedsonaf@gmail.com/ [3]
Link: https://github.com/Rust-for-Linux/linux/issues/2 [4]
Link: https://github.com/rust-lang/rust/pull/120062 [5]
Link: https://github.com/rust-lang/rust/pull/120055 [6]
Link: https://reviews.llvm.org/D86310 [7]
Link: https://github.com/rust-lang/rust/pull/117772 [8]
Link: https://github.com/rust-lang/rust-clippy/pull/10903 [9]
Link: https://github.com/rust-lang/rust/pull/119365 [10]
Link: https://github.com/rust-lang/rust/issues/119364 [11]
Link: https://lore.kernel.org/rust-for-linux/ZWipTZysC2YL7qsq@Boquns-Mac-mini.home/ [12]
Link: https://github.com/rust-lang/rust/issues/119618 [13]
Link: https://github.com/rust-lang/rust/pull/120932 [14]
Link: https://github.com/rust-lang/rust/pull/120932/files#diff-e6fc1622c46054cd46b1d225c5386c5554564b3b0fa8a03c2dc2d8627a1079d9 [15]
Link: https://github.com/rust-lang/rust/issues/120969 [16]
Link: https://github.com/Rust-for-Linux/linux/issues/354 [17]
Link: https://github.com/rust-lang/rust/pull/121202 [18]
Link: https://github.com/rust-lang/rust/pull/121237 [19]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240401212303.537355-4-ojeda@kernel.org
[ Added a few more details and links I mentioned in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>