Re: [PATCH 1/4] rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()

[Miguel Ojeda]

On Thu, Jul 31, 2025 at 5:49 PM Danilo Krummrich <d...@kernel.org> wrote:
>
> +impl Kmalloc {
> +    /// Returns a [`Layout`] that makes [`Kmalloc`] fulfill the requested 
> size and alignment of
> +    /// `layout`.
> +    pub const fn aligned_layout(layout: Layout) -> Layout {

I think this `const fn` here was removed when applying to make it work
with older compilers, right?

I was fixing another `rusttest` thing and noticed while applying
these. I had a patch to fix it, since we can actually just use the
feature, and then I noticed it wasn't in the tree. Since I have it, I
am attaching it for reference in case the now-stable feature is
needed, e.g. if you want to make that `const fn` again.

Cheers,
Miguel

From 3cf24f05614e33493ba086bab1f02f701c894807 Mon Sep 17 00:00:00 2001
From: Miguel Ojeda <ojeda@kernel.org>
Date: Sat, 16 Aug 2025 22:20:29 +0200
Subject: [NOT PATCH] rust: kernel: use `const_alloc_layout` feature to fix Rust
 < 1.85 builds

Before Rust 1.85.0 (released 2025-02-20), `pad_to_align` was not available
in const contexts:

    error: `core::alloc::Layout::pad_to_align` is not yet stable as a const fn
       --> rust/kernel/alloc/allocator.rs:122:9
        |
    122 |         layout.pad_to_align()
        |         ^^^^^^^^^^^^^^^^^^^^^
        |
        = help: add `#![feature(const_alloc_layout)]` to the crate attributes to enable

Since the `const_alloc_layout` feature [1] is already stable [2] and is
available in our minimum Rust version too, simply add it to the list of
the ones we use.

Link: https://github.com/rust-lang/rust/issues/67521 [1]
Link: https://github.com/rust-lang/rust/pull/134768 [2]
Not-Fixes (patch was changed on apply): fde578c86281 ("rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()")
Link: 20250731154919.4132-2-dakr@kernel.org/">https://lore.kernel.org/rust-for-linux/20250731154919.4132-2-dakr@kernel.org/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/lib.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ed53169e795c..94d174ed7b8b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -31,6 +31,9 @@
 #![feature(const_ptr_write)]
 #![feature(const_refs_to_cell)]
 //
+// Stable since Rust 1.85.0.
+#![feature(const_alloc_layout)]
+//
 // Expected to become stable.
 #![feature(arbitrary_self_types)]
 //
--
2.50.1